Which method for quantifying urinary albumin excretion gives what outcome? A comparison of immunonephelometry with HPLC

Background. Microalbuminuria has recently been identified as an independent risk factor for cardiovascular disease in the general population. Immunochemical urinary albumin assays only detect immunoreactive intact albumin. High performance liquid chromatography (HPLC) is able to detect both immunoreactive and immunounreactive intact albumin. We compared both measurement methods respectively in subjects with normo-, micro-, and macroalbuminuria in the general population. Methods. We used 24-hour urine samples that were collected within the framework of the second screening for the PREVEND Study, a prospective cohort study, on albuminuria in the city of Groningen, The Netherlands. Results. With nephelometry as immunochemical reference method, we classified 986 subjects as normoalbuminuric, 283 as microalbuminuric, and 43 subjects as macroalbuminuric. The mean+/-SD albumin concentration was 6.8+/-4.3 mg/L for nephelometry in the urine samples of the 998 subjects with a concentration <20 mg/L according to nephelometry versus 17.6 +/- 10.3 mg/L for HPLC (P<0.001, HPLC 159% higher). These values were 58.9+/-40.6 mg/L for nephelometry versus 74.0+/-51.8 mg/L for HPLC (P<0.001, N=280, HPLC 26% higher) in the concentration range between 20 to 200 mg/L, and 436.3 +/- 371.8 mg/L for nephelometry versus 399.1 +/- 329.2 mg/L for HPLC above 200 mg/L (P=0.048 N=34, HPLC 8.5% lower). Associations of 24-hour urinary albumin excretion with cardiovascular risk factors were generally somewhat stronger for nephelometry than for HPLC. Logistic regression analyses with an abnormal ankle-brachial index as outcome parameter revealed adjusted odds ratios of 1.78 (95% CI 1.01-3.12, P<0.05) and 4.67 (95% CI 1.68-12.9 P<0.05) respectively for micro- and macroalbuminuria as determined by HPLC, compared to 1.37 (95% CI 0.77-2.41, P=NS) and 3.85 (95% CI 1.53-9.67. P<0.05) respectively for nephelometry. The ROC-curve showed similar sensitivity and specificity for both methods (P=0.25). Conclusion. The use of HPLC for determination of urinary albumin concentrations reveals higher values compared to nephelometry, especially in the lower concentration range, resulting in a higher prevalence of microalbuminuria. With HPLC compared to nephelometry, we found a 21% higher independent odds ratio for microalbuminuria with the presence of peripheral vascular disease, and a 30% higher independent odds ratio for macroalbuminuria. This higher prevalence of microalbuminuria, accompanied with a similar absolute risk for peripheral vascular disease compared to patients with microalbuminuria detected by nephelometry, suggests HPLC to identify more people at risk, which is of great importance, especially when screening in large populations is concerned.