Amphetamines

Dose and frequency of use are the dominant determinants of how long amphetamine remains detectable. A single therapeutic dose of a prescription product typically produces urinary detection for 24–48 hours at the SAMHSA 500 ng/mL screening cutoff. Sustained daily prescription use produces consistent positive results across the dosing interval. Heavy illicit use, particularly of higher-dose preparations or via routes that produce higher peak plasma concentrations, can extend detection toward the upper end of the published 5–7 day chronic-use range. Programs interpreting amphetamine results in patients on prescription therapy should expect consistently positive screens during active treatment.

Route of administration affects pharmacokinetics. Oral amphetamine — the standard for prescription use — produces a relatively predictable plasma concentration profile with peak levels reached within 1–3 hours and steady-state behavior on chronic dosing. Inhaled (insufflated) and intravenous use produce more rapid peaks and shorter apparent durations of action, which is part of what makes those routes more associated with non-medical use. Lisdexamfetamine (Vyvanse) is a special case because it is a prodrug that must be metabolized in red blood cells to liberate active d-amphetamine; this produces a slower, smoother onset and a longer effective duration, but a drug test will still detect the released d-amphetamine just as it would for any other amphetamine product.

Urinary pH has an unusually large effect on amphetamine clearance. Amphetamine is a weak base, and acidic urine substantially increases the proportion eliminated unchanged through the kidneys, shortening the detection window. Alkaline urine reduces renal clearance and extends the window. This effect is large enough to be clinically meaningful — donors with a consistently acidic urine pH may clear amphetamine in 1–2 days, while donors with a consistently alkaline pH may remain positive for 3–4 days from the same dose. Hepatic metabolism plays a smaller role; the major metabolic pathways involve CYP2D6 hydroxylation.

Body composition, age, sex, and concurrent medications all contribute smaller modifying effects. Renal impairment extends the detection window by slowing elimination. Concurrent medications that affect urinary pH (acetazolamide, sodium bicarbonate, ammonium chloride) can shift detection windows in either direction. Programs interpreting borderline results in patients with complex medication regimens should consider these factors in MRO review rather than treating amphetamine results as a simple positive/negative determination.

SAMHSA's Mandatory Guidelines for Federal Workplace Drug Testing Programs specify the cutoffs that federally regulated employers must use for amphetamines. The initial urinary immunoassay screening cutoff is 500 ng/mL, and the confirmation cutoff by GC/MS or LC-MS/MS is 250 ng/mL for amphetamine and for methamphetamine reported separately. SAMHSA also requires that any methamphetamine confirmation positive contain at least 100 ng/mL of amphetamine as a corroborating metabolite, a requirement designed to distinguish true methamphetamine use from inadvertent ingestion of pharmaceutically inert l-methamphetamine present in certain over-the-counter nasal decongestants.

MDMA (methylenedioxymethamphetamine) is grouped administratively with amphetamines in the SAMHSA panel but is reported separately on confirmation at its own 500/250 ng/mL screening/confirmation cutoffs. Programs that want explicit MDMA coverage should verify that their device's MDMA channel is configured accordingly. Most commercial amphetamine immunoassays cross-react with MDMA at varying degrees, but confirmation testing should always be used to differentiate the specific compound responsible for a presumptive positive.

Non-federal programs and clinical settings are free to use the SAMHSA cutoffs or to set their own. Lower cutoffs (300 ng/mL screening, for example) increase sensitivity to recent use but also increase the rate of presumptive positives that require MRO review. Magenta's CLIA-waived dip cards and cups are calibrated to the SAMHSA 1000 ng/mL or 500 ng/mL amphetamine cutoff depending on the device configuration; talk to us about specific cutoff options for your program.

Lateral-flow immunoassay is the standard format for point-of-care amphetamine screening. The Magenta amphetamine analyte — whether on a single-analyte dip card or on a multi-panel cup — uses an anti-amphetamine antibody that produces a result based on cumulative immunoreactive amphetamine in the specimen. As with other Magenta point-of-care devices, no test line indicates a positive result at or above the device's cutoff, a visible test line indicates negative, and the control line confirms reagent function. Result time is approximately five minutes.

Amphetamine immunoassays have a well-known cross-reactivity profile that distinguishes them from most other SAMHSA analytes. Pseudoephedrine and ephedrine — both widely available in over-the-counter cold and allergy products — can produce presumptive positives at sufficient doses, particularly with the older-generation amphetamine antibodies. Phentermine (a prescription appetite-suppressant), bupropion (Wellbutrin, used for depression and smoking cessation), trazodone, and ranitidine have all been documented in case reports as sources of presumptive amphetamine positives. The l-methamphetamine present in certain over-the-counter Vicks Vapor Inhaler products can also produce presumptive positives, which is the reason SAMHSA requires the 100 ng/mL amphetamine corroborating metabolite on methamphetamine confirmation. Methylphenidate (Ritalin, Concerta) does not cross-react reliably with amphetamine immunoassays at clinical concentrations and is essentially a separate analyte for testing purposes.

Confirmation testing by GC/MS or LC-MS/MS is the gold standard for resolving any presumptive amphetamine positive. Confirmation identifies the specific compound (amphetamine, methamphetamine, MDMA, MDA) by its mass spectrum and quantifies it precisely, and can also differentiate the d- and l-isomers — clinically important because the l-isomer is present in Vicks Vapor Inhaler at concentrations that would not be present in legitimate prescription amphetamine therapy. SAMHSA-certified laboratories perform this confirmation testing on every screening positive in federally regulated programs and on most non-federal programs that follow SAMHSA-style procedures.

Medical Review Officer review is essential for amphetamine results because the cross-reactivity profile and the prevalence of prescription amphetamine therapy together generate a substantial rate of legitimate-medical-use positives. The MRO contacts the donor, requests documentation of any prescription amphetamine, methamphetamine (rare), or MDMA-containing medications, and verifies the prescription with the prescriber and pharmacy as appropriate. A confirmed positive that is fully explained by a valid prescription is generally reported to the employer as negative under federal rules, though prescription amphetamine use that may impair safety-sensitive duties can trigger separate evaluation under fitness-for-duty processes.

Specimen validity testing accompanies amphetamine screens just as for other analytes. Creatinine and specific gravity identify dilute specimens; pH and oxidant adulterant panels add additional protection. Magenta's adulteration-panel cups integrate these checks alongside the analyte channels. Notably, attempts to manipulate urinary pH to shorten the apparent amphetamine detection window can themselves be detected through pH testing, and unusually acidic or alkaline specimens generally trigger laboratory recollection.