Access to the PPM Journal and newsletters is FREE for clinicians.
9 Articles in Volume 14, Issue #6
Migraine Treatment From A to Z
Alternative Medicine in Chronic Migraine 2014: What Clinicians Need to Know
Hormone Abnormalities in Uncontrolled Chronic Pain Patients: Use of Hormone Profiles
Interpreting Negative Urine Drug Test Results
Case Challenge: Chronic Opioid Use Causing Adrenal Insufficiency
Editor's Memo: Toxic Insurance Plans
Guest Editor's Memo: The Forgotten Patients: Those Who Benefit From Opioid Treatment
Ask the Expert: Multiple Benzo Prescriptions
Ask the Expert: Burning Foot Syndrome

Interpreting Negative Urine Drug Test Results

Urine drug monitoring is one of many tools available to clinicians for the assessment and management of patients on chronic opioid therapy.

When a clinician orders urine drug testing for a patient prescribed chronic opioid therapy (COT), it is essential that the results are interpreted correctly because they often have significant clinical implications. A positive test result showing drugs and/or metabolites found in urine is easily understood. A negative result, while initially simple to understand, can be more complicated and requires the clinician to be aware of several factors that can cause the urine sample to test negative for a compound. For example, a positive drug test for oxymorphone, a metabolite of oxycodone, can be misinterpreted as possible misuse of a “nonprescribed” medication. On the other hand, negative urine drug test results are clinically meaningful, especially when the possibility exists that the patient may be misusing, abusing, or diverting their medications. For the purpose of this article, the authors will focus on the implications of negative test results.

Ruling Out Abuse

The Centers for Disease Control and Prevention have declared prescription drug abuse to be an epidemic in the United States. Drug abuse permeates all sections of society: male and female, rich and poor, young and old. Clinicians who prescribe opioids must make every effort to ensure that the prescriptions given to a patient are used by that individual and not misused. Thus, when a clinician receives urine drug test results with a negative result, it is imperative that the potential reasons are clearly understood so that the appropriate clinical decision can be made. 

Over the last few years, states have enacted mandates at the clinician level to prevent misuse, abuse, and diversion, including the adoption of prescription drug monitoring programs, controlled substance treatment agreements with patients, and urine drug testing. As a result, clinicians have needed to familiarize themselves with drug metabolism and excretion times in order to interpret test results more effectively. Some potential explanations for why a prescribed drug might not be detected in a patient’s urine sample include medication hoarding or diversion, the time between last dose and sample collection, self-escalation or binge use, failure to take the medication or taking only as needed, rapid metabolism, drug-drug interaction, and potential lab error.2 Other factors for a negative result include the integrity of the specimen submitted, analytical testing factors within the laboratory including false negatives, and steps within sample processing. Given the consequences surrounding negative urine drug test results, it is important to fully identify all possible causes for a negative drug test result. 

Specimen Integrity

Specimen validity testing (SVT) is a laboratory means to assess whether the submitted urine sample is consistent with characteristics of human urine. Generally, these tests involve the evaluation of the following parameters: pH (ie, the alkalinity or acidity of a sample), amount of creatinine excreted, determination of specific gravity, and a test for oxidants. A sample may fall under the following categories: normal, dilute, invalid, substituted, or adulterated. Ko et al summarized each of these scenarios in “Focus on Screens: Specimen Validity Testing,” published in Practical Pain Management in June 2013.3 An abnormality in SVT can account for a negative test result because the specimen may be very dilute or contain an additive that could have resulted in drug degradation below the level of detection.

Analytical Testing Factors

Evaluating the laboratory’s established cutoff level is a critical step when interpreting a negative urine drug result. Cutoffs are administrative values chosen by the laboratory and often are based on multiple factors, such as analysis type (screening test via an immunoassay technique versus confirmation test via a mass spectrometer), specimen type, purpose of the testing (workplace versus treatment of COT patients), and the instrument’s capabilities and limitations (confirmation with a mass spectrometer versus tandem mass spectrometry). In addition, Point of Care cups for rapid screening may produce negative results for a prescribed drug because the urine level of the parent drug or metabolite may be below the threshold or cutoff level.

