BACKGROUND: In the search for more powerful tools for diagnoses of endemic diseases in resource-limited settings, we have been analyzing technologies with potential applicability. Increasingly, the process focuses on readily accessible bodily fluids combined with increasingly powerful multiplex capabilities to unambiguously diagnose a condition without resorting to reliance on a sophisticated reference laboratory. Although these technological advances may well have important implications for the sensitive and specific detection of disease, to date their clinical utility has not been demonstrated, especially in resource-limited settings. Furthermore, many emerging technological developments are in fields of physics or engineering, which are not readily available to or intelligible to clinicians or clinical laboratory scientists.
CONTENT: This review provides a look at technology trends that could have applicability to high-sensitivity multiplexed immunoassays in resource-limited settings. Various technologies are explained and assessed according to potential for reaching relevant limits of cost, sensitivity, and multiplex capability. Frequently, such work is reported in technical journals not normally read by clinical scientists, and the authors make enthusiastic claims for the potential of their technology while ignoring potential pitfalls. Thus it is important to draw attention to technical hurdles that authors may not be publicizing.
SUMMARY: Immunochromatographic assays, optical methods including those involving waveguides, electrochemical methods, magnetorestrictive methods, and field-effect transistor methods based on nanotubes, nanowires, and nanoribbons reveal possibilities as next-generation technologies.
In a recent review (1), Hawkins and Weigl set the criteria for applicability of tests to low-resource settings using the acronym ASSURED: Affordable, Sensitive, Specific, User-friendly, Rapid and robust, Equipment-free, and Deliverable. We add to the list stability to extremes of temperature and humidity, low weight, minimal waste, and the possibility of being manufactured in a developing country. We further add the ability to perform multiplex assays, but have not included nucleic acid–based assays in this review, except when the technology used has direct relevance to immunoassays.
As part of our strategy, we initially performed an internet search for companies with names already known to us and reviewed publications on the company web sites or publications found by using the corresponding address fields in the Web of Science online database. We further selected important reports according to the high citation frequencies in the Web of Science. For example, at the end of 2011 a report by Patolsky et al. (2) had been cited 432 times. We explored report citations for relevant technologically oriented publications and consulted bibliographies in those reports where relevant. We limited the analysis to the peer-reviewed literature, with the exception of patents included to illustrate historical aspects. A review of the entire field was beyond the scope of this review, and there may be commercial developments that are inaccessible if held as trade secrets. There may also be further issues related to sample preparation that are intrinsic to the sample type and also beyond the scope of this review.
We analyzed specific embodiments and formulated some general conclusions concerning the possibilities for use in point-of-care settings of the technology categories we have covered.
Self-performing assays are the ideal tools for use in resource-limited settings because they require only sample addition, and then a series of reactions are initiated that result in a readable result. Lateral flow immunochromatographic assays (LFIA)2 flow immunochromatographic assay; FET, field-effect transistor; GMR, giant magnetoresistive.. are the best-established methods in this genre. In the classical sandwich format, the sample solubilizes and mobilizes a labeled antibody, and together the sample and antibody are chromatographed along a membrane and bind to a capture site, where the label forms a visible deposit if captured. No wash step is required because the captured label concentrates sufficiently above the background of unbound label to become visible. We recently published an extensive review of the literature on LFIA (3). The impact of the label on the limit of detection was apparent. For example, Parpia et al. (4) used a carbon label to develop a pediatric test for determining the presence of HIV p24 antigen in newborn heelstick blood samples, for which conventional HIV antibody assays would be confounded by the presence of maternal antigen. These investigators achieved limits of detection of approximately 2 pmol/L. This result was for 1 analyte only. LFIA does not require a skilled operator and has the potential for inexpensive high-volume manufacturability. Thus this test lends itself to performance in resource-limited settings. However, we found no information on feasibility of transplanting the manufacture of the membrane to a resource-limited setting.
Leung et al. (5) described an LFIA method for the early detection of myocardial infarction by duplex analysis of heart fatty acid–binding protein and a semiquantitative bar-code assay for C-reactive protein as a cardiac risk marker. This exploratory assay has not been established as having clinical utility and so far has been described only in a research publication.
