Serum creatinine is the most commonly used biomarker of kidney function. The assay is inexpensive and rapidly available in all hospital laboratories. In the steady state, the reciprocal of serum creatinine correlates with GFR, and in children height factored by serum creatinine (the Bedside Schwartz equation) provides an excellent surrogate for measured GFR. Thus, increases in serum creatinine are considered clinically to indicate decreased GFR, as would be expected from nephrotoxins, obstruction, or acute kidney injury. Although cystatin C has been used as a preferable or alternative indicator of GFR, the assay is not readily available in many hospital laboratories in the US, so its usefulness in detecting acute changes in GFR is limited. Cystatin C is freely filtered like creatinine, but there is normally no cystatin C left in the final urine, due to tubular uptake and degradation. In this report, the rapid availability of cystatin C facilitated a diagnosis. Imaging showed a novel fluid collection around the kidney collecting system in a 3-month-old boy. If urine were the source of this new collection, the physicians reasoned that the serum creatinine would rise, whereas there would be no change in serum cystatin C. To be sure, there was no baseline cystatin C, because kidney dysfunction was not anticipated, but the normal cystatin C value in the setting of a clearly increased creatinine value indicated the likelihood of extravasation of urine. Dissociation between creatinine and cystatin C values was also reported in an 8-year-old girl 1 week after renal transplantation with a defective anastomosis at the vesicoureteral junction (1). These and other studies indicate the need for hospitals performing kidney transplants, urological surgeries, and other tertiary procedures to have a readily available assay for cystatin C.
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 October 27, 2016.
- Accepted for publication November 2, 2016.
- © 2016 American Association for Clinical Chemistry