Previous reports revealed poor performance in identifying
drugs of abuse users through first-level workplace drug testing (WDT), based on
urine samples. In a cross-sectional study, we evaluated: (i) the effect of
creatinine normalization of drug values from diluted urine samples (creatinine
levels ≤20 mg/dL) on the prevalence of drug users; (ii) the independent
procedure-related predictors of positivity and dilution.
Workers' urine samples were collected at the workplace or at
our certified laboratory between 2008 and 2012. All samples were analysed for
drugs of abuse by immuno-enzymatic method in our laboratory, according to the
Italian WDT law. Detectable drugs of abuse concentrations lower than the positive
cutoff values were normalized based on mean levels of urinary creatinine.
Detectable concentrations of drugs were confirmed by GC/MS. Multivariate
logistic regression was used to detect independent procedure-related predictors
of positive and diluted urine samples.
Of the 3080 urine samples screened, 51 (1.7%) were found
positive for some drugs of abuse (26 cannabinoids and 16 cocaine) and 116
(3.8%) were diluted. Seventeen out of 23 diluted urine samples with detectable
concentrations of cannabinoids or cocaine were found positive after urine
creatinine normalization and GC/MS confirmed both negative and positive
results. This increased the percentage of positivity for cannabinoids and
cocaine from 1.35% to 2.09% (+55%, p=0.0005), which is closer to the expected
prevalence of drug users based on Italian self-reported surveys. Collection of
samples in the laboratory was an independent predictor of positivity (OR=2.33,
95%CI 1.27-4.28) and diluted urine sample (OR=1.65, 95%CI 1.04-2.61).
Efficacy of first-level WDT could be improved by
well-controlled pre-analytical procedures and urine creatinine normalization of
detected concentrations of drugs of abuse.
1Toxicology Laboratory, Occupational and Preventive Medicine, Varese Hospital, Varese, Italy.
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