Cancer incidence in patients with a high normal platelet count: a cohort study using primary care data

A platelet count >400 × 109/l (i.e. thrombocytosis) is a recently discovered risk marker of cancer. The risk of undiagnosed cancer in patients with thrombocytosis is 11.6% for men and 6.2% for women, well above the 3% risk threshold set by National Institute for Health and Care Excellence (NICE) for cancer investigation. Patients with a platelet count at the upper end of the normal range (325-400 × 109/l) could be at increased risk of undiagnosed malignancy.

To quantify the risk of an undiagnosed cancer in patients with a platelet count at the upper end of the normal range.

A primary care-based cohort study using Clinical Practice Research Datalink (CPRD) data from 2000 to 2013. The study sample comprised 2704 individuals stratified by platelet count: 325-349 × 109/l; 350-374 × 109/l; 375-399 × 109/l. Incident cancer diagnoses in the year following that platelet count were obtained from patient records.

Cancer incidence rose with increasing platelet count: 2.6% [95% confidence interval (CI) 1.9 to 3.6] in subjects with a count of 325-349 × 109/l, 3.7% (95% CI 2.5 to 5.3) in subjects with a count of 350-374 × 109/l and 5.1% (95% CI 3.4 to 7.5) in those with a count of 375-399 × 109/l. Colorectal cancer was most commonly diagnosed in all three groups. Cancer incidence was consistently higher in males than in females.

These results suggest that clinicians should consider cancer in patients with a platelet count >375 × 109/l, review reasons for testing and any additional reported symptoms. Until these results are replicated on a larger scale, recommendations for clinical action cannot be made.