Received on March 22, 2008
Accepted on June 19, 2008

General Clinical Chemistry

Comparison between Immunoturbidimetry, Size-Exclusion Chromatography, and LC-MS to Quantify Urinary Albumin

¹ Department of Internal Medicine, Division of Nephrology and Hypertension, Mayo Clinic College of Medicine, Rochester, MN
² Department of Laboratory Medicine and Pathology, Mayo Clinic College of Medicine, Rochester, MN
³ Department of Internal Medicine, Division of Nephrology and Hypertension, Division of Endocrinology, Diabetes, Metabolism and Nutrition, Department of Biochemistry and Molecular Biology, Mayo Clinic College of Medicine, Rochester, MN
⁴ Department of Internal Medicine, Division of Nephrology and Hypertension, Department of Laboratory Medicine and Pathology, Mayo Clinic College of Medicine, Rochester, MN

^* To whom correspondence should be addressed. E-mail: Larson.Timothy{at}mayo.edu.

BACKGROUND: The accurate and precise measurement of urinary albumin is critical, since even minor increases are diagnostically sensitive indicators of renal disease, cardiovascular events, and risk for death. To gain insights into potential measurement biases, we systematically compared urine albumin measurements performed by LC-MS, a clinically available immunoturbidimetric assay, and size-exclusion HPLC.

METHODS: We obtained unused clinical urine samples from 150 patients who were stratified by degrees of albuminuria (<20 mg/L, 20–250 mg/L, >250 mg/L) as determined by the immunoturbidimetric assay used in our clinical laboratory (Roche Hitachi 912). Urine albumin was then remeasured via LC-MS and HPLC (Accumin^TM) assays.

RESULTS: The immunoturbidimetric assay, calibrated using manufacturer-supplied serum-derived calibrators (Diasorin), underestimated albumin compared with LC-MS. After calibration with purified HSA, this immunoturbidimetric assay correlated well with LC-MS. HPLC overestimated albumin compared with both LC-MS and immunoturbidimetry. The current LC-MS and HPLC assays both performed poorly at concentrations <20 mg/L.

CONCLUSIONS: Efforts are needed to establish gold-standard traceable calibrators for clinical assays. LC-MS is a specific method to quantify albumin in native urine when concentrations exceed 20 mg/L, and therefore could be employed for standardization among assays.