Blood tests are usually the first step in diagnosing myositis and a regular part of living with it. Here is what each test measures, and what the numbers do and do not mean.
Eat and take your medications as normal unless told otherwise. Do tell your team about any hard exercise in the days before the test, because it can change the results.
When muscle is inflamed, some of its contents leak into the bloodstream. Blood tests measure those leaked proteins, called muscle enzymes, to gauge how irritated the muscle is. Other blood tests look for the immune fingerprints of myositis, the antibody panel, or keep watch over your general health during treatment.
Creatine kinase (CK, sometimes written CPK) is the main muscle enzyme. In active myositis it is often high, sometimes many times the normal limit, and it tends to fall as treatment calms the inflammation. Your team follows the trend over months rather than reacting to any single number.
Context matters when reading a CK. Levels are naturally higher in men and in people of African or Caribbean ancestry, a hard workout can raise CK for several days, and a minor bump on one test often means nothing. CK also lags behind how you feel in both directions: it can improve before your strength does, and your strength can improve while the number is still settling. How you are moving and functioning matters as much as the value on the page.
AST and ALT come from muscle as well as liver. In active myositis they rise for the same reason CK does: they are leaking from inflamed muscle. It is common for a lab report, or even a well-meaning clinician outside the clinic, to flag them as a liver problem. Your Centre team expects this pattern and can tell the two apart. If a new doctor is worried about your liver, remind them you have myositis, it genuinely changes how those numbers are read.
Some forms of myositis run with normal or only mildly raised CK. This is common in inclusion body myositis, and it happens in dermatomyositis too, where the skin disease can even appear with little or no measurable muscle involvement. Enzymes can also settle later in the course of disease while symptoms continue.
So if your bloodwork looks reassuring while you feel weaker, you are not imagining things and you have not been ruled out. This is exactly why diagnosis leans on the full set of tools: strength testing, the antibody panel, EMG, MRI, and when needed a muscle biopsy. Say clearly to your team how your function has changed. That report matters as much as the lab report.
Once treatment starts, regular bloodwork changes jobs: as well as following the myositis, it keeps your medications safe. Depending on what you take, your team monitors blood counts, liver and kidney function, and sugar levels at set intervals. Each of our medication guides explains the monitoring that goes with that drug. Keeping these appointments is one of the simplest things you can do to keep treatment on track.
Yes. A blood draw carries only minor risks, a small bruise or brief light-headedness. If needles make you anxious, tell the lab staff; looking away and staying hydrated genuinely help.
Results go to your Centre team, who read them against your history, your medications, and your last results. If you follow your own numbers through a patient portal, remember that a value flagged in red is not always a problem in myositis, and bring anything that worries you to your next visit rather than carrying it alone.
This guide is for learning, not medical advice. Reference ranges differ between labs, and what your numbers mean depends on your whole situation. Always review your results with your care team.
Written by Angelo Papachristos, BScPT, MBA, ACPAC, Advanced Practice Physiotherapist, Toronto Myositis Centre. Medically reviewed by Dr. Ophir Vinik, MD, FRCPC, Rheumatologist and clinical lead of the Myositis Clinic at St. Michael's Hospital. Last updated: August 2026. References are listed at the end of this page.
1. Lundberg IE, Fujimoto M, Vencovsky J, et al. Idiopathic inflammatory myopathies. Nature Reviews Disease Primers. 2021;7(1):86.
2. Lundberg IE, et al. 2017 EULAR/ACR classification criteria for adult and juvenile idiopathic inflammatory myopathies and their major subgroups. Annals of the Rheumatic Diseases. 2017;76(12):1955-1964.
3. Schmidt J. Current classification and management of inflammatory myopathies. Journal of Neuromuscular Diseases. 2018;5(2):109-129.
4. McHugh NJ, Tansley SL. Autoantibodies in myositis. Nature Reviews Rheumatology. 2018;14(5):290-302.