Incomplete recovery of prescription opioids in urine using enzymatic hydrolysis of glucuronide metabolites

Confirmation of opioids in urine samples of clinical patients requires liberation of opioids from their glucuronide conjugates. Both acid hydrolysis and enzyme hydrolysis using β-glucuronidase from various sources have been reported, with the latter approach prevailing in most clinical toxicology laboratories. The goal of this study was to compare the efficiency of acid versus different enzyme hydrolysis methods in recovering morphine and common semisynthetic opioids from glucuronide standards and 78 patient urine samples that were screened positive for opioids as a class. Specimens were analyzed with a validated gas chromatography-mass spectrometry (GC-MS) procedure. With the exception of oxycodone, the results indicated that the majority of opioids tested were extensively glucuronide-conjugated in urine. Significantly, acid hydrolysis liberated > 90% of morphine and hydromorphone from their glucuronide standards but enzyme hydrolysis had lower and variable efficiency, depending on the opiate type and the enzyme source. In patient specimens, much higher concentrations of free codeine, morphine, hydromorphone, and oxymorphone were obtained with acid hydrolysis than with various enzyme methods. Incomplete hydrolysis using β-glucuronidase could lead to false-negative results for many opioids when urine is tested for drugs of abuse. We conclude that acid hydrolysis is the method of choice for GC-MS confirmation of urine opioids.