Objective. To quantify differences in results obtained by immunoassays (IAs) and tandem mass spectrometry (MSMS) for cortisol and free thyroxine (FT4). Design & Patients. Cortisol was measured over 60 minutes following a standard ACTH stimulation test ( ); FT4 was measured over time in two cohorts of pregnant ( ), and nonpregnant ( ) women. Measurements. Samples were analyzed with both IA and MSMS. Results. Results for cortisol by the two methods tended to agree, but agreement weakened over the 60-minute test and was worse for higher (more extreme) concentrations. The results for FT4 depended on the method. IA measurements tended to agree with MSMS measurements when values fell within “normal levels’’, but agreement was not constant across trimester in pregnant women and was poorest for the extreme (low/high) concentrations. Correlations between MSMS measurements and the difference between MSMS and IA results were strong and positive ( ; all ). Conclusions. IA and MSMS provide different measures of cortisol and FT4 at extreme levels, where clinical decision making requires the greatest precision. Agreement between the methods is inconsistent over time, is nonlinear, and varies with the analyte and concentrations. IA-based measurements may lead to erroneous clinical decisions. 1. Introduction In this paper, we compare the agreement [1–7] of immunoassay (IA) and tandem mass spectrometry (MSMS) in the measurement of the concentrations of two analytes, cortisol, and free T4. Both analytes are critical for normal growth and development, and survival. Thyroid hormone regulates the expression of multiple genes and is necessary to maintain normal function in virtually all organ systems of the body [8]. Cortisol regulates normal responses to stress, and is important for vascular reactivity, carbohydrate metabolism, and immune function [9]. Cortisol measurement is the basis for diagnosing adrenal insufficiency or conditions of glucocorticoid excess. Mismeasurement of cortisol could lead to withholding treatment for patients with adrenal insufficiency or Cushings syndrome, which could have life-threatening consequences. Similarly, incorrect diagnosis of these conditions exposes patients to the toxic side effects of steroid or steroid-lowering therapies, without any expected benefit. FT4 is one of the analytes used to titrate the treatment of hypothyroidism and hyperthyroidism. Erroneous analyte values could lead to mismanagment of these disorders. This is a particular concern in vulnerable populations such as the young, the elderly, those who are pregnant, those
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