Biosite has commercialized a family of tests under the name Triage, which have evolved over a period of years from membrane-based, non–self-performing assays to capillarity-driven self-performing assays. These techniques are therefore distinct from LFIA. The Triage Drug Screen (6) and Cardiac Panel (7) are multiplex immunoassays that are currently commercially available for point-of-care use. The Triage tests are more complex than standard LFIA. In the earliest embodiment of the Drug Screen (6), a competitive immunoassay was used, in which colloidal gold-labeled drug-analog tracer was bound to an antibody and so was not available for binding to antidrug on the solid phase. When drug was present, the competition prevented formation of the complex, so the label became available for binding to the solid phase. In this way, 7 separate drug assays were performed simultaneously. Sample and lyophilized reagents were first mixed in a well for a fixed time and then distributed along a membrane, where specific drug antibodies were immobilized in defined locations. The membrane was then washed to reveal the presence of a drug above a defined threshold as a visible band. The Biosite Triage Cardiac Panel (7) appears to be more technologically advanced than the first reported drug screen (6). Three steps have been reduced to a 1-step addition of whole blood or plasma. The sample migrates through a filter into a reaction chamber containing dry fluorescent-labeled antibodies. The sample remains in the chamber for a time determined by a reaction gate. The reaction gate becomes hydrophilic with contact with the sample, and the liquid is then conducted by capillary action through grooves that transport the liquid past the capture antibodies into a used reagent reservoir. Sandwich immunoassay formation takes place at the locations of immobilized capture antibodies, and the excess sample serves to wash out excess label. The results are read in an automatic reader. A more recent version of the Triage Drug Screen (8) appears to follow the same technology as the Cardiac Panel and uses a simple competition format. The Triage Panel tests bring the simplicity of LFIA with multiplex capability, but the internal complexity may limit use in resource-limited settings because of high cost and limited availability.
Hong et al. (9) have developed microfluidic devices in which reagent streams and flow are controlled by a combination of fluidic microcircuits and surface tension barriers. However, the use of these devices for clinically relevant analytes has not been demonstrated, and the only multiplex use attempted was with dual fluorescent labels.
Two groups of investigators have reported microfluidics-based systems directed at resource-limited areas. These investigators attempted to achieve enhanced sensitivity with silver-enhanced gold nanoparticle labeling to attain ELISA sensitivity without the complication of dealing with enzymes and substrates. Sia et al. (10) demonstrated the feasibility of a low-cost transmission-based detection system and microfluidic chip, but did not show how reagent addition would be performed in the resource-limited environment. Chin et al. (11) used a vacuum pulled by a syringe and individual reagent additions separated into slugs by air gaps. Their report provides data on the performance of this system in the field, but the requirement for manual control of 14 successive reagents may still be a challenge.
Considerable efforts have been made to fabricate low-cost microfluidic systems that use chromatographic flow through paper or membranes. In 1990, one of us first described a method for creating barriers in a membrane to create liquid microcircuits and programmable microfluidics (12). We created flow channels by laser-etching barriers within nitrocellulose membranes backed with plastic. Recently, nitrocellulose-based systems have been described for variously shaped networked systems for controlled flow delivery of reagents (13). Three-dimensional systems have also been created by layering paper with controlled flow paths and communication between layers for multiple samples and multiple analytes (14). Neither of these approaches (13, 14) has shown feasibility data for immunoassays.
Melin et al. (15) have described a lateral flow system designed to control flow and avoid membrane issues, with the membrane replaced by an injection-molded, patterned-surface pillar chip (“4castchip”) developed by Åmic, a wholly owned subsidiary of Johnson and Johnson Nordic. The described system performs duplex assays for C-reactive protein and B-type natriuretic peptide, both cardiac risk markers, and consists of a sample zone, a reaction zone, and a wicking zone. Plasma sample and label are mixed and applied to the sample zone. The liquid migrates past capture antibodies at a reaction zone, and a wicking zone removes the excess liquid. The signals in the reaction zone are read with a fluorescence reader. The C-reactive protein analysis is performed as a competitive assay to bring the sensitivity into a clinically relevant range on the undiluted sample. The pillar chip can in principle be manufactured on a much larger scale and less expensively than membranes. The requirement to mix sample and label off-line limits its applicability to use in resource-limited settings, and so far only a duplex approach has been demonstrated.
Doering et al. (16) described a triplex lateral flow immunoassay for respiratory viruses with 3 assays in 1 capture bar. The detection antibodies were labeled with nanogold coated with silica and specific Raman reporter dyes, commercialized by Oxonica, that use surface-enhanced Raman resonance spectroscopy to magnify the signal. Illumination is with a monochromatic laser in the infrared, where there is little background from biological materials. The method requires a laser and a spectrometer to read the results. No experimental details or data from clinical specimens has been published.
