Decreasing the Lower Limit of Quantitation for Urine Albumin Improves Clinical Utility.

Background: Urine albumin-to-creatinine ratio (uACR) is a screening assay for chronic kidney disease (CKD). A value of >30 mg/g is flagged abnormal, but lower ratios have prognostic implications. Thus, to maximize diagnostic utility, urine albumin (uAlb) should be measurable to 3 mg/L to match the lowest creatinine concentration generally utilized (10 mg/dL). Most uAlb assays have lower limits of quantitation (LLOQs) 2- to 4-fold higher. We sought to determine the performance characteristics of a commonly used uAlb assay at 3 mg/L and to evaluate the clinical screening impact of reducing the LLOQ.

Methods: Urine was serially diluted to assess uAlb linearity and precision for concentrations near the claimed LLOQ (12 mg/L). Samples (n = 30) with uAlb <12 mg/L were compared between laboratories. Sequential samples (n = 1239) were evaluated for clinical impact of reducing the measuring range to 3 mg/L.

Results: The assay was linear to 1.6 mg/L. Interday precision at 3.7 mg/L and 4.3 mg/L was 7.7% and 8.6%, respectively. Minimal bias was observed between labs (y = 1.091x - 0.75; average bias = -0.13 mg/L). Clinical validation demonstrated 501 of 1239 samples (40.4%) had uAlb <12 mg/L. Using 11.9 mg/L as the numerator for samples with uAlb <12 mg/dL and urine creatinine >10 mg/L, 107 of 499 (21.4%) would have a ratio flagged abnormal at >30 mg/g. Using the numeric value for these samples to 3 mg/L reduced alarm to <1%.

Conclusions: A uAlb LLOQ of 3 mg/L improves screening utility of uACR by simplifying reporting and clinical interpretation when uAlb is low and provides clinical information for prognostic tools developed for people at risk of CKD.