Moody D E, Monti K M, Spanbauer A C
Center for Human Toxicology, Department of Pharmacology and Toxicology, University of Utah, Salt Lake City 84112-9457, USA.
J Anal Toxicol. 1999 Oct;23(6):535-40. doi: 10.1093/jat/23.6.535.
Stability is an important consideration in the use of specimens for accurate determination of analyte concentrations. To determine the long-term stability for analytes routinely analyzed by mass spectrometry in this laboratory, quality-control (QC) results were plotted versus time. The time required for the initial concentration to reach a specified level of deviation (i.e., 15%) was then determined from the slopes. QCs were prepared at 1-3 concentrations in drug-free matrix and stored at approximately -20 degrees C; urines were fortified with 1% sodium fluoride; plasmas (except for cocaines) were prepared with heparin. For cocaine and metabolites, the plasma was either fortified with 2% sodium fluoride or with 2% sodium fluoride and 1 mg% physostigmine after adjustment of the plasma pH to 6.0. In urine, amphetamine, methamphetamine, codeine, morphine, benzoylecgonine (BZE), and 11-nor-9-carboxy-delta9-tetrahydrocannabinol (THCA) slopes did not exceed a 15% deviation before 852 days. Cocaine, however, reached a 15% reduction at 165 days. When cocaine and BZE were prepared in plasma with just 2% sodium fluoride, negative slopes reached 15% deviation within 154 and 111 days, respectively. Further fortification with physostigmide and adjustment of the pH extended this time frame significantly. Delta9-Tetrahydrocannabinol (THC) and THCA in plasma had negative slopes that deviated by 15% just prior to one year. l-Alpha-acetylmethadol (LAAM), methadone, and their N-demethylated metabolites in urine did not have any negative slopes exceeding 15% before 686 days. Several of the compounds had positive slopes. Those for 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine reached the 15% mark within 98 days. Those for LAAM, norLAAM, and dinorLAAM were concentration dependent. The 25-ng/mL controls reached 15% at 158-216 days. The 700-ng/mL controls reached 15% at 784-1340 days. In plasma, only naltrexone and buprenorphine displayed negative slopes at all three concentrations, reaching the 15% mark as early as 576 and 272 days, respectively. LAAM, norLAAM, dinorLAAM, ibogaine, 6-beta-naltrexone, risperidone, and 9-OH-risperidone did not exceed a 15% deviation before 416 days. To attempt to validate this method, two sets of clinical plasma samples that had been analyzed for buprenorphine were reanalyzed 644 and 869 days after the initial analyses. Those reanalyzed after 644 days were not statistically different from initial analyses, whereas those stored for 869 days were statistically different (p < 0.05). As the average time to reach 15% deviation for the three concentrations of buprenorphine QCs was 782 days, this suggests that extrapolation of QC results gathered over time may provide a reliable method to estimate long-term stability limits for drugs stored under the same conditions as the QC samples.