Optical Immunosensor Systems
Several groups have described optical biosensor systems that directly detect binding events and do not require label. The obviation of the label requirement results in a potential simplification of the assay system so that it can also be regarded as self-performing. The Biacore system, recently reviewed by Gopinath (17), uses surface plasmon resonance at a gold film surface, and changes in refractive index are measured. This system has found extensive application for the determination of binding interactions but is strictly intended for laboratory use. There is a wealth of publications on the determination of kinetic and binding constants by use of this technology, and limits of detection would relate directly to the affinity of the antibody used. This system performs only monoplex-binding interactions, although through automation it can be used for high-throughput parallel screening of multiple binding interactions.
Gauglitz et al. (18) demonstrated the possibility of performing immunoassays by reflectance spectroscopy. The binding reaction occurs on the surface of an interference filter, and the optical thickness of the interference filter is modified by a macromolecular binding reaction. The spectral shift as a function of time was used to determine the binding reaction. Zavali et al. (19, 20) have described a system that uses white light reflectance spectroscopy to demonstrate IgG binding to anti-IgG. The detection of a 150 pmol/L concentration was shown within 1 minute. The obviation of label and wash steps results in a very simple protocol, and these authors have described conditions for regeneration of the probe, thus minimizing the number of disposables. However, no application to clinical samples or multiplexing capability has been demonstrated.
The ForteBio Octet uses similar white light spectroscopy (21,–,23), but appears to be a simplification in that it interrogates directly internal reflection from the tip of a probe at the end of an optical fiber. The incident light and reflected light interfere, and the interference is affected by the optical thickness of the biological layer on the probe tip (biolayer interferometry). The wavelength shift can be used to measure the binding in real time. Disposable probes are used in groups of 8 in wells of a microtiter plate, thus giving potentially 8-plex assays. Because this is a dip-and-read method, neither reagent additions nor a fluidic system is required. Binding events are determined only at the optical surface, so the system is insensitive to sample matrix effects. Applications to date have been for laboratory use, not clinical applications. The use in a resource-limited field environment is likely to be limited by the need for a spectrometer.
Axela has developed a label-free optical sensor system, the dotLab system, which is based on spots made up of lines that create an optical grating (24). The grating is illuminated by laser and the diffraction pattern generated by total internal reflection through the transparent supporting medium. The intensity of the diffraction pattern changes with bound species on the lines. The lines are printed avidin to which will bind preformed immune complexes including biotin label. The device comprises a microchannel containing a row of 8 circular spots, so in principle, 8-plex immunoassays might be performed. The diffraction pattern is read with a charge-coupled device camera. The system is run by a programmable fluidic controller and has the options of enzyme label amplification as well as label-free detection. The full potential of this system in a clinical setting is yet to be determined.
Some years ago, one of us published a system for detection of binding events with an evanescent waveguide-directed illumination and light-scattering label (25, 26). MBio Diagnostics, a subsidiary of Precision Photonics, have developed a similar system that uses a fluorescent label and complementary metal oxide semiconductor camera detection (27), and have published a demonstration experiment with a manual assay and 5 hepatitis C virus and HIV antibodies. The system is a potentially low-cost approach, but applicability to a resource-limited environment has yet to be demonstrated.
A group at Phillips has developed an opto-magnetic biosensor (28, 29) in which magnetic microparticles are deposited on one surface of a cuvette, and the opposing surface is interrogated by frustrated total internal reflectance. Plasma was added directly to the cuvette and the immunoreaction on the waveguide surface was accelerated by an external magnetic field. The procedure was 1 step. No washes were required. The results were read in real time by a charge-coupled device. This system therefore has many capabilities suitable for resource-limited settings, but the ability to manufacture it on a large scale and the cost are unknown.
A group at Purdue University has developed a spinning-disk technology that they call BioCD (30, 31). This group uses interferometric data with argon laser illumination reflected from a 120-nm silicon dioxide surface. Scanning the disk before and after binding reactions gives a differential thickness map that relates to the bound material. These investigators have described a 96-well format, in which the wells are created by rings of hydrophobic ink and therefore can accommodate individual samples, or a 25 000-spot array without the options of subjecting the spots to different solutions. They have shown decreased signal to noise, and consequent decreased limits of detection, by averaging data from multiple spots. This technology is, in principle, an immunosensor not requiring a label. For sufficient sensitivity with analytes such as prostate-specific antigen, a sandwich assay is performed in such a way that the second antibody functions as a mass label. Although this system in principle lends itself to high multiplex assays, the disk does not correspond to a standard CD, protocols are multistep in spite of being potentially label free, the surface must be washed and dried to read it, and the apparatus uses an argon laser and sensitive optics. The method thus does not lend itself to field use in a resource-limited setting. Quadraspec was founded to commercialize this spinning-disk system. The same group published a variant of this technology (32), in which the sample is illuminated with a light-emitting diode through a microscope objective, and the interference between the incident and reflected beams is used to compute the optical thickness of the layer. Differences are enhanced by computing the difference between adjacent images. This variant is a simplification of the BioCD detection system, with a compromise in the limit of detection.
