Urine drug screening is a procedure designed to detect specific substances or their metabolic byproducts (metabolites). The results are used in medical, legal, and employment contexts, making the integrity of the collected specimen highly important. Laboratories must ensure the sample analyzed is the one provided by the individual and that its chemical composition accurately reflects the body’s output at the time of collection. Any compromise to the sample’s stability or chain of custody can render the entire test invalid.
Immediate Integrity Checks
The first line of defense against sample tampering or substitution occurs immediately at the collection site. Trained collectors establish a verifiable “chain of custody,” which is a chronological paper trail documenting every transfer of the sample from the donor to the testing laboratory. The donor must provide an adequate quantity of urine, typically at least 45 milliliters, necessary for both initial screening and confirmatory testing on a split specimen.
The most common integrity check is temperature verification, performed within four minutes of collection. A temperature strip confirms the specimen is within the acceptable human body temperature range, usually between 90°F and 100°F. If the temperature falls outside this window, it indicates the sample was substituted or improperly stored. A rejected temperature reading triggers a collection under direct observation or a cancellation of the test.
Biological and Chemical Degradation
Once a urine sample leaves the body, its chemical stability immediately declines due to biological and environmental factors. The most significant process is the degradation of urea, the primary nitrogenous waste product, which breaks down into ammonia and bicarbonate. This hydrolysis is caused by urease, an enzyme produced by common bacteria introduced into the collected sample.
The conversion of urea to ammonia rapidly increases the sample’s alkalinity, causing the pH level to rise beyond the normal physiological range. This change in pH can interfere with chemical assays used in laboratory screening, potentially leading to inaccurate results or the degradation of drug metabolites. For example, the primary metabolite of cannabis is susceptible to adsorptive loss, a stability concern amplified by temperature and pH.
Laboratories also monitor the concentration of creatinine, a waste product of muscle metabolism normally excreted at a constant rate. Improper storage allows bacterial action to break down creatinine, lowering its concentration. Low creatinine levels flag a diluted sample, a common method of masking drug use, but can also result from poor storage conditions.
Recommended Storage Time Limits
The time a urine sample remains suitable for drug testing depends heavily on the storage method, as temperature directly influences the rate of degradation. At room temperature, a sample is viable for less than two hours before bacterial growth and urea hydrolysis compromise its integrity. This rapid degradation requires same-day transport and analysis for most non-regulated testing scenarios.
To slow biological processes and maintain the chemical profile, refrigeration is the standard short-term storage method. Guidelines recommend storing the sample between 35°F and 46°F, extending viability to between 24 and 48 hours. This timeframe allows for transportation to a certified testing laboratory without major loss of metabolite concentration.
For long-term storage, such as retaining a split specimen for potential re-analysis, freezing is the preferred method. Storing the sample below 0°F can preserve metabolites and chemical markers for several months, sometimes up to four months. Freezing has limitations, as the process of freezing and thawing can cause components to precipitate or slightly affect the stability of fragile drug metabolites. Laboratories must employ specific procedures to ensure frozen samples are properly prepared and analyzed to account for these physical changes.
Sample Rejection and Repercussions
When a collected specimen fails integrity checks, it is reported as “invalid” or “adulterated.” An invalid result means the laboratory could not obtain a definitive result due to an abnormal concentration of an endogenous substance, such as creatinine or pH, which may indicate dilution or poor storage. An adulterated result suggests a deliberate act, where a substance has been added to the urine to interfere with the testing process, such as a severely abnormal pH.
In either scenario, the compromised sample results in a cancelled test, which is problematic for the donor and the organization. A cancelled test often requires an immediate re-test, which may be conducted under direct observation if tampering is suspected. For employment-related testing, a cancelled result can carry administrative consequences, including the withdrawal of an employment offer or removal from a safety-sensitive position until a valid result is obtained.