Testing methods may be limited by the availability of compounds used as reference standards. Reference standards are known materials used by laboratories to compare patient samples, and often are accompanied by certificates detailing the prepared concentration and purity. These standards allow for monitoring the most reliable target compound for evaluating the presence or absence of drug. If the parent drug or best metabolite to monitor adherence is not used, a false negative result is possible. Recently, this occurrence was demonstrated when patients were tested for the antipsychotic medication aripiprazole (Abilify). In testing urine samples from patients clinically assessed as adherent, the parent compound aripiprazole was detected only 50% of the time and its metabolite dehydroaripiprazole only 8% of the time. OPC3373, a lesser known metabolite, was identified above the cutoff level in 93% of samples.4

Some immunoassay tests are subject to the “hook effect,” a phenomenon that can result in a false negative test result.5 Immunoassays are based on the binding of an antibody and antigen. When a drug is present in such high concentrations that there are no more binding sites left on the antibody, the sample may present with falsely low values. If the low value is below the test cutoff, the result will be negative. Although this is rare, it cannot be ruled out, nor can it be predicted. Fortunately, the manufacturers of immunoassay testing kits have made design improvements over the years and the incidence of such a phenomenon has been greatly reduced.

Another point to consider is the degree of cross-reactivity between the target compound and structurally related compounds. For example, Fileger et al demonstrated how a sample containing lorazepam or 7-aminoclonazepam (primary metabolite of clonazepam) may result in a false negative benzodiazepine test result because these compounds are known to have a low cross-reactivity with many traditional benzodiazepine immunoassay kits.6 Therefore, when a patient is prescribed a drug for which an assay has limited cross-reactivity, it is important that the laboratory conduct further testing, as is accomplished with a mass spectrometer.

In addition, drug degradation in a urine sample may result in a negative test. The stability of some drugs may be compromised after sample collection if the sample is not appropriately refrigerated pending analysis. Drug stability may be impacted further if the sample is subjected to high temperatures. For example, 6-monoacetylmorphine (6-MAM), a unique metabolite of heroin, lacks stability and is subject to in vitro hydrolysis. In the body, heroin is rapidly metabolized to 6-MAM, which is further metabolized to morphine.7 This also can occur in the sample after specimen collection. In fact, one study documented that a 6-MAM positive urine sample was hydrolyzed completely to morphine within 7 days of storage at room temperature.8

Sample Processing

As stated by Alexander Pope, “To err is human.” Laboratory testing, be it routine urinalysis or specialized drug testing, is not exempt from human error. A critical part of reducing human error is the automation of various testing procedures, use of sample barcoding, and effective quality assurance programs.

To understand why a patient’s sample fails to contain an expected substance and/or contains an unexpected substance, the laboratory must review all points in the specimen processing, starting with specimen collection.

Staff members need to begin with the date of collection to ensure the sample was properly collected and sealed, and not mixed with another patient’s sample. Typically, the patient has initialed the strip that has been placed over the sealed cup. Depending on the laboratory’s specimen retention policy, the sample should be verified for patient initials and sample identification numbers. If questions arise about the sample, laboratory personnel, in conjunction with a clinic member, should discuss whether a review of the data or a sample retest is appropriate. If a sample is retested, the new report can be compared to the original report.

In some cases, it may not be necessary to retest a sample. For example, the chart review of a patient whose sample is negative for a prescribed opioid may reveal that the last dose was several days prior to sample collection, and the prescription was written on an “as needed” basis.


Urine drug monitoring is one of many tools available to a clinician for the assessment and management of patients on COT. When a negative test result occurs for a prescribed medication, further evaluation is warranted. Besides a patient not taking their medication as prescribed, clinicians should be aware of other potential factors that can produce a negative result. To further assist in interpreting laboratory results, practitioners should contact the testing facility for assistance or to request retesting of a sample.

Last updated on: January 3, 2019
close X