Quest Diagnostics Amphetamine Urine Drug Screen Reporting Cutoff
The standard immunoassay screening cutoff for amphetamines on Quest Diagnostics urine drug screens is 1000 ng/mL, which is the federally recommended concentration for workplace and forensic testing in the United States. 1
Standard Cutoff Concentrations
- The United States uses a recommended screening cutoff of 1000 ng/mL for amphetamine immunoassays in regulated forensic urine drug testing (FUDT), while Europe uses 500 ng/mL 2, 1
- Switzerland previously used 300 ng/mL but this resulted in an unacceptably high rate of false-positive results (specificity only 86%), requiring expensive confirmatory testing 2
- Increasing the cutoff from 300 ng/mL to 500 ng/mL improves specificity from 86% to 99%, while sensitivity only decreases from 97% to 91% 2
- At 1000 ng/mL cutoff, specificity reaches 100% but sensitivity drops to 81% 2
Clinical Implications of the 1000 ng/mL Cutoff
- The 1000 ng/mL screening cutoff is too high to consistently detect single low-dose amphetamine administration - only 19% of urine specimens screened positive following a single 5 mg oral dose of d-amphetamine 1
- Following a 10 mg dose, 67% of specimens were positive, and following a 20 mg dose, 88% were positive at the 1000 ng/mL cutoff 1
- Peak urinary amphetamine concentrations following a 5 mg dose ranged from 620 to 3160 ng/mL, with considerable overlap between different dose levels 1
- Time to peak concentration varies widely (2 to 18 hours post-administration), and amphetamine excretion increases with increasing urine flow and decreasing urine pH 1
Confirmatory Testing Requirements
- All positive immunoassay screening results are presumptive only and require confirmation by gas chromatography-mass spectrometry (GC-MS) before clinical decisions are made 3, 4
- Immunoassay screening has variable positive predictive value due to cross-reactivity with structurally related compounds including pseudoephedrine (found in over-the-counter cold medications), MDMA, MDA, PMA, ephedrine metabolites, and beta-phenethylamine 3, 4, 5, 6
- False-positive amphetamine screens account for 3.9-9.9% of all positive amphetamine immunoassay results in clinical laboratories 7
- Serial dilution testing can enhance the positive predictive value of presumptive positive results, with an optimal slope cutoff of 320 providing 96% sensitivity and 90% specificity for identifying true (meth)amphetamine-containing specimens 5
Important Caveats
- Approximately 80% of administered Adderall (amphetamine salts) is eliminated in urine as unchanged amphetamine, so patients on prescribed amphetamine will legitimately test positive 4
- Standard amphetamine panels do not detect methylphenidate (Ritalin), so a positive result cannot be explained by methylphenidate use 3, 8, 4
- High concentrations of less reactive cross-reacting compounds may mask low concentrations of actual amphetamines in the immunoassay 5
- A positive screening result only indicates amphetamine exposure at some time prior to specimen collection - it cannot distinguish therapeutic use from abuse, determine timing of use, or quantify the dose administered 1