From the Guidelines
Morphine can cause a positive drug screen for codeine due to their chemical similarity and the limitations of standard drug tests. This is because morphine and codeine are both opioids that can produce metabolites which cross-react with the antibodies used in immunoassay drug screens designed to detect codeine 1. According to a 2022 study published in The Lancet Child and Adolescent Health, standard urine tests for opiates can detect morphine and codeine, but may not distinguish between them 1.
Key Points to Consider
- Morphine is metabolized in the body and can produce metabolites that cross-react with the antibodies used in immunoassay drug screens designed to detect codeine.
- Codeine itself is metabolized to morphine in the body, further complicating the distinction between the two substances.
- If you are taking prescribed morphine and need to undergo drug testing, it's essential to inform the testing facility or your healthcare provider about your medication beforehand.
- More specific confirmatory testing methods like gas chromatography-mass spectrometry (GC-MS) can differentiate between these compounds if needed, but these tests are typically only performed after an initial positive screening result and are more expensive and time-consuming.
Clinical Implications
It's crucial for healthcare providers to be aware of the potential for false positives when interpreting drug test results, especially in patients taking prescribed opioids like morphine. Informing patients about the possibility of false positives and the importance of disclosing their medication use can help prevent misunderstandings and ensure accurate test results. Additionally, using more specific confirmatory testing methods like GC-MS can help differentiate between morphine and codeine, but these tests should be used judiciously due to their higher cost and complexity 1.
From the Research
Morphine and Codeine Interaction
- Morphine preparations are known to have small amounts of codeine as an impurity, estimated to be about 0.04% 2
- This impurity can lead to a positive drug screen for codeine in patients prescribed morphine, with 24 out of 535 pain patients testing positive for codeine >20 ng/mL 2
Urine Drug Testing
- A study of 8600 urine specimens found that increasing the opiate screening and confirmation cutoff value to 2000 ng/mL reduced the confirmed-positive rate for codeine and morphine by over 300% 3
- Another study found that 11% of patients prescribed morphine only had urine samples that tested positive for codeine, with the codeine concentration being less than 1% of the morphine concentration 4
Codeine Positivity in Urine
- A study of 80 urine samples found that the percentage of codeine-positive results resulting from morphine use was higher than previous estimates, with careful analysis of urine drug testing results helping to determine patient compliance with pain management regimens 4
- Measuring free drug only can lead to false negative results, highlighting the importance of detecting glucuronide metabolites in urine drug testing 5
Morphine and Codeine Concentration Ratios
- A study found that the morphine/codeine ratio in blood can be used to distinguish between heroin use and medication with codeine, with a ratio greater than unity suggesting heroin use 6
- The ratio of codeine to morphine (C/M) in urine can also help determine if patients are compliant with their pain management regimens 4