There's no lab test for depression — the diagnosis is made by looking at your symptoms and figuring out whether you meet the diagnostic criteria. Physical conditions can, however, mimic depression symptoms or induce depression, and to "qualify" for a diagnosis of depression, your doctor first needs to rule those out. If you have one of these conditions, it doesn't necessarily mean you're not depressed, but it may lead to changes in the way depression is treated. This means lab tests could be part of your diagnostic process after all.
A thyroid test
Hypothyroidism — also called underactive thyroid or a slow thyroid — happens when your body doesn't make enough thyroid hormone. The symptoms might take a while to become apparent, and also vary from one patient to the next. Depression is, however, one of them; in fact, hypothyroidism is the medical condition most often linked to this mood disorder. Several other symptoms, like fatigue, general aches and pains, constipation, and weight gain are also often associated with depression. This is why your doctor will often want to order a thyroid function test to look for elevated TSH when you present with depressive symptoms.
A complete blood count
A complete blood count is basically just a "blood test" that looks for abnormalities. Your doctor will want to know if you could be suffering from anemia — most often iron-deficiency anemia — since it has been linked to depression, and especially in older adults. Anemia can also lead to symptoms similar to those of depression, such as fatigue and irritability. A vitamin B12 or folate deficiency (pernicious anemia) is another possible cause, and it is often associated with memory problems and fatigue.
Kidney function tests
Not only are people with kidney disease more likely to suffer from depression, some of the symptoms — fatigue, drowsiness, and confusion — can give the impression that someone is depressed even if they're not. What's more, the dose and kind of antidepressant people with kidney disease are prescribed has to be different because of their pre-existing medical condition. This is why your doctor may order kidney function tests, concretely creatinine and blood urea nitrogen.
Liver function tests
Substance abuse, including alcoholism, is one of the differential diagnoses of depression — meaning your doctor will want to rule them out before they make a diagnosis. It is, of course, very much possible to have a substance abuse problem and also to be depressed, but in this case your doctor will need to know more about your liver function in order to decide what medications to prescribe.
A blood glucose test
People living with diabetes have been found to have almost double the risk of depression than the general population. This is a problem because living with depression worsens the prognosis, in part because depressed people with diabetes are less likely to take their medications as they should. If you have not already been diagnosed with diabetes but have symptoms of it, your doctor may ask you to participate in a fasting blood glucose test.
A cortisol test
Cushing's disease, also called Cushing's syndrome, is a condition in which your body produces too much cortisol for long periods of time. It is often associated with depression as well as weight gain, severe fatigue, and lowered libido — symptoms sometimes seen in people with major depressive disorder as well.
Your doctor may want to have your cholesterol patients tested, both because high cholesterol can interfere with prescription medications and because low plasma cholesterol levels have been found to be associated with a higher suicide risk in people suffering from major depression.
Other diagnoses sometimes confused with depression — and how your doctor can rule them out
- Some medications lead to depressive symptoms — and while they're real, the approach to treatment will be different; switching your medications where possible. Isotretinoin (Accutane), levodopa, and interferon are examples of medications that are well-known to have the ability to lead to depression.
- Alzheimer's or other forms of depression — it can sometimes be difficult to tell the difference between depression later in life and the onset of dementia, and if this applies to you, your doctor will conduct a neuropsychological exam and perhaps make use of brain imaging tests such as an MRI scan.
- Heart disease — many heart disease patients suffer from depression, and your doctor may want to order an electrocardiogram (ECG).
- Obstructive sleep apnea is a condition in which patients stop breathing for short periods of time while they're sleeping and that's most common in obese people. Many of the people with sleep apnea who report excessive daytime sleepiness are found to suffer from depression — but research has also revealed that treating the sleep apnea relieves depressive symptoms. In order to diagnose obstructive sleep apnea, a sleep study is carried out.