Serum samples from 9 healthy controls and from subjects with primary hyperparathyroidism (n = 5), Paget disease (n = 3), pregnancy (n = 5), glucocorticoid therapy (n = 5), postmenopausal osteoporosis (n = 10), and renal failure (n = 10) were used to assess the clinical agreement among eight commercially available assay kits for osteocalcin (OC). These kits differ in their assay configurations (six radioimmunoassays, two immunoradiometric assays), standards (five bovine, three human), and antibodies (six polyclonal, two monoclonal). Individual results were divided by the mean OC of the control subjects for each assay and expressed as percentage deviations. The expected wide variation in absolute OC concentrations between kits was only partially reduced by this transformation. Agreement was equally poor when absolute OC concentrations were compared with the reference ranges quoted by the manufacturers. The discordance was particularly marked in renal failure, presumably because of immunoreactive fragments, and in osteoporosis. Systematic differences could not be attributed to assay format, species source of standard, or antibody specificity. We conclude that results cannot be compared between assays even when normalized against healthy subjects, and that standardization is needed.
We measured ionized calcium concentrations in whole blood from 91 patients who had no clinical or biochemical evidence of disturbed calcium homeostasis and who had a wide range of serum albumin concentrations. We used both a standard Ciba-Corning 634 analyzer, which has a membrane-restricted saturated KCl reference electrode bridge, and a modified instrument with a 150 mmol/L NaCl bridge. After adjusting the externally standardized values from each instrument for their least-squares regressions on pH, there was a significant correlation between ionized calcium and albumin only with the standard analyzer. In contrast, only values from the modified instrument correlated with serum chloride; this was not explained by ionic strength or organic anion interferences. We conclude that there is unlikely to be any major advantage in using a membrane-restricted isotonic NaCl reference electrode for in vitro clinical measurements, although it may be of value for in vivo monitoring. The importance of measuring serum albumin when using most commercial ionized calcium analyzers is emphasized.
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