The BioCD system above uses spinning disks that resemble CD technology because CD technology is so widespread, mass produced, and low cost for the music industry. In addition, several attempts have been made to use optical scanning of spinning disks as the basis for immunoassays. A group in Madrid has been developing technology based on standard disks (33,–,38). In the most recent version (38), 6 environmentally related small molecules are determined in a competition assay with haptens directly linked to the plastic surface, and colloidal gold antibody is used for label with silver intensification. The intensity is determined directly by the CD head reader with DVDs. Li et al. (39) have devised a method in which an unmodified disk with a prerecorded audio file can be read with existing error-detection software to determine locations on disk where there is a binding reaction. Although there are clear advantages in using the existing CD/DVD technology, at present the process of printing reagents is not at the same production scale, and the protocols require multiple reagent treatments that do not lend themselves to use in the field.
A group in Athens has developed a capillary-based immunosensor system for performing multiplex assays by locating binding sites at specific locations within a capillary, illuminating with a laser, and reading a fluorescence wave guided through the capillary walls (40,–,42). The first antibody is mixed with the sample, flowed through the capillary, and followed by a fluorescent-labeled antibody, and the capillary is interrogated with the laser. Although there is a simplification achieved by flow through a capillary, there are challenges in depositing capture probes at defined locations within the capillary. Members of the same group (43, 44) have described an integrated device with an LED and detector coupled through a waveguide, which detects fluorescent label-binding events on the surface of the waveguide. The device comprises 9 LEDs and sensor surfaces, and a single detector. The Athens group has used the monolithic device for multiplex detection of PCR products, although it could equally well be used for immunoassays. The entire device can be fabricated by standard microfabrication technology and is proposed for use as a point-of care system. Further work would need to be done to demonstrate fabrication costs within the range of requirements for resource-limited settings, and applicability to relevant diseases.
An electrochemical technology has been developed at the Fraunhofer Institute, and a device has been commercialized by eBiochip and referred to as the eMicroELISA (45). The device has been used for both multiplex nucleic acid detection and immunoassays, but all applications include an enzyme-labeled antibody-binding step. For example, Quiel et al. (46) have described its use for detection of bacterial toxins. The device consists of 16 electrode-array positions, each electrode array being made up of interdigitated gold elements (45). Capture antibodies are located on the electrodes and an automated ELISA is programed into the device. Detection is via enzyme labels with p-aminophenol as an electroactive product. This causes redox recycling at the electrodes and results in an electrical signal that corresponds to the amount of bound product. Total assay time is 26 min. The use of the system in point-of-care or field applications has been proposed (47) on the grounds of its low cost and simplicity of operation compared with comparable systems. Whether this can be borne out in practice for resource-limited applications remains to be seen.
Another amperometric array system has been developed by Genefluidics (48). The device consists of 6 individually addressable gold electrodes coated onto a plastic base, and a pneumatically and hydraulically controlled microfluidic system that incorporates a cartridge preloaded with reagents; this device may be controlled by a smartphone. Horseradish peroxidase provides the label for immunoassays or nucleic acid hybridization assays. The reduction of tetramethylbenzidine by horseradish peroxidase provides a redox current, which is interpreted by the amperometric detection system. Assay time may be under 1 h. Clinical validation has been mostly with nucleic acid–based systems (49). An earlier version was described for the use of a combination of proteomic biomarkers (thioredoxin and interleukin-8) and 4 mRNA biomarkers (SAT, ODZ, interleukin-8, and interleukin-1b) in saliva as a test for oral cancer (50). The system is deemed applicable to near patient testing or resource-limited settings (48), but so far there are no data on assays in resource-limited settings, and no cost analysis has been provided.
