A negative urine test and a positive hair test need not answer the same exposure question. A 2013 study of children referred after suspected household methamphetamine exposure compared three biological matrices and illustrates why the specimen, analytical threshold and available denominator belong beside every result.
Who was examined, and which specimens were available?
Castaneto and colleagues studied 91 children referred for medical and forensic evaluation. All three matrices were available for 67. Within that complete-specimen subgroup, 12 were negative in every matrix, 41 were positive in one, 12 in two and two in all three. These counts concerned any qualifying target analyte, not methamphetamine alone.
The research panel included methamphetamine, amphetamine, MDMA, MDA and MDEA. Urine and oral fluid were analyzed by gas chromatography–mass spectrometry; hair was analyzed by liquid chromatography–tandem mass spectrometry. This was an observational detection study, not controlled administration to children.
Why do the analytical limits matter?
The study’s limits of quantification were 25 ng/mL for urine and 15 ng/mL for oral fluid. Hair limits differed by analyte: methamphetamine, MDMA and MDEA at 0.1 ng/mg, amphetamine at 0.025 ng/mg and MDA at 0.02 ng/mg. Results at or above the applicable limit were counted as positive.
The hair denominator measures mass per mass; urine and oral-fluid results measure mass per volume. Comparing the numerical values alone cannot establish that one assay was a specified number of times more sensitive. The matrices and their incorporation and elimination processes differ.
What did discordant specimens show?
All five positive oral-fluid specimens and 18 of the 19 positive urine specimens also had positive hair findings. Hair therefore added exposure information that the shorter-window specimens often did not show. That is a finding about this selected study population and its methods, rather than a universal diagnostic-performance estimate.
A negative result means the specified targets were not quantified at the method’s reporting threshold in that specimen at collection. It cannot establish that exposure never occurred, and it cannot reconstruct what a different specimen would have shown at an earlier time.
Did washing hair prove ingestion?
The laboratory washed the hair before analysis. That procedure is relevant to contamination assessment, but it does not by itself identify the route or absorbed dose. The authors discussed environmental contact, possible ingestion and adolescent self-administration as explanations requiring consideration.
De-identification also limited detailed age-based interpretation. The study could not fully distinguish accidental or environmental exposure from self-administration among older children. A scientifically careful account should retain that limitation rather than assign one exposure history to every positive sample.
Can a hair result date a specific event?
Not from this study alone. The investigators collected proximal hair segments, but variability in children’s hair growth and other biological factors complicates calendar interpretation. A positive segment cannot automatically identify a particular day, household incident or person responsible for the exposure.
Likewise, detection is not a measurement of clinical injury, impairment or the amount administered. Those questions require other evidence. The useful contribution here is the side-by-side specimen comparison and the demonstration that a single negative matrix could leave important exposure questions unresolved.
Source
Castaneto MS and colleagues. Identifying methamphetamine exposure in children. Therapeutic Drug Monitoring. 2013;35(6):823–830. DOI: 10.1097/FTD.0b013e31829685b2. Full text; bibliographic record.