Joshua Gunn, PhD
Because of the advanced way in which urine samples are tested toxicologically now, there’s really no arguing the results. However, in the past, many providers who were doing some form of drug testing were utilizing instant testing devices in the office things like urine cups, dipsticks, and other point-of-care tests that we now realize are not ideal for this because of their limitations and the possibilities of false positives and false negatives.
Properly administered, what can urine and blood testing tell a clinician about his or her pain patients? What are the best uses for each of these testing modalities?
Urine drug testing is our best friend when it comes to general compliance monitoring because drugs stay in the urine a lot longer than they stay in the blood. Typically, an opioid will only remain detectable in the blood for several hours, but it’s going to remain detectable in the urine for several days. Urine affords us a standard window of detection that is ideal for compliance monitoring, because if you’re seeing a patient for the first time, you not only want to know what’s in their system now, you also want to know whether they’ve been using substances or illicit drugs in the three or four days leading up to their visit.
One limitation of urine drug testing is that it provides no information on how much drug has been taken. However, for illicit drugs or non-prescribed drugs, this issue really doesn’t come into play it’s more a matter of determining whether they’re in the patient’s system or not. But providers are also concerned that their patients may be taking only some of their prescribed pain medication; they may be taking only one pill of their prescribed medication each morning in order to pass their urine drug test and then selling the rest. Because there is no linear relationship between what’s detected in the urine and how much drug was actually ingested, as long as the drug shows up in their urine, that patient is going to appear compliant. If we want to know how much drug was taken, we have to go back to pharmacokinetics and look at steady-state blood levels.
As a provider, if your main concern is illicit drug use or non-prescribed drug use, you’re going to use urine testing because it gives you that extended window of detection to identify those things that should not be there. If your major concern is to determine how much prescribed medication the patient is actually taking, to ensure they’re not taking too little or overmedicating, that’s when you’re going to use blood testing.
On an initial visit, urine is going to be your best friend. However, if you start that patient on a prescribed controlled substance and their urine tests continually come back positive for the correct drug or the correct metabolite, but you have suspicions about how they’re actually taking it, then further down the line you may consider a blood draw just to ensure that they’re actually taking the medication as you’re prescribing it. We’re never going to replace one test with the other, but used together they can be very useful because blood picks up where urine leaves off.