Before You Write That Prescription: Guidelines for Ordering Lab Tests
Mental health clinicians often ask for a single, black-and-white answer to the question: Which lab tests should I order before starting this particular medication? Inevitably, the answer is that it depends. However, there are some guidelines that are useful.
When it comes to ordering lab tests, it sometimes feels like we are “damned if we do, and damned if we don’t.” Order too many tests, and you cost the health system money for potentially little gain—and your tests could yield false-positive abnormalities that lead to further unnecessary testing. Order too few tests, and you may miss a genuine problem.
If you want to order the appropriate tests, I recommend that you keep four key factors in mind: 1) Anticipated likelihood of an abnormal result; 2) Cost of the test; 3) Potential seriousness of the abnormality that may be found; and 4) The potential to treat the abnormality, if found.
The probability of an abnormal test—often referred to as the pretest probability—should influence your decision. If a positive result is extremely unlikely, then ordering a test for everyone will make each positive result expensive—depending on how expensive the test is. In an effort to determine the cost-effectiveness of lab tests, researchers from Spain retrospectively reviewed all lab tests done on 510 patients hospitalized at a psychiatric hospital in Madrid from 2000–2005. By dividing the costs of all tests by the number of abnormal results, they determined the “direct cost” of a given abnormal test (Arce-Cordon R et al, Psychiatr Serv 2007;58(12):1602–1605).
This direct cost (in 2005 dollars) varied considerably among psychiatric inpatients, eg., an abnormal creatinine was only 52 cents, an abnormal GGT (gammaglutamyltransferase) was $2.36, and an abnormal thyroid stimulating hormone (TSH) was $47.40. Whether these costs are acceptable is a judgment call and depends on whether a patient is likely to actually benefit. For example, paying $47.40 for an abnormal TSH may be a bargain if it helps you identify hypothyroidism. Similarly, iron deficiency in a person with restless leg syndrome is a problem that, if identified, can be easily remedied. At the other extreme, ordering a complete blood count for a patient starting mirtazapine is probably not cost-effective, since agranulocytosis is an extremely rare side effect of that antidepressant.
The pre-test probability of an abnormal test results varies with patient characteristics. In general, the literature shows that among psychiatric patients, lab tests are most likely to be valuable in the elderly, the poor, those with a history of substance abuse, and those who are disoriented or otherwise appear to have a mental disorder secondary to another medical condition (Anfinson TJ and Kathol RG, Gen Hosp Psychiatry 1992;14(4):248–257).
Thus, we should keep in mind that any laboratory testing recommendations are a framework of suggestions and not rigid guidelines to be followed unvaryingly.
Reasons to order lab tests in psychiatric patients
Deciding which lab tests to order in a particular situation becomes clearer if we keep in mind why we are ordering the tests. Here are four common reasons why psychiatrists might order labs on their patients.
1. Assess your patient’s general health. While you probably don’t consider yourself to be a primary care physician, you may have patients who rarely, if ever, see any other doctor for regular checkups. One study, for example, found several abnormalities to be surprisingly common among psychiatric inpatients: hepatic impairment (66%), hepatitis B (16%), and HIV (14%) (Lopez-Castroman J et al, Int J Psychiatry Med 2012;44(3):211–224). At least one criterion for metabolic syndrome was met in 68% of these patients! Even if you order labs to evaluate general health status, the person should also be referred to a primary care physician as soon as possible since preventive healthcare goes beyond simply ordering a few laboratory tests. We should also try to establish a simple and regular system to share lab test results with the primary care physician in both directions.
Mago,, R. (2017). Before You Write That Prescription: Guidelines for Ordering Lab Tests. Psych Central. Retrieved on March 23, 2017, from https://pro.psychcentral.com/before-you-write-that-prescription-guidelines-for-ordering-lab-tests/0018275.html