Objective: The recommended reference range for serum C-reactive protein (CRP) concentrations is usually not adjusted for age and sex. We sought to determine if age, sex, and race or ethnicity influence the distribution of CRP values, and if upper reference limits of CRP should be adjusted by demographic factors.
Methods: Interviews, physical examinations, and blood draws were performed on > 22,000 individuals age > or = 4 yrs representative of the noninstitutionalized population of the United States, as part of the Third National Health and Nutrition Evaluation Survey (NHANES III). Serum CRP concentrations were measured by nephelometric immunoassay.
Results: The 95th percentile value of CRP in the overall population was 0.95 mg/dl for males and 1.39 mg/dl for females, and varied with age and race. For ages 25-70 yrs, the age adjusted approximate upper reference limit (mg/dl) was CRP = age/50 for males, and CRP = age/50 + 0.6 for females. The upper limits for Mexican-Americans and non-Hispanic whites were similar, whereas for non-Hispanic black adults the approximate upper limit was CRP = age/30 for males and CRP = age/50 + 1.0 for females. Even after accounting for identified inflammatory conditions, demographic factors influenced the reference limits of CRP. The 95th percentile values were uniformly lower in children than in older adults.
Conclusion: Demographic factors, including age, sex, and race, should be used to adjust the upper reference limit for CRP. Clinicians should be aware of these factors when using CRP values to assess inflammatory diseases.