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Diurnal variation of biopyrrin excretion in random urine specimens is not corrected by creatinine

Circulating bilirubin is thought to function as a physiological antioxidant. One of the decomposition products of this process is the biopyrrins, which include two regioisomers: biotripyrrin‐a (1,14,15,17‐tetrahydro‐2,7,13‐trimethyl‐1,14‐deoxy‐3‐vinyl‐16H‐tripyrrin‐8,12‐dipropionic acid) and biotripyrrin‐b (1,14,15,17‐tetrahydro‐3,7,13‐trimethyl‐1,14‐deoxy‐3‐vinyl‐16H‐tripyrrin‐8,12‐dipropionic acid). We measured biopyrrins in random urine specimens and investigated whether the biopyrrin values obtained were valid when expressed as a ratio of the creatinine (Cr) concentrations. All of the random urine specimens collected over 48 hr were from presumably healthy adults. We measured the biopyrrins by means of an enzyme‐linked immunosorbent assay (ELISA) using an anti‐bilirubin monoclonal antibody. When the values were expressed in terms of the ratio to Cr, the within‐day coefficient of variation (%CV) of the excretion of biopyrrins was reduced to 27%±10% (P<0.05) from 59%±27%. However, assay values on random or spot urine specimens were unreliable because of the large %CV. The biopyrrin concentrations only in the first‐morning‐urine specimens in terms of both absolute amounts and ratios to Cr significantly reflected those in a 24‐hr urine specimen (P<0.001). Concentrations in a random urine specimen voided at the second collection or later did not correlate with the concentration in a 24‐hr urine specimen (P>0.05), even if their values were corrected by Cr. The amounts of biopyrrins excreted in 24‐hr urine specimens were significantly correlated with the 24‐hr cortisol excretion (P<0.001) but not to uropepsin (P>0.05). J. Clin. Lab. Anal. 21:1–7, 2007. © 2007 Wiley‐Liss, Inc.