Why is Creatine Kinase (CK) Low? Causes, Symptoms, and What to Do

A low creatine kinase (CK, also called CPK) result usually means much less than a high one. Most often it simply reflects having less muscle: being inactive, older or of slight build, or being female. CK is also lower in pregnancy and has been found lower in some inflammatory diseases such as rheumatoid arthritis and lupus. A low CK does not cause symptoms by itself, and in a person who feels well it rarely needs any action.

What creatine kinase is

CK is an enzyme found mostly in skeletal muscle, with smaller amounts in heart muscle and the brain. A little leaks into the blood all the time; when muscle is damaged, much more leaks out. That is why a high CK is used to detect and follow muscle injury and muscle diseases, while a low CK mainly tells you about how much muscle there is. Blood CK reflects muscle mass, which is why men usually have higher values than women.

The test can be split into three forms (isoenzymes):

• CK-MM, mostly from skeletal muscle
• CK-MB, mostly from heart muscle
• CK-BB, mostly from the brain and smooth muscle

Typical values

CK is reported as enzyme activity in units per litre (U/L, also written IU/L). Ranges vary between laboratories and depend on sex, age, muscle mass and activity level, so always compare your result with the range printed on your own report.

The last four figures come from a European neurology guideline (EFNS). They are not upper limits of normal: they are 1.5 times the upper limit, the level above which the guideline suggests looking further into a high CK in someone without symptoms. The same review notes that standard laboratory ranges are often set too low and flag healthy people as "high". Because many laboratory ranges start close to zero, a result marked "low" depends heavily on where your laboratory set its lower limit.

Causes of a low CK

Common and usually harmless

• Low muscle mass or an inactive lifestyle. Less muscle means less CK leaking into the blood.
• Female sex and older age. Women have lower values than men on average, and CK declines slightly with age.
• Pregnancy. In one study of 124 pregnant women, the average CK was less than half that of teenage girls, and lower than in non-pregnant adult women.

Linked with medical conditions or treatment

• Inflammatory rheumatic diseases. In a study of 498 patients, CK was lower in rheumatoid arthritis, lupus and spondyloarthritis than in comparison patients. The lower the CK, the higher the inflammation markers tended to be.
• Corticosteroid treatment. The same study linked lower CK with the dose of prednisolone, a steroid medicine, and with lower body mass.
• Other connective tissue diseases. A study in JAMA also found low CK common in Sjögren's syndrome, and noted that low values occurred even in people not taking steroids.

An overactive thyroid has been linked with somewhat lower CK on average, but CK is usually normal in hyperthyroidism and cannot be used to judge thyroid function. Low CK is not a recognised sign of a specific disease on its own. Its main practical importance is the opposite of reassurance in one situation: in people with connective tissue diseases, a low or normal CK does not rule out muscle inflammation (myositis).

Symptoms

A low CK causes no symptoms. If you have muscle weakness, tiredness or pain, these come from whatever underlying condition is present, not from the CK value. Tell your doctor about such symptoms even if your CK looks low or normal.

High CK and dermatomyositis

Dermatomyositis is a disease that causes muscle inflammation together with a skin rash: a purple colour of the upper eyelids and a purple-red rash on the face, knuckles, neck, shoulders, chest or back. Weakness, stiffness or soreness of muscles, shortness of breath and difficulty swallowing can occur.

In dermatomyositis a high CK means muscle fibres are being damaged by the inflammation, so enzyme leaks into the blood. CK that stays high or keeps rising may mean the damage is continuing, which is why doctors repeat it to follow the disease. However, CK can be normal in about 10% to 26% of people with dermatomyositis, so a second muscle enzyme, aldolase, is often measured too. Diagnosis also relies on myositis-specific autoantibodies, electromyography (EMG), MRI of the muscles and muscle or skin biopsy, and the work-up usually includes screening for cancer and checks of the lungs.

Other common reasons for a high CK are much more ordinary. Strenuous exercise can raise CK up to 30 times the upper limit within 24 hours, with a gradual fall over the following 7 days. Statin medicines can raise CK 2 to 10 times the upper limit, and an underactive thyroid can cause a mild to moderate rise.

