Ascorbate interference in the estimation of urinary glucose by test strips.

Currently used test strip methods for the detection of glucose in urine are influenced by ascorbate and may thus give false negative results, e.g. in screening for diabetes. Six different test strips for urine glucose were evaluated for interference by ascorbate in vitro. Interference by ascorbate varied markedly, being highest at low glucose concentrations. Interference coefficients for the individual tests were calculated to serve as an approximate index of interference by ascorbate. A new test (BM 33.071, Boehringer Mannheim GmbH, currently used in Combur-9-Test/Chemstrip-9 and other multiple test strips of Boehringer Mannheim) was clearly much less influenced as no urine containing 5.5 mmol/l glucose was read as negative even at very high ascorbate concentration. Readability of test strips differed due to patchy colour reactions. Precision was good within-test strip and within-urine but markedly less between urines.