This case illustrates that initial laboratory results suggestive of SIADH do not rule out other potential causes of hyponatremia, such as hypothyroidism and hypocortisolism. To date, SIADH has been a diagnosis of exclusion, but advances in mass spectrometry offer prospects for accurately measuring ADH in the future.
When analyzers yield low sodium values, an initial priority is to exclude factitious hyponatremia. When sodium is measured by indirect methods, pseudohyponatremia due to a high serum protein or lipid content does not usually lower the sodium concentration to the degree observed in this case; however, the multiple potential sources of large errors in sodium measurements should be considered (e.g., sample dilution by intravenous fluids, storage of blood samples at reduced temperature, instrument failure, plugging of sample probes with clots, separator gel or cryoglobulins, sampling errors from inadequate samples or from bubbles in samples, or interfering substances) (1)(2)(3). Electrode methods can be affected by surfactants such as alcohols or other compounds used as skin disinfectants or by surfactants from other sources, such as catheter surfaces(3). Fortunately, well-trained laboratory staff can detect many of these errors before the reporting of results by repeating the analyses, by comparing the present value with previous results, and by evaluating other test results for the same sample. Sample re-collection may be required. In this case, the very low osmolality independently supports a severe electrolyte deficiency. The albumin and hemoglobin values suggest that no substantial sample dilution occurred. Fig. 1 indicates the analysis of additional samples that further rules out factitious hyponatremia.
Affecting women more often than men, autoimmune polyglandular syndrome type 2 is a polygenic disorder with onset in childhood or early adulthood; it is characterized by primary adrenal insufficiency or adrenal autoantibodies plus either autoimmune thyroid disease or type 1 diabetes (4). Because primary adrenal insufficiency typically involves destruction of the adrenocortical glomerulosa and fasciculata, measurement of renin and aldosterone would have been of interest in this case.
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Authors’ Disclosures of Potential Conflicts of Interest: No authors declared any potential conflicts of interest.
Role of Sponsor: The funding organizations played no role in the design of study, choice of enrolled patients, review and interpretation of data, or preparation or approval of manuscript.
- © 2009 The American Association for Clinical Chemistry