Increase and decrease in formic acid concentration in urine samples stored at room temperature.

: Formic acid concentrations are not stable in urine samples stored at room temperature. Formic acid may increase or decrease due to bacterial contamination. Enterococci and E. coli produce formic acid under anaerobic conditions at pH values of more than 6.0. In urine sample stored at room temperature, E. coli caused a decrease in formic acid. The main substrate of formic acid formation by bacteria seems to be citric acid. Formic acid in urine is stable at room temperature for at least 7 days when concentrated acetic acid (50 microliters/10 ml) or 50 g/l thymol in isopropanol (50 microliters/10 ml) is added.