2. Rule out medical or substance use issues that may be contributing to your patient’s symptoms. Some examples of typical tests based on the patient’s presentation can be found in the table “Potential Laboratory Tests for Some Psychiatric Symptoms” (p. 2). While this is not a complete list, these are common scenarios. It’s important to keep in mind that these tests are unlikely to be helpful in most patients you see, particularly if you have an outpatient practice. For example, ordering a TSH for every single one of your patients with depression is not reasonable. However, in patients who have not responded to multiple medication trials, or who have pronounced somatic symptoms, such tests are likely to have a higher yield.
3. Make sure to check if your patient has liver or renal disease, or is pregnant. It is important to ask patients specifically whether they have ever had any liver or kidney disease because these may affect the pharmacokinetics of most medications that we prescribe. If a patient has liver or kidney disease, we should make sure that we have results of hepatic or renal function tests, respectively, done within the previous year. Finally, if there is any chance that the patient may be pregnant, be sure to get a pregnancy test.
4. Get baseline values for things that could go wrong later. The tests that we should run before starting particular medications also depend on what particular adverse effects a medication can cause. (For exceptions to this, see #5: the “Don’t” point.)
Fasting serum glucose and serum sodium are usually done as part of a basic metabolic panel (formerly called a chem 7), and platelet count is done as part of a complete blood count. In the table “Lab Monitoring for Psychiatric Medications” (p. 3), hepatic function tests and serum creatinine are recommended above and beyond general medical evaluation for those medications that are particularly known to cause impairment of liver or kidney function.
5. Don’t test for things that are a) exceedingly rare, b) unpredictable, or c) unpreventable due to how rapidly they occur. For example, as noted previously, checking a complete blood count before starting mirtazapine is not required. Agranulocytosis, even if it is associated with mirtazapine (which is debatable), is extremely rare. In any case, it would occur relatively quickly and getting a blood count at baseline and then infrequently would be unlikely to help. Note that while the prescribing information for mirtazapine has a bolded warning about the possibility of agranulocytosis, it does not suggest that we routinely monitor white cell counts. Rather, it only suggests that if a patient on mirtazapine presents with symptoms and signs of infection (eg, sore throat, fever, stomatitis), a white cell count should be obtained.
Another example is checking serum amylase and lipase before starting divalproex. This is not recommended because the baseline value is extremely unlikely to be abnormal, pancreatitis is rare, and it occurs relatively quickly rather than gradually progressing. Thus, occasionally checking amylase and lipase will probably not allow for early detection of pancreatitis. Amylase and lipase should, of course, be tested promptly if clinical symptoms suggestive of pancreatitis appear.
Putting it all together: A practical approach for ordering labs
In our busy practices, we may not have the time to scrutinize each and every potential reason to order individual labs, so it makes sense to have a policy for “routine” screening. This does not mean that you have to be the one to order these tests. In fact, most of these will have already been obtained by primary care doctors. The table “Lab Tests to Consider Documenting in Patients’ Charts” (p. 6) lists the basic screening tests that I like to see in the charts of all of my patients. These, along with the tests recommended based on the specific medication that the patient will take, are recommended as an approach to prudent prescribing without wasteful, unnecessary testing.