The Abbott I-Stat is a handheld analyzer that includes immunoassay capability, for example for cardiac troponin I. The system uses an alkaline phosphatase–labeled sandwich immunoassay, electroactive product, and amperometric readout (51). Although this device achieves clinically useful analytical sensitivity for a commercial product, the complexity and cost would appear to limit its utility for resource-limited settings for immunoassays. It also has no claims of multiplex capability.
Field-Effect Transistor Technology
A field-effect transistor (FET) consists of 3 terminals; the source, drain, and gate. The current is conducted by one type of carrier (electrons or holes), depending on the type of FET (n-channel or p-channel). Charge at the gate surface causes charges or free holes to be repelled from the region of the substrate under the gate, leaving behind a carrier-depletion region. When sufficient charge is present, the source and drain regions are electrically bridged. FETs or ion-sensitive FETs have the advantage over electrochemical methods in that no label is needed. The devices are based on transistors in which the current is affected by the surface charge. When a bound protein has significant charge, the binding reaction is determined directly as a function of the charge of the bound species. The disadvantage is that the effect will be critically dependent on ionic and pH conditions, and so there will be a susceptibility to matrix effects. A further consideration is that the system is sensitive to the spatial distribution of the charge, so that the further the bound molecule extends from the surface, the smaller the effect (52). Park et al. (53) have described an application for monitoring C-reactive protein. All data were collected from solutions in buffer and no attempt was made to use serum or plasma as sample. No multiplex format was shown. Bian et al. (54) described an FET-based system for duplex determination of hemoglobin and hemoglobin A1c and proposed its use as a bedside system. No data on patient samples were provided, and the only data were for diluted analyte in buffer.
FET devices have the advantage that sensitivity can be increased by reduction of scale. A device using nanowires configured as FETs was described by Patolsky et al. (2). Their device consisted of silicon nanowires spread between source and drain at 2-μm separation. Single virus-binding events were detected stochastically. The same group (55) also described a multiplexed system for cancer markers, prostate-specific antigen, carcinoembryonic antigen, and mucin-1. However, the assays were all performed in diluted buffer, and the control for effect of real sample was done with spiked dialyzed goat serum. Thus, in spite of the potential for a very simple, exquisitely sensitive system that uses no label, the issue of ion content and pH of the clinical sample remains unanswered. Further enhancement of sensitivity was shown by Gong (56) to be achievable by focusing the analyte onto the nanowires by dia-electrophoresis, by applying an alternating current field to the system. Gong used prostate-specific antigen as a model system and extended the limit of detection from 1fmol/L to 1amol/L. This low detection was also achieved with pure protein diluted into low ionic strength buffer. No attempt was made to collect data from clinical samples.
In principle, nanoscale devices can be manufactured on a large scale and at a very low cost. The fabrication methods used in some FET devices (2, 55, 56) have been described (57). The nanowires are fabricated separately in a liquid phase in a 7-step process, and the integration into the final device is another 9-step process. Thus, this multistep production might limit the potential of nanoscale devices for inexpensive large-scale production.
Carbon nanotubes have also been proposed as an alternative to nanowires. The dimensions are smaller than nanowires, so they have a potentially higher sensitivity (58). Briman et al. (59) have also attempted to avoid the requirement of low ionic strength by using a network of nanotubes as a capacitive device rather than a transistor. They showed feasibility of measurement of prostate-specific antigen down to 1 μg/L in calf serum.
Alternative fabrication methods for nanowires and nanoribbons have been described. These are methods of top-down fabrication, in which the wires are printed by direct lithography rather than chemical growth and transfer to a chip. Stern et al. (60) fabricated nanowires with a trapezoidal section by selective etching of a silicon-on-insulator surface. Conditions were described for selective functionalization of the nanowire and a model system with streptavidin binding to a biotinylated surface. Significant electrical response was observed down to 10 fmol/L. Furthermore, concentrations of 100 fmol/L were detected with binding of IgG to anti-IgG. Although this system might lend itself to large-scale inexpensive fabrication, the authors demonstrated only monoplex testing performed in low ionic strength buffer. There were no data for clinical samples. Elfstrom et al. (61) have also demonstrated experiments with avidin–biotin using nanoribbons, but found a limit of detection of 1 pmol/L.
Chua at al (62) created a multiplexable top-down microfabrication system consisting of 36 clusters of 5 nanowires each. However, they did not attempt to address multiplex testing. They established limits of detection for a cardiac tropinin I concentration of 1 fg/mL in low ionic strength buffer or 30 fg/mL in undiluted serum. However, the serum was previously desalted. A requirement for sample desalting would be disadvantageous in resource-limited settings.
