Bedside Snapshot
- Normal (Adult)
- Lab-specific upper limit
One 2026 UK study: 170 U/L (female) and 314 U/L (male, White), higher in Black and Asian groups - Critical Values
- No universal critical value. Typical severe threshold: >5,000 U/L (>83 µkat/L)
- High Suggests
- Rhabdomyolysis: crush, long lie, seizure, heat illness, stimulants
- Exertion, viral myositis
- Statin or other drug myopathy
- Endocrine or inherited myopathy
- Low Suggests
- Rarely clinically significant. A normal value does not exclude rhabdomyolysis if drawn within 12 hours of injury
- Act Now If
- CK at least 5 times ULN with dark urine, hyperkalemia or rising creatinine: monitor, 12-lead, check K⁺, start isotonic fluid; IV calcium for hyperkalemic ECG changes; notify receiving/medical control
- Watch Out For
- Calcium for asymptomatic hypocalcemia, and routine bicarbonate or mannitol, do not help injured muscle; give calcium for hyperkalemic ECG changes. Repeat CK before reassuring a patient with a normal early value
Reference & Critical Ranges
Lab-specific ranges: Every lab sets its own interval, by analyzer and by population. Your lab report's range is the one that counts. The values below are typical.
Reference ranges
| Population | Conventional | SI | Notes |
|---|---|---|---|
| Adult female | Upper limit set by lab. One 2026 UK multi-ethnic study: 170 U/L (White), 188 U/L (Asian), 389 U/L (Black), age 13 and older | Multiply U/L by 0.0167 for µkat/L | UK Pathology Harmony interval 25 to 200 U/L (derived from White cohorts). Limits differ by sex and ethnicity; check your lab's own interval |
| Adult male | Upper limit set by lab. Same study: 314 U/L (White), 357 U/L (Asian), 757 U/L (Black) | Multiply U/L by 0.0167 for µkat/L | UK Pathology Harmony interval 40 to 320 U/L (derived from White cohorts). Higher than female values in every ethnic group in that study |
| Children | Age-specific lab interval; not established on this page | As above | Use the pediatric interval from your lab |
Severity & critical values
| Level | Conventional | SI | What it means |
|---|---|---|---|
| Mild | Above the upper limit, under 5 times it (about 1,000 U/L) | Above the upper limit, under about 16.7 µkat/L | Nonspecific; exercise, drugs, endocrine causes and minor muscle injury |
| Rhabdomyolysis | 5 times the upper limit or more (about 1,000 U/L) | 16.7 µkat/L or more | Most studies define rhabdomyolysis at this level |
| Critical | Above 5,000 U/L | Above 83 µkat/L | Severe rhabdomyolysis in many cohorts; kidney-injury and electrolyte risk rises |
| Very high | Above 16,000 U/L | Above 267 µkat/L | Renal failure more likely at this level (loose correlation, per AAST 2022) |
Critical values: There is no universal critical value. The typical severe threshold depends on the lab: CK above 5,000 U/L, or 5 times the upper limit with myoglobinuria, hyperkalemia or rising creatinine.
Conversion: U/L × 0.0167 = µkat/L (1 µkat/L = 60 U/L)
Multiple of the upper limit
CK ÷ upper limit of normal = × ULN
Rhabdomyolysis is usually defined at CK above 5 times the upper limit of normal. AAST 2022 also accepts CK above 1,000 U/L. Here's the catch: ULNs differ by up to about four-fold between groups in one UK cohort. A fixed 1,000 U/L cut-off can misclassify some patients. Use the ULN from the same report.
Causes of Abnormal Results
High (CK at least 5 times the upper limit)
| Mechanism | Common causes | Bedside clue |
|---|---|---|
| Direct muscle trauma | Crush, compression, long lie, multisystem trauma, compartment syndrome | Swollen, tense or pale limb; dark urine |
| Exertion | Marathon, military training, extreme prolonged exercise | Muscle pain and weakness over days |
| Seizure | Generalized or prolonged seizure, status epilepticus | Recent seizure; post-ictal muscle pain |
| Drugs and toxins | Alcohol, recreational stimulants, statins; venom from spider bites, wasp stings or snakes | Agitation, tachycardia, fever, or a recent drug history |
| Heat and hyperthermia | Heat stroke, neuroleptic malignant syndrome, malignant hyperthermia | Hyperthermia with rigidity; see the dantrolene page for MH |
| Infection and inflammation | Viral myositis (influenza), bacterial or parasitic infection, inflammatory myopathy | Fever, myalgia, recent viral illness |
| Endocrine and inherited | Hypothyroidism or hyperthyroidism, diabetic complications, inherited metabolic myopathy | Chronic or recurrent CK elevation without trauma |
Statin with muscle symptoms: Check CK and creatinine. Statin-associated rhabdomyolysis is often defined at CK above 10 times the upper limit of normal. That is a higher bar than the 5 times used for other causes. Markedly raised CK, or rhabdomyolysis suspected? Stop the statin and evaluate. That is what the 2018 AHA/ACC cholesterol guideline advises.
