Intra individual variability in markers of proteinuria for normal subjects and those with cadmium induced renal dysfunction: interpretation of results from untimed, random urine samples

The project aimed to help interpretation of urinary protein measurements, namely -2-microglobulin, retinol-binding protein, albumin and total protein in untimed, random urine samples as indicating significant changes in renal tubular reabsorption and glomerular permeability in an individual. A standard methodology used in clinical laboratory medicine was applied to calculate the intra-individual biological variation for these analytes. This parameter in conjunction with a laboratory's analytical variation allows definition of uncertainty about a single urine protein measurement, significant changes above normal variation in serial measurements within an individual and a defined level of maximum acceptable analytical imprecision. Repeat urine samples were obtained over a period of one week from a group of cadmium-exposed workers, 90% of whom had long-term tubular proteinuria, and a group of five unexposed volunteers with normal renal function. Dilute samples defined as having creatinines less than 3 mmol l-1 were excluded, as were urines with pH less than 5.5 for -2-microglobulin. Samples were analysed twice after randomisation in large batches. There was no evidence of any diurnal variation in the four protein measurements from samples collected between early morning and 16:00 hours. Creatinine or specific gravity correction of urine results for all four proteins only marginally reduced the uncertainty associated with an individual measurement asreflecting the true excretion value. For those subjects with defined tubular proteinuria, variability in retinol-binding protein excretion was less than that for -2- microglobulin. About 30% of the samples had urine pHs of 5.5 or less where -2- microglobulin degradation occurs. Using our laboratory analytical precision the minimum changes between serial creatinine-corrected measurements that are needed to be considered statistically significant (p < 0.05) is 110% for retinol-binding protein, 177% for -2-microglobulin, 70% for total protein, and 81% for albumin. Unlike published data for cadmium and mercury, the use of creatinine or specific gravity correction of random untimed urine samples for the urinary proteins does not make a large improvement to the interpretation of data by reducing the uncertainty associated with a measurement. There are significant advantages to the use of retinol-binding protein in contrast to -2- microglobulin as an indicator of renal tubular damage. Levels for defined acceptable analyti