Importance — Individuals with newly diagnosed cancer often have a high platelet count (thrombocytosis). Whether thrombocytosis is associated with the presence of an undiagnosed cancer remains unknown.
Objective — To assess whether a new diagnosis of thrombocytosis is associated with a subsequent risk of cancer among adults.
Design, Setting, and Participants — This population-based retrospective cohort study was conducted using linked laboratory data from Ontario, Canada, from January 1, 2007, to December 31, 2017, with follow-up until December 31, 2018. The study cohort included adults aged 40 to 75 years on the date of a routine complete blood count (CBC) test (index test) who had a normal platelet count in the 2 previous years and no history of cancer. Data analysis was performed in December 2020.
Exposures — Exposed individuals were those with a platelet count greater than 450 × 109/L. Matched unexposed control individuals had a platelet count within the reference range (150 × 109/L to 450 × 109/L) reported within 30 days of the exposure.
Main Outcomes and Measures — Incident cancers within 5 years after diagnosis of thrombocytosis. Absolute and relative risks for cancer associated with thrombocytosis were estimated for all cancers and for cancers at specific sites.
Results — Of the 3 386 716 Ontario residents with a recorded routine CBC test result, 53 339 (1.6%) had thrombocytosis and a prior normal platelet count. Among individuals with thrombocytosis, the median age was 59.7 years (interquartile range, 50.2-67.4 years) and 37 349 (70.0%) were women. Among the 51 624 individuals with thrombocytosis included in the matched analysis, 2844 (5.5%) had received a diagnosis of a solid cancer in the 2-year follow-up period and 3869 (7.5%) had received a diagnosