The impact of patient ethnicity on cancer incidence following platelet count and C-reactive protein tests in English primary care: a cohort study of 5 million patients

BackgroundPlatelet count and C-reactive protein (CRP) are blood tests commonly used in primary care as part of diagnostic work up for symptomatic patients. Abnormal results of these tests can indicate an undetected cancer; however, it is not known whether the association between an abnormal test result and cancer risk varies by patient ethnicity. MethodsThis cohort study used routinely collected primary and secondary health care records in England with linkage to national cancer registry data. Included patients had a record of ethnicity, no prior malignancy, a platelet count or CRP record between 1st January 2010 and 31st December 2017, and were aged 40 years or over at the time of that test. Ethnicity was categorised as White, Asian, Black, Other, and Mixed. Multi-level logistic regression models estimated cancer incidence within one-year of testing, adjusted for age, sex, comorbidities, BMI, deprivation, and year of test. ResultsAmong 4,948,342 patients with a platelet record and 811,559 with a CRP record, one-year cancer incidence was highest among White patients and lowest among Asian patients. Following a normal platelet count, cancer incidence was 1.3% (95% CI 1.3-1.3%) for White patients and 0.63% (0.60-0.66%) for Asian patients; following thrombocytosis, incidence increased to 4.1% (4.0-4.2%) and 1.8% (1.5-2.0%), respectively. After a normal CRP result, cancer incidence was 1.5% (1.4-1.5%) for White patients and 0.79% (0.71-0.88%) for Asian patients, rising to 3.6% (3.5-3.7%) and 1.9% (1.7-2.2%) for a high CRP result, respectively. No significant interactions were found between ethnicity, blood test result, and overall cancer diagnosis, and similar diagnostic odds ratios (dOR) were observed across all ethnic groups. However, for colorectal cancer, Black patients with abnormal results showed higher diagnostic odds ratios (dOR) compared with White patients, relative to a normal result. The dOR for thrombocytosis was 11.1 (7.8-15.6) for Black patients versus 5.7 (5.4-6.0) for White patients (interaction p-value <0.001), and for raised CRP was 4.1 (2.6-6.6) for Black patients versus 2.5 (2.3-2.7) for White patients (interaction p-value=0.043). ConclusionThis large primary care study underscores the need for ethnically diverse cohorts when evaluating diagnostic tests to avoid widening healthcare inequalities.