CONVENTIONAL DIPSTICKS IN THE SCREENING OF MICROALBUMINURIA. AN OLD FASHION TEST OR A FRUITFUL TOOL?

Background: Recently, the use of dipsticks based on immunometric reactions for urinary albumin determination is widely supported for both screening programs and cardiovascular risk assessment. However, the relationship between albumin and total protein urinary concentration (Alb-U, PT-U) became linear just over both the range of cut-off levels of microalbuminuria (20 mg/L) and the first protein turning point of conventional dipstick. In the present study we evaluated the diagnostic capabilities of the two PT-U turning point (trace, 150 mg/L; 1+, 300 mg/dL) of conventional dipsticks (Aution Sticks 10A, Menarini Diagnostics Italy) for cut-off values of microalbuminuria for possible application in epidemiological studies. Methods. The study was a prospective, observational study of outpatients (n=534) referred to our unit for Alb-U evaluation. A preliminary investigation of the Alb-U/PT-U relationship was performed in a group of 80 samples with normal urinary electrophoresis. PT-U was measured with Pyrogallol-red complex method (ADVIA 2400 system, Siemens Healthcare Diagnostic, USA). Alb-U was measured with immuno nephelometric method (Immage 800, Beckman-Coulter, California, USA). Diagnostic accuracy of dipsticks for Alb-U (20 mg/L) was calculated with the ROC curve analysis. Results. Alb-U and PT-U concentration were closely correlated (y=6.43x-37.11; n=80; r=0.92; p<0.01) when the whole range of values was considered. However, linearity was lost below 150 mg/L of PT-U where albumin was only a minor component of total proteins (5-35%). A proportion of 30% subjects (138 out of 454) had U-Alb>20 mg/L. ROC curve analysis revealed AUC 88% with sensitivity and specificity 88 and 81% and 34 and 99% at trace and 1+ respectively. Conclusions. Alb-U/PT-U relationship is linear over Alb-U 20 mg/L and PT-U 150 mg/L concentrations. Trace reading of PT-U conventional dipstick have a sensitivity (88%) which can be acceptable for screening of microalbuminuria in epidemiological studies where low cost is a priority, such as in developing countries. This value is however limited for diagnostic purpose in the single patient.