Biological variability of albumin excretion rate and albumin-to-creatinine ratio in hypertensive type 2 diabetic patients.

The importance of measuring microalbuminuria is well established. However, only scanty data are available concerning the biological variability of albumin excretion in type 2 diabetic subjects. We report our experience from a large clinical trial of a new antihypertensive drug (Lercanidipine) designed to reduce albumin excretion and blood pressure in type 2 diabetic patients with hypertension and microalbuminuria. Eighty seven patients with persistent microalbuminuria were studied within 1 year of the clinical trial. The measurements were performed on blood and timed urine samples frozen at -80 degrees C and shipped to a central laboratory unit. Preliminary experiments were performed to assess albumin stability in urine under various conditions (4 degrees C, -20 degrees C and -80 degrees C), particularly with regard to the albumin/creatinine ratio. Urine samples can be stored up to 3 weeks at 4 degrees C or up to 2 months at -80 degrees C. The biological variability of the albumin excretion rate was 25.7%, while that of the albumin/creatinine ratio was 13.4%. These data are useful in defining the analytical goals of imprecision for microalbuminuria (CV = 13% for albumin, and CV = 6% for albumin/creatinine ratio). No correlation between albumin/creatinine ratio and HbA1c was found in the cohort of 61 microalbuminuric patients who completed the trial. The results of this study confirm that the albumin/ creatinine ratio is much more suitable for monitoring albumin excretion in longitudinal studies than the albumin excretion rate.