The impact of the assay for measuring albumin on corrected ('adjusted') calcium concentrations

The K/DOQI guidelines recommend the use of albumin-corrected calcium (Ca), phosphate, parathyroid hormone (PTH) and calcium-phosphate product as therapeutic targets. The two most common assays for measuring albumin yield discordant results in uraemic patients, the Bromcresol purple (BCP) method providing lower albumin values than the Bromcresol green one (BCG). The aim of this study was to assess the impact of the assay on corrected Ca and, thus, on reaching recommended K/DOQI targets for corrected Ca and CaxP product.

We measured plasma albumin (both by BCG and BCP), total Ca and phosphate in all our chronic hemodialysis (HD) patients. Total Ca was corrected ("adjusted") for albumin level by a formula proposed by the K/DOQI.

89 patients were included, aged 71.2 +/- 11.5 years, on chronic hemodialysis for 29 (1-362) months. Albumin level was 3.78 +/- 0.24 g/dL by BCG and 3.12 +/- 0.27 by BCP ( p < 0.0001). Based on BCG albumin levels there were 12 cases of "hypocalcaemia" (<8.6 mg/dL), 3 cases of "hypercalcaemia" (>10 mg/dL) and 74 cases with "normal" Ca. The corresponding albumin levels were 3.9 +/- 0.2; 3.1 +/- 0.6 and 3.8 +/- 0.2 g/dL, respectively. According to BCP albumin levels, only one patient was labelled as hypocalcaemia , 21 as hypercalcaemia et 67 as "normal" adjusted Ca (albumin 3.1; 3 +/- 0.3 and 3.2 +/- 0.3 g/dL, respectively). Depending on the use of BCG or BCP, a discrepancy was thus observed in 29 cases (32.6%): 18 cases were classified as hypercalcaemia when albumin was measured by BCP but were considered normal using BCG, whereas 11 cases classified as hypocalcaemia with BCG had normal adjusted Ca with BCP. Concerning CaxP product, 7 discrepancies were detected.

The choice of either BCG or BCP has a major impact on albumin-adjusted Ca and thus on reaching K/DOQI targets for Ca and CaxP product. Clinicians should take this fact into account for the interpretation of laboratory values and the prescription of drugs related to mineral metabolism and dialysate calcium concentration. The type of assay used for the measurement of albumin should also be recorded and its impact taken into account (or corrected) in multicentric studies and registries.