Low
Low CK rarely matters clinically. Drawn early, it does not exclude rhabdomyolysis. CK takes hours to rise.
Interpretation & Bedside Approach
Step-by-step approach
- Is it real? Check the specimen and whether the result fits the history. Repeat if the pattern does not match the story.
- How sick is the patient? Check the ECG, potassium, creatinine, urine color and urine blood. Dark urine with a positive blood dipstick and few red cells suggests myoglobinuria.
- Acute or trend: CK rises during the first 12 hours, peaks within about 3 to 5 days and returns to baseline over 6 to 10 days. An early normal value does not rule out evolving rhabdomyolysis; repeat it.
- Calculate: Express CK as a multiple of the upper limit of normal from the same lab.
- Classify: Use the McMahon score at admission. A score of 6 or more predicts need for high-volume fluid, kidney replacement therapy and death.
- Escalate or dispose: Treat hyperkalemia and fluid need first, then admit to a unit with dialysis capability when kidney injury or high-risk scores are present.
Pattern and what it suggests
| Pattern | Suggests |
|---|---|
| CK at least 5 times ULN with pain, weakness or dark urine | Rhabdomyolysis; start the fluid plan |
| CK above 5,000 U/L | Severe rhabdomyolysis; higher risk of kidney injury and hyperkalemia |
| CK 1 to 5 times ULN, no injury | Nonspecific: exercise, statin, endocrine, viral causes; repeat with trend |
| CK rising after a stable start | Ongoing muscle injury or compartment problem; reassess |
Here's what people miss: CK also rises after myocardial injury. Diagnose MI with troponin, not CK. See the troponin page.
Management Priorities
Emergency: Rhabdomyolysis (CK at least 5 times ULN with dark urine, hyperkalemia or rising creatinine)
- Monitor: Cardiac monitor, 12-lead ECG, potassium, creatinine and urine output. Look for compartment syndrome as well.
- Stabilize the myocardium: IV calcium for hyperkalemic ECG changes. The dose lives on the calcium gluconate page.
- Fluids early: Isotonic crystalloid, titrated to urine output. Lactated Ringer's or saline, either is acceptable.
- Shift potassium: Move it with insulin and dextrose, plus nebulized albuterol, per the potassium page.
- Stop the cause: Treat hyperthermia. Stop seizures and nephrotoxic drugs. Bring surgery in early for compartment syndrome.
Medications
| Drug | Adult dose | Pediatric dose (max) | Onset / key notes |
|---|---|---|---|
| Normal saline or lactated Ringer's (rhabdomyolysis) | Start about 400 mL/hr IV (AAST 2022 range 200 to 1,000 mL/hr; protocol-dependent). Titrate to a urine output of 1 to 3 mL/kg/hr, not above about 300 mL/hr of urine output | Per pediatric protocol and weight-based dosing | Titrate the infusion to urine output, not to a fixed rate cap. Some crush protocols use higher early rates (1 L/hr for 2 h, then 500 mL/hr); these were not trial-based. Stop for crackles, falling SpO₂ or edema |
| Sodium bicarbonate (not routine) | Not routine; AAST 2022 and EAST 2022 advise against routine use | Per protocol only | Lowers ionized calcium and adds sodium. Use for severe acidosis per protocol |
| Mannitol (not routine) | Not routine; EAST 2022 conditionally advises against | Per protocol only | Use a diuretic only for fluid overload |
Avoid calcium for asymptomatic hypocalcemia: Calcium may settle into injured muscle. Treat hypocalcemia only when it causes symptoms, or when it is part of hyperkalemia with an ECG change.
Definitive care & disposition
- Kidney replacement therapy: AAST 2022 sees no role for it in preventing AKI. Reach for it on the usual indications: refractory hyperkalemia, acidosis, volume overload or oliguric AKI. See hemodialysis and CRRT.
- Compartment syndrome: identify and release it early.
- ICU admission: hyperkalemia, AKI, McMahon score 6 or more, or ongoing muscle injury. Follow CK until it is clearly falling.
Pediatric fluid and drug calculations: weight-based dosing.
Doses are typical references. Doses vary by protocol; follow local protocols, medical direction and institutional policy.
Special Populations
| Population | What changes | Clinical implication |
|---|---|---|
| Neonates & infants | Age-specific interval; no range on this page | Use the lab's neonatal interval and seek pediatric input early |
| Children & adolescents | Causes differ from adults: trauma, viral infection, drugs and exercise predominate | Interpret with the cause; use the pediatric interval and pediatric fluid dosing |
| Pregnancy | No pregnancy-specific interval in the sources reviewed | Interpret against the adult female range and involve obstetric care |
| Older adults | Age older than 55 is an independent risk factor for rhabdomyolysis after trauma; McMahon score adds age points | Lower threshold for monitoring and careful fluid titration |
| Black and Asian patients | Upper limits are higher in Black and Asian groups in one 2026 UK study | Using a White-population limit can flag normal results as high; check the lab's ethnicity-specific interval |
Ethnicity: In some populations, higher CK is usually benign. Hold it against the lab's interval before you call it injury.
Interfering Factors & Pitfalls
Factors that change the result
| Factor | Effect | Why | Avoid / fix |
|---|---|---|---|
| Recent exercise | ↑ False + | Muscle injury from training raises CK for days | Note exertion in the history; repeat with trend |
| Statins and other drugs | ↑ | Drug-induced myopathy (statins are a listed cause) | Review medications and check CK with symptoms |
| Ethnicity and sex | False + | Limits differ by group in one UK study | Use the lab's sex- and ethnicity-specific limit |
| Early sample | False − | CK rises over the first 12 hours | Repeat later if the story fits |
| Late sample | False − | CK peaks at about 3 to 5 days and falls over 6 to 10 days | Compare with the trend, not one value |
Common misinterpretations
- Normal CK rules out rhabdomyolysis: Not when it was drawn early. Repeat it.
- High CK means heart attack: Troponin decides MI, not CK.
- Calcium protects injured muscle: It can make the injury worse. Avoid it for asymptomatic hypocalcemia. Give it for hyperkalemic ECG changes.
Physiology & Test Basics
What it measures
Total creatine kinase (CK) measures the activity of this enzyme across muscle, brain and heart, all together. Damaged muscle cells let it climb, which makes it the marker for rhabdomyolysis. The CK-MB and myoglobin pages cover the cardiac fractions.
Physiology & regulation
- In muscle cells, CK turns creatine phosphate into ATP.
- Muscle injury lets intracellular CK leak into the blood.
- After muscle injury, CK takes days to clear. In rhabdomyolysis it peaks at about 3 to 5 days. Back to baseline takes about 6 to 10 days.
How it is measured
Serum CK on a routine chemistry analyzer. Use the tube type the lab specifies. Fast only if the lab asks you to. Turnaround is usually hours.
Why it matters
In emergency and critical care, CK answers one question. Is the muscle injury bad enough to drive kidney injury and hyperkalemia? Serial values show whether that injury is still ongoing.
Clinical Pearls
Prehospital: Crush or long-lie patient? Start the fluid plan early, under protocol or medical control. Watch the ECG and potassium when the trapped limb comes free. Entrapment protocols are on the rhabdomyolysis and crush injury page.
Nursing titration: Titrate fluid to urine output. Stop for crackles, falling SpO₂ or edema.
Repeat, do not reassure: Drawn early and normal? That does not exclude rhabdomyolysis. Repeat it.
Ethnicity-specific limits: Check the lab's ethnicity-specific upper limit before you call a result high.
Trend over value: Repeat CK daily until it is clearly falling. The peak comes days after the injury.
References
- Chavez, L. O., Leon, M., Einav, S., & Varon, J. (2016). Beyond muscle destruction: a systematic review of rhabdomyolysis for clinical practice. Critical Care, 20, 135. https://doi.org/10.1186/s13054-016-1314-5
- AAST Critical Care Committee (Kodadek, L. and colleagues). (2022). Rhabdomyolysis: an American Association for the Surgery of Trauma Critical Care Committee clinical consensus document. Trauma Surgery & Acute Care Open, 7(1), e000836. https://doi.org/10.1136/tsaco-2021-000836
- Eastern Association for the Surgery of Trauma (Sawhney, J. S. and colleagues). (2022). Management of rhabdomyolysis: a practice management guideline from the Eastern Association for the Surgery of Trauma. American Journal of Surgery, 224(1 Pt A), 196–204. https://doi.org/10.1016/j.amjsurg.2021.11.022
- Grundy, S. M., et al. (2019). 2018 AHA/ACC/multisociety guideline on the management of blood cholesterol. Circulation, 139, e1082–e1143. https://doi.org/10.1161/CIR.0000000000000625
- Kalaria, T., Gama, R., et al. (2026). Age, sex and ethnicity changes in creatine kinase and sex- and ethnicity-specific reference intervals of creatine kinase. Clinical Medicine, 26(4), 100596. https://doi.org/10.1016/j.clinme.2026.100596
Medical Disclaimer
- For Educational Purposes Only: This content is intended for educational reference and should not be used for clinical decision-making.
- Not a Substitute for Professional Judgment: Always consult your local protocols, institutional guidelines, and supervising physicians.
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