Cycle Effects Are Not Universal: A Case Study of Urinary C‐Reactive Protein Concentrations in Rural Polish and Polish American Samples

ABSTRACT Objectives We need to better understand how the menstrual cycle interacts with other biological systems, such the inflammation and immune response. One way to study this interaction is through C‐reactive protein (CRP). Studies of CRP concentrations across the menstrual cycle have been inconsistent. This study explores menstrual cycle CRP variation in two geographically different samples of Polish and Polish American individuals. Methods Analyses were conducted on 76 Polish and 22 Polish American daily urine samples collected on the first day of menstruation until the start of their next period. CRP, estrone‐3‐glucuronide, and pregnanediol‐3‐glucuronide were assayed. Sample‐specific linear mixed models were used to examine cycle effects and median CRP concentrations across cycle phases and between the start of menses and remainder of the cycle were compared using Kruskal‐Wallace and Dunn tests. Results Polish and Polish American samples had distinct menstrual cycle CRP phenotypes. The Polish sample did not show cycle effects. The Polish American LMM demonstrated that CRP decreases after the first 3 days of menses (estimate −0.17, t‐value −5.2). The KW and Dunn tests supported this. CRP concentrations were higher during the early follicular (median 0.406, p