Cancer incidence following a high-normal platelet count: cohort study using electronic healthcare records from English primary care

Background A raised platelet count (thrombocytosis) measuring >400 × 109/l is associated with high cancer incidence. It is uncertain whether platelet counts at the upper end of the normal range (high-normal: 326–400 × 109/l) are also associated with cancer. Aim To investigate cancer incidence following a normal platelet count in primary care. Design and setting A prospective cohort study was undertaken using data from the Clinical Practice Research Datalink and National Cancer Registration and Analysis Service, dating from 1 May 2005 to 30 April 2014. Method One-year cancer incidence was estimated for 295 312 patients with normal platelet counts (150–400 × 109/l). Patients with platelet counts >325 × 109/l were oversampled to maximise precision of estimates of cancer incidence. All patients were aged ≥40 years with no prior cancer diagnoses. The effects of age, sex, and smoking were explored. Non-melanoma skin cancers were omitted from exclusions and incidence. Results One-year cancer incidence increased greatly with age, male sex, and higher platelet count. Males aged ≥60 years with a high-normal count had an incidence of 4.2% (95% confidence interval [CI] = 4.0 to 4.4). The highest incidence of 6.7% (95% CI = 5.3 to 8.4) was found in males aged ≥80 years, who had platelets in the range of 376–400 × 109/l; this was 3.1 percentage points higher than the incidence for patients in the same age group with lower-normal counts of 150–325 × 109/l. Risks for all female subgroups were <3%. Patients with high-normal platelet counts were most at risk of lung and colorectal cancers and, in general, had advanced-stage cancer at diagnosis. Conclusion Platelet counts at the high-normal range in males aged ≥60 years may be indicative of an underlying malignancy, and referral for further investigation should be considered.