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

Background: 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.

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

Methods: 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.

Results: 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.

Conclusion: 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.