What Are the Normal Serum Creatine Kinase Values for Skeletal Muscle? A Worldwide Systematic Review

Background: Serum creatine kinase (CK) has been used as a diagnostic marker for neuromuscular disorders since 1959. As manufacturer-provided normative data indicate, CK levels can be elevated in normal individuals. Recent evidence suggests these data often underestimate true CK values, which are influenced by age, race, gender, and other physiological factors. Therefore, establishing a reliable normal range for CK is crucial before further investigation, particularly in oligo/asymptomatic patients. Objective: This systematic review aims to determine the normal CK levels across various populations. Methods: We systematically reviewed observational studies with a cross-sectional, descriptive design. We included studies focusing on healthy adults (> 18 years) of any race who aimed to establish normal CK values, including at least 100 participants. We utilized the following databases: PubMed, Embase, and Cochrane. Results: CK values typically exhibit a non-Gaussian distribution. Black individuals demonstrate significantly higher CK values compared to Caucasians and Asians. The upper reference limits for CK in Caucasian and Asian males range from 227–440 U/L, while for Black males, it is between 520–810 U/L. The upper reference limits for females range from 135–248 U/L and up to 354 U/L, respectively. Conclusions: Normal CK values might be higher than previously suggested, especially among specific racial groups. Each laboratory should ideally determine its reference values for CK that reflect its local population.