An alternative approach to the use of nanowires has been the introduction of a nanogap below the gate between the source and the drain electrodes. The capacitance in the nanogap affects the current between source and drain. The device has been called dielectric-modulated FET (63), and its feasibility with avidin–biotin binding has been demonstrated. The method is not sensitive to the charge of the bound species, as is the case for the standard ion-sensitive FETs. The technology was further improved (64) by the use of a double gap instead of single gap, and fabricated 6 × 6 arrays. Multiplexing was not attempted, but arrays were used to improve the statisitics of the results by averaging. Antigen (avian influenza virus AIa antigen) was bound selectively in the gap by use of linkage with silica-binding protein (the origin and use of which was not described), followed by specific anti-AIa. The device was dried before electrical measurements. No limit of detection was shown and no attempt was made to analyze clinical samples. Thus, although this device has been reported to be useable in point-of care settings, this work appears to be very preliminary.
Giant Magnetoresistive Sensor Technology
The giant magnetoresistive (GMR) sensor principle depends on the realignment of the spin of electrons in a magnetic field. Because of the quantum state, a small change in the magnetic field results in a large decrease in electrical resistance, hence the name giant. This principle has been used for many years in the encoding of computer hard drives. Srinivasan et al. (65) developed a GMR-based sensor, emphasizing the need for small size (13 nm) and uniformity of the magnetic nanoparticle label. The nanoparticles were also cubic in shape. The authors developed a sandwich assay system for interleukin-6 and showed a linear dependence on concentration and limit of detection of about 2 × 106 molecules. In further work, the same group demonstrated the use of a competition format to detect interleukin-6 down to concentrations of 50 fmol/L in human serum that had not been subjected to dialysis (66). Given the large-scale production of GMR devices for computer drives, it should be possible to manufacture these sensors on a large scale and at a low cost. The procedure, however, requires thorough washes after each binding reaction.
The group at Philips, in addition to the optical method described above (28, 29), described a GMR system based on the use of 300-nm magnetic particles (67). This group showed detection of parathyroid hormone at picomole per liter concentrations in buffer. The assay was a rapid 1-step reaction requiring no washes. No data were provided for clinical samples.
A group at Stanford has developed a GMR system based on 50-nm magnetic particles. Hall et al. (68) described their system as having an optimized electrical detection system and showed triplex results for their 64-element arrays. These authors also showed a method for improved reproducibility by building in correction for temperature variation (69). They found no advantage in magnetically directing particles to the sensor, and showed a lower limit of detection of 5 fmol/L for analyte diluted in buffer. No data were provided for clinical samples. A report by Hall et al. (68) gives a useful literature review of characteristics of GMR detection systems, although the criteria did not include issues of cost, manufacturability, or ease of use.
The various categories of technology are summarized in Table 1. It can be seen that LFIA has potential for high sensitivity given the choice of a suitable label, and is currently the preferred option. Although manufacturing is taking place in countries with emerging economies such as India and China, to our knowledge the manufacturing process has not been transplanted to resource-limited countries. Optical biosensors are attractive because of the possibility of true dipstick protocol, but the reflectance spectroscopy approach appears to be limited to the 10−10 mol/L range. Furthermore, although the requirement for a spectrometer appears to be a hurdle, one cannot rule out advances in a technology that would lead to the production of an extremely simple low-cost spectrometer. The adaptation of CD technology does not appear to bring any great advantage. Electrochemical methods bring a potential for higher sensitivity, but the requirement of on-board reagents for enzyme immunoassay to produce an electroactive product would be an added cost and complication, as well as a limitation to multiplex capability. The electronics could be low cost and even adapted from mobile phone technology. The variants of FET technology can bring extraordinarily high sensitivity and manufacturing could be large-scale, low-cost microfabrication, but the inherent sensitivity to salt ions may be the Achilles heel. GMR technology may be optimized to achieve ultrahigh sensitivity with multiplex capability, but sensitivity to temperature may still be an issue. In summary, a multitude of exciting technologies is emerging, but most have drawbacks of one kind or another. LFIA remains the current preferred option.
2 Nonstandard abbreviations:
- lateral flow immunochromatographic assay;
- field-effect transistor;
- giant magnetoresistive
Author Contributions: All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article.
Authors' Disclosures or Potential Conflicts of Interest: No authors declared any potential conflicts of interest.
- Received for publication September 27, 2011.
- Accepted for publication January 5, 2012.
- © 2012 The American Association for Clinical Chemistry