Urine albumin measurement: effects of urine matrix constituents

There is increasing clinical interest in accurate measurement of urine albumin as an early indicator of kidney disease. However, urine is a highly variable sample matrix that may exert matrix effects on assays for urine albumin.

Variation in urine composition was assessed for components routinely measured in the clinical laboratory over 1 year. Solutions of representing different concentrations of urine components were prepared and spiked with a constant amount of albumin. Fourteen urine specimens of variable composition were ultrafiltered and spiked with a constant amount of albumin. Concentrations of albumin in solutions were determined with an immunoturbidimetric assay on the Beckman LX20 analyzer and with a competitive immunoassay on the Siemens Immulite analyzer.

Variation in concentrations of most constituents altered measured urine by <10%. High NaCl concentration increased assay values by 10% or more. Addition of 0.5% Triton X-100 increased values in the LX20 assay by about 20%. Most ultrafiltered urine specimens had <10% effect on measured albumin at a concentration near 20 mg/l, but one specimen was noted to decrease measured albumin by approximately 20% in the LX20 assay.

Urine matrix components exert small but potentially significant effects on assays for urine albumin. Urine matrix effects should be considered in the design and evaluation of assays for albumin.