Method-to-method variability in urine albumin measurements

Background: Urine albumin (uALB) is a useful marker in diagnosis and treatment of renal microvascular disease. Beckman Coulter recently re-formulated their uALB reagent for the AU series of instruments to increase the analytical measurement range (AMR).

Methods: Precision, linearity, reportable range, and analytical sensitivity for the reformulated reagent were determined using the AU680. In addition, the re-formulated AU reagent was compared to the previous generation AU reagent and to the Siemens Vista using residual urine specimens. The hook effect was evaluated on five instruments by spiking serum into albumin-free urine to generate a range of albumin concentrations.

Results: Precision and linearity within the AMR were confirmed, along with accuracy of dilutions to extend the reportable range. For patient sample correlation, the re-formulated reagent demonstrated a positive 11% bias relative to the original AU reagent and a negative 11% bias relative to the Vista. Concentrations of uALB >3000mg/dL produced falsely low results for both AU reagents. The DCA Vantage assay "hooked" at even lower uALB concentrations.

Conclusions: The re-formulated AU uALB reagent met the manufacturer claimed performance characteristics. The AU and DCA Vantage were the only instruments of those tested affected by the hook effect in the concentration range evaluated. uALB assays are clearly not standardized, yet clinical guidelines dictate result interpretation. The method-to-method biases we observed here have the potential to lead to clinically significant post-analytical errors in uALB interpretation.