Urine Versus Blood for Laboratory Testing of Pain Management Patients
Because of the risk of addiction or abuse, it’s common practice to monitor pain management patients for compliance with therapy. Unfortunately, some pain management patients have been known to take additional illicit drugs, while others who are already addicted to illicit drugs may feign pain to acquire additional prescription opioids, complicating testing.
Drug screening rarely progresses to follow-up testing, which can lead to patients being falsely accused of taking illicit drugs in the case of a false-positive screening result, or of being noncompliant with therapy in the case of a false-negative screening result. To alleviate this serious situation, some laboratories have turned to using mass-spectrometry methods as the first line of testing and encouraging additional testing of any unexpected result. A randomly collected simple urine screening for a panel of drugs of abuse is the most common method of monitoring pain management patients. Other specimens include serum or plasma, oral fluid, and hair. Of these, oral fluid is being used with increasing frequency, while serum or plasma and hair are used in specific situations.
Urine is the most commonly used specimen for drug testing because of ease of collection. Other advantages to using urine for drug testing are the relatively high concentrations of many drugs and metabolites in urine, as well as the extended drug detection window of urine versus blood. However, there are some important limitations to using urine for drug testing. The concentrations of drugs in urine cannot be used to determine degree of toxicity or dose taken. Additionally, urine specimens are easily tampered with.
Serum is the preferred specimen for therapeutic drug monitoring, because drug concentrations in the specimen reflect the drug’s disposition at collection time. When the time and amount of the previous dose are known, the measurement can be compared with a predicted blood concentration of the drug. Given the collection method and the fact that it is by nature observed, serum specimens are very difficult to adulterate and there is little chance of a false-positive result from contamination.
However, for serum testing to be useful, the patient needs to be prescribed and comply with a daily therapy regimen. Serum drug testing is not useful for monitoring for patients taking infrequent, low-dose, or as-needed courses of pain management drugs.
More information is available in CLSI document C63 for labs that conduct testing for pain management programs, including information on specimen collection, testing methodologies, and results reporting and interpretation.