title: Inhibition by ascorbic acid (vitamin C) of chemical detection of blood in urine.
abstract: Review of results of 9,620 consecutive urinalyses revealed 892 urines that contained microscopic evidence of erythrocytes and 98 for which there were discrepancies between microscopic evidence of hematuria (greater than 20 erythrocytes/high-power field) and chemical detection of blood (0 or "1+" when results should be "3+"). Eleven specimens (from ten patients) showed negative results of chemical tests for blood and greater than 40 erythrocytes/high-power field (HPF). Nine of these patients were receiving ascorbic acid supplementation. In a random sample of 20 patients with greater than 40 erythrocytes/HPF and strongly positive tests for blood, none was receiving ascorbic acid, a significant difference by chi-square analysis. In a prospective study, low levels of ascorbic acid inhibited chemical detection of blood. At 25 mg/dl ascorbic acid, 10--20 erythrocytes/HPF could not be detected; at 35 mg/dl ascorbic acid, greater than 20 erythrocytes/HPF were undetectable. For quantitation of the low level at which ascorbic acid inhibits chemical detection of blood, fresh urine specimens should be analyzed. Ascorbic acid is oxidated in vitro to products that partially inhibit detection of blood yet do not assay as ascorbic acid.
mesh_terms: D001205:Ascorbic Acid / Q000494:pharmacology* / Q000652:urine; D004912:Erythrocytes; D006417:Hematuria / Q000175:diagnosis*; D006801:Humans
chemical_list: D001205:Ascorbic Acid