Urine pH: the Effects of Time and Temperature after Collection

Abstract

The Mandatory Guidelines for Federal Workplace Drug Testing Programs provide criteria for specimen validity testing, including urine pH cut-offs, to report a urine specimen as adulterated or invalid. Since the urine pH criteria for invalid classifications, > or = 3 and < 4.5 or > or = 9 and < 11, became effective in November 2004, a number of specimens with results within the upper invalid limits, typically in the range of 9.1 to 9.3, have been reported with no evidence of adulteration. This study evaluated the hypothesis that these pH findings were the result of exposure to increased environmental temperatures during specimen standing and transport. Indeed, increased storage temperatures were associated with increased urine pH, with the magnitude of the change related to both storage time and temperature. The pH values of specimens stored at -20 degrees C are relatively stable, whereas pH results > 9 are achieved at storage temperatures of room temperature or higher. It is noteworthy that no condition(s) produced a specimen with a pH > 9.5. Degradation of nitrogenous urine analytes is most likely responsible for the noted increases in pH. These findings are intended to supplement information used by the Medical Review Officers who are responsible for interpreting such marginally invalid pH results.

Zopiclone degradation in biological samples : Characteristics and consequences in forensic toxicology

“…Previous studies have shown that urinary pH increases with increasing temperature [155]. In this study (Paper III) the measured pH lasted up to twelve weeks at 4˚C and at -20˚C and with different initial pH values.…”

Review of the Diagnostic Evaluation of Normal Anion Gap Metabolic Acidosis

“…Because urine can achieve a hydrogen ion concentration 1,000× greater than blood, its pH ranges from 4.5 to 8 (average pH 5-6) [36] . Though the gold standard of measurement is with a pH electrode, dipsticks offer the convenience of cost and ease of use [37] .…”

“…Highly diluted urine and a very low concentration of urinary sodium may interfere with the achievement of a minimum pH despite a normal renal acidification function. Often, the urine specimen becomes contaminated preanalytically with bacteria during collection [36] . While midstream urine collection after cleansing of the genitalia may be the preferred method, most urine collections are made without this practice.…”

“…Both the male and female urethras are colonized with microorganisms. While urine collected by conventional voiding is often bacterially contaminated, even midstream random urine collections, whether with or without prior cleansing of external genitalia, have bacterial contamination rates of up to 30% [36,38] . Left standing, the pH of a bacterial-contaminated, unpreserved urine specimen will continue to increase.…”

Urine-sampling methods for environmental chemicals in infants and young children

“…Urine specific gravity does appear to be more stable than creatinine over storage temperatures ranging from À20 to 931C (Cook et al, 2007). More research is needed to evaluate various approaches for accounting for urine dilution in neonates and young children.…”