Trends in C reactive protein testing: a retrospective cohort study in paediatric ambulatory care settings.

Objectives: This study aims to investigate C reactive protein (CRP) testing practices in paediatric ambulatory care across British primary care and accident and emergency (A&E) departments.

Design, Setting, Participants: This retrospective cohort study included children <18 years old having ≥1 CRP test at primary care or A&E departments in Oxfordshire between 2007 and 2021.

Outcomes: We estimated the frequency and annual changes in CRP testing in both settings and evaluated referral and admission patterns based on CRP levels: low (<20 mg/L), intermediate or high (≥80 mg/L).

Results: Over 15 years, 91 540 CRP tests were requested in 63 226 children, with 33 882 (53.6%) in primary care and 29 344 (46.4%) in A&E. Both settings showed rising trends in test requests, with average annual percentage change of 3.0% (95% CI 1.2% to 4.7%) in primary care and 11.5% (95% CI 8.6% to 14.6%) in A&E. The proportion of intermediate/high-test results remained stable. In primary care, 50 709 (95.8%) of CRP tests were <20 mg/L, with 99.0% of these children managed at home. High and intermediate CRP values increased odds of referral versus low CRP (OR adjusted for age=21.80; 95% CI 16.49 to 28.81 and 4.77; 3.78 to 6.02, respectively). At A&E, 27 610 (71.5%) children had CRP<20 mg/L, of whom 42.5% were admitted while 3776 (9.8%) had CRP≥80 mg/L with 57.9% admission rate. High and intermediate CRP values increased odds of admission versus low CRP (OR adjusted for age=1.90; 95% CI 1.78 to 2.04 and 1.39; 1.32 to 1.46, respectively).

Conclusion: There are rising trends of CRP test requests in paediatric ambulatory care settings, with no evidence of increases in proportion of intermediate/high-test results in primary care. Low CRP values at primary care were linked to children managed at home, while almost half of children with low CRP values at A&E were admitted to the hospital.