Understanding Invalid Drug Test Results

An invalid result for a urine drug test indicates that the sample is neither positive nor negative and could not be tested by the laboratory. Reasons why a drug test may be invalid are many and varied.

To better understand this result, it helps to know the steps involved in the urine drug testing process.

Testing Performed at the Lab

When the lab first receives the sample it does an inspection to ensure that true human urine is submitted for testing. This involves inspecting the color of the sample, noting any debris or abnormal appearance, and making sure bottle A and bottle B have a similar appearance.

Once the urine passes this visual inspection, it then undergoes series of validity tests which ensure the sample meets criteria established for normal human urine. These tests detect synthetic urine, adulterants, and specimens substituted with fluid other than human urine. These test for specific gravity, creatinine, pH, and oxidants (mainly nitrite).

If the sample passes these validity tests it then moves on to the screening test (immunoassay) and ultimately to the confirmation test (usually GC/MS) as necessary. On occasion, there can be substances in the urine that interfere with the screening or confirmation test, such as certain medications or attempts at adulteration.

Thus, if the urine sample is not able to be tested fully - either because it failed the validity tests or it could not be tested during the screening or confirmation test due to interference - the result is classified as "Invalid".

MRO Review of Invalid Results

The next phase occurs when the MRO reviews these invalid results to determine if there is a medical explanation for the labs findings. Below are a few examples of how an invalid test may be medically explained:

1. High nitrite concentration due to a urinary tract infection (UTI) or certain medications.
2. Low specific gravity due to over hydration before the test or the use of diuretics.
3. High pH, if between 9-9.5, usually due to bacterial degradation of urine during transit, especially if it is in transit for more than 24 hours and ambient temperature is high.
4. Immunoassay interference due to certain medications such as Flagyl (Metronidazole) or Cipro (Ciprofloxacin).
5. Prolonged and/or improper storage of samples in transit to the lab for testing. Once a sample is collected, it must be stored at room temperature - 18-25°C (64-77°F). If these conditions are not maintained while in transit to the lab, due to shipping delays or issues with storage conditions, the specimen viability may be impacted.

Once the MRO receives an invalid drug test result, the donor/subject will be contacted by the MRO to determine if medical explanation exists for the result.

Based on the MRO interview with the donor, the following actions may occur:

1. If there is medical explanation for the invalid result, the test is cancelled by the MRO and no further action is required unless a negative result is needed for a pre-employment, return to duty, or follow up test type. If that is the case, the employer may elect to have the donor complete testing again, but it is not collected under observation.
2. If the MRO does not find a medical explanation for an invalid result, the MRO cancels the test and recommends immediate recollection under observation. Under Department of Transportation (DOT) guidelines the employer is not supposed to attach any consequence to this cancelled test other than retesting the donor under observation.

Invalid results can be complex and varied depending on the reasons cited above. Interpretation of these results by the MRO is important in determining the next steps that follow such a result.

References:

1. The Medical Officer's Manual by Dr Robert Swotinsky, MD, MPH. 6th edition.
2. https://www.transportation.gov/odapc/part40/40-159

Posted: October 5, 2021