High CK-MB

CK-MB comes mostly from the heart. After a heart attack it rises within 3 to 6 hours, peaks at 12 to 24 hours and returns to normal 12 to 48 hours after the damage if no further injury occurs. Other causes include inflammation of the heart muscle (myocarditis), heart surgery, chest trauma, electrical injury, defibrillation, pulmonary embolism and heart failure. Today a troponin test is used more often than CK for a suspected heart attack, because troponin is better at detecting heart muscle damage.

What usually happens next

• Low CK in someone who feels well: usually no further testing. Your doctor will look at it together with your other results and your health history.
• Low CK with muscle symptoms or a rheumatic disease: your doctor may look for myositis with aldolase, autoantibody tests, EMG or MRI rather than relying on CK.
• High CK: usually repeated after a period of rest. One review recommends 7 days without exercise; your doctor may also ask you to avoid intense exercise and alcohol for a few days before the test. Companion tests often include thyroid tests (TSH), kidney function (creatinine) and urine tests.
• High CK-MB or chest symptoms: troponin and an ECG.

Frequently asked questions

What does low creatine kinase mean? Most often it means you have relatively little muscle mass or are not very physically active. It is also normal for CK to be lower in women, in older people and in pregnancy. It is far less clinically important than a high CK.

Are low CK levels dangerous? A low CK is not harmful in itself and causes no symptoms. It becomes relevant only alongside other findings, for example in someone with a rheumatic disease and muscle weakness, where a low CK should not be taken as proof that the muscles are healthy.

Can you have muscle disease with low or normal CK? Yes. In connective tissue diseases a low or normal CK does not rule out myositis, and in dermatomyositis CK is normal in about 10% to 26% of patients. Doctors therefore also use aldolase, antibody tests, EMG, MRI or biopsy.

What does a high creatine kinase level mean in dermatomyositis? It means the inflammation is damaging muscle fibres. A falling CK during treatment generally reflects less ongoing damage, while CK that stays high or rises may mean the damage continues. Your rheumatologist reads it together with your strength and other tests.

What does high CK-MB mean? It points to injury of heart muscle, most importantly a heart attack, but also myocarditis, heart surgery or chest trauma. Troponin is now the main test for a heart attack.

Do I need to avoid exercise before a CK test? Yes, intense exercise can raise CK sharply for several days. Your doctor may ask you to avoid intense exercise and alcohol for a few days beforehand. No fasting is generally needed.

When to see a doctor

A single low CK in someone who feels well is not a reason for concern; it is worth mentioning at your next routine visit. It is worth discussing with your doctor soon if you have muscle weakness, unexplained muscle pain, a new rash over your eyelids or knuckles, difficulty swallowing, or dark urine, or if a high CK was found while you take a statin. Seek urgent care for chest pain, severe breathlessness or fainting.

Sources

1. Creatine Kinase — MedlinePlus, U.S. National Library of Medicine
2. CPK isoenzymes test — MedlinePlus, U.S. National Library of Medicine
3. Dermatomyositis — MedlinePlus, U.S. National Library of Medicine
4. Approach to asymptomatic creatine kinase elevation — Cleveland Clinic Journal of Medicine
5. Creatine Kinase (CK) Test — Testing.com
6. Serum creatine kinase levels in pubertal, mature, pregnant, and postmenopausal women — Journal of Medical Genetics (PMC)
7. Serum creatine kinase in patients with rheumatic diseases — Clinical Rheumatology (PubMed)
8. Clinical significance of low creatine phosphokinase values in patients with connective tissue diseases — JAMA (PubMed)
9. Disease spectrum of myopathies with elevated aldolase and normal creatine kinase — PMC
10. The relationship between serum creatine kinase and thyroid hormones — Clinica Chimica Acta (PubMed)
11. Troponin Test — MedlinePlus, U.S. National Library of Medicine

Decisions about your health should always be made in consultation with your healthcare provider.