Creatine kinase (CK) is an enzyme found mainly in muscle cells and heart cells, with small amounts in brain cells. Cells release CK into the bloodstream when they are damaged and during normal cell turnover. Many Black men have been told by their doctors that they have a substantially “elevated CK level.” Most are puzzled by the high readings, and unfortunately, many medical providers can’t explain them.
What many medical providers don’t know is that African Americans tend to have significantly higher baseline CK levels. Black Americans can have dramatically higher CK levels in their blood, and Black men have the highest levels of all.
Research confirms: higher CK in Black Americans
A national study of more than 10,000 healthy adults from the University of Pennsylvania found consistently higher CK levels in African Americans, and even higher levels in Black men. Black men were more than eight times as likely as white men to have a CK level flagged as “abnormal.” Because CK comes mainly from muscle, many believed the greater muscle mass of African American men explained the difference. But after further analysis, muscle mass did not explain the gap.
Normally, elevated CK in the blood can be linked to strenuous exercise, heavy alcohol use, some medications, toxins or poisons, and especially a heart attack. CK levels are often checked in people who go to the emergency department with chest pain that may be heart-related. In that setting, an elevated CK can suggest damage to the heart muscle.
Some people have elevated CK while taking statin medications. In others, an elevated CK is linked to undiagnosed rheumatologic disorders called idiopathic inflammatory myopathies.
Higher CK normal range in Black men and women
The important message is that the “normal range” is significantly higher for both African American men and women. The table below, from University of Pittsburgh researchers writing in the Cleveland Clinic Journal of Medicine, shows the CK levels at which further testing is recommended for a person with no muscle aches and a normal physical exam. They follow European neurology guidelines that call for a work-up only when CK is at least 1.5 times the upper limit of normal for a person’s sex and race. For a Black man, that threshold is 1,200 IU/L, more than twice the 504 IU/L threshold for a white man.
In everyone, CK levels decrease with age, likely because of the muscle loss that comes with aging.
The graph below, from the national study, shows the very different range and spread of CK levels by race and ethnicity, with Black men and women substantially higher than all other groups.
The authors reported the following:
“The most notable factor that contributed to higher CK levels was Black race. Previous studies suggested that racial differences in CK may not be due to differences in height, weight, or body mass, but did not evaluate other body composition measures. This study evaluating BMI, waist circumference, and arm circumference, provides additional evidence that racial differences are not explained by differences in muscle mass. Higher CK levels among black individuals might instead be due to differential production or clearance of CK.”
George, Michael D. MDa,*; McGill, Neilia-Kay MDa; Baker, Joshua F. MD, MSCEa,b,c. Creatine kinase in the U.S. population: Impact of demographics, comorbidities, and body composition on the normal range. Medicine: August 2016 – Volume 95 – Issue 33 – p e4344 doi: 10.1097/MD.0000000000004344
Why CK runs higher in Black Americans
The authors suggest a fundamental difference in how African Americans either produce creatine kinase or clear it from the blood . . . and the difference is significant!
The differences in CK levels in African Americans have been known for more than 40 years, yet most laboratories still have not adjusted their “normal range.” When these so-called lab “abnormalities” show up, medical providers start a range of tests and suggest treatments that imply something is wrong, when in reality they are simply seeing a normal variation.
The precision medicine connection
High CK in Black Americans is a clear example of why one lab range doesn’t fit everyone. Here is how precision medicine applies.
1. Know your own normal
In the national study, the upper limit of normal (97.5th percentile) for CK was about 1,000 IU/L in Black men compared with 382 IU/L in white men, and 487 IU/L in Black women compared with 295 IU/L in white women. Most lab reports still use a single range. If your CK is flagged, ask whether it is high for someone of your sex and ancestry, and keep a record of your baseline level when you are well. That is not a race “correction” that hides disease. It keeps a healthy person from being labeled sick.
2. Statins and your genes
An elevated CK can lead doctors to stop a cholesterol-lowering statin, even though statins prevent heart attacks and strokes, which strike African Americans hard. The gene variant most strongly linked to statin muscle problems, SLCO1B1 c.521C, is found in about 17% of people of European ancestry but only 1% to 2% of African Americans. In one study of more than 11,000 patients, Black patients were less likely to have side effects from simvastatin. So a high CK with no symptoms should not be the only reason to stop a statin. Talk with your doctor about the dose or a different statin instead.
3. Sickle cell trait and extreme exercise
About 1 in 13 Black babies is born with sickle cell trait. In a study of nearly 48,000 Black U.S. Army soldiers, those with the trait had a 54% higher risk of exertional rhabdomyolysis, a dangerous breakdown of muscle during intense exercise, but no higher risk of death. If you have sickle cell trait, stay hydrated, build up training slowly, and stop and get help right away for severe muscle pain or weakness during a workout.
4. Know the safety limits
A higher normal range applies only to people who feel well. Get checked promptly if a high CK comes with muscle pain, weakness, dark or cola-colored urine, chest pain, or a recent new medicine. If you have no symptoms, doctors often repeat the test after 7 days without exercise before doing more. Further testing, including thyroid tests, is reasonable if your CK stays above about 1.5 times the upper limit for your sex and race.
Knowing these differences in the care of African Americans leads to better care and much less worry for patients and providers alike.
