Before your blood test
Does prednisone affect blood test results?
Short answer
Yes, and the changes are real rather than lab errors. Prednisone, dexamethasone and other glucocorticoids raise blood glucose within hours (and A1c over weeks), raise the white blood cell count — mostly neutrophils — while lowering lymphocytes and eosinophils, and suppress your body's own cortisol so that a cortisol test taken on steroids reads low and cannot be interpreted. They can also raise triglycerides and lower potassium, and mask the fever and inflammation markers (CRP, ESR) that would normally signal infection. Take the dose as prescribed; never stop steroids abruptly for a test. Tell the clinician the drug, dose and how long you have taken it, and schedule cortisol or adrenal testing only with the prescriber's guidance.
Medically reviewed by Youmna DiStefano, MD · Last reviewed October 2026
Which results does it affect?
| Marker | Direction | How long | What to do |
|---|---|---|---|
| Glucose (fasting and random) | raises | Within hours of a dose; afternoon rise typical with morning prednisone | Expected on steroids; A1c over weeks reflects it [source] |
| White blood cells (neutrophils) | raises | Within hours to days; persists while taking | A high WBC on steroids is usually the drug, not infection — the differential helps [source] |
| Lymphocytes and eosinophils | lowers | While taking | Expected; an eosinophil count of zero on steroids is normal [source] |
| Cortisol (serum, salivary, urine) | lowers | Suppressed while taking and for weeks to months after a long course | Do not test cortisol on steroids without the prescriber's plan; some steroids also cross-react with the assay [source] |
| CRP and ESR | lowers | While taking | Inflammation markers can look reassuring despite infection [source] |
| Potassium | can lower | With higher doses | Relevant if also on diuretics [source] |
| Triglycerides | can raise | With ongoing use | Note steroid use when lipids are read [source] |
Why do steroids change so many results?
Glucocorticoids are the body's stress hormone in pill form, and the lab picture is the stress picture: the liver releases more glucose and tissues take up less, so glucose climbs; neutrophils are pushed from the vessel walls into the circulating blood, so the white count rises even though no new cells are being made; lymphocytes and eosinophils move out of the blood, so they fall. None of these are assay errors — the numbers are accurate measurements of a body on steroids — which is why the medication list matters more than any prep rule.
Inhaled, nasal and topical steroids have far smaller effects than tablets, but high-dose inhaled steroids and long courses of potent creams can still suppress cortisol.
Which tests should wait until you are off steroids?
Cortisol and anything built on it. A morning cortisol, an ACTH stimulation test or a Cushing's screen taken while on prednisone cannot be read — the drug suppresses your own production and, for some steroids, shows up in the assay itself. After a course of more than a few weeks, recovery of the adrenal axis can take weeks to months, so testing is timed by the prescriber. A glucose or A1c on steroids is still useful — steroid-induced high blood sugar is real and sometimes needs treatment — as long as the reader knows about the steroid. For a CBC, the differential tells the story: a high neutrophil count with low lymphocytes and no eosinophils is the steroid signature.
The tests this affects — order without a doctor's visit
Prices are cash-pay; every order adds one $6 lab processing fee. Physician authorization is included; no insurance or referral needed.
| Test or panel | Type | Price | Fasting | Results |
|---|---|---|---|---|
| Glucose, Fasting | Test | $24.99 | Yes | 1–3 business days |
| CBC with Differential | Test · 14 biomarkers | $21.99 | No | 1–3 business days |
| Hemoglobin A1c | Test | $21.99 | No | 2–4 business days |
| Comprehensive Metabolic Panel | Test · 14 biomarkers | $22.99 | Yes | 1–3 business days |
Common questions
Can I take prednisone before a blood test?
Take it as prescribed and never stop it abruptly for a test. For a fasting draw, have the blood drawn first and take the dose afterwards if the timing allows; tell the clinician the dose and duration.
Does prednisone raise white blood cell count?
Yes, within hours. Neutrophils rise while lymphocytes and eosinophils fall, so a high WBC on steroids usually reflects the drug rather than infection — the differential and your symptoms decide.
Can I have a cortisol test while on prednisone?
Not usefully. Prednisone suppresses your own cortisol and some steroids cross-react with the assay, so the result cannot be interpreted. Cortisol or adrenal testing on or after steroids is timed by the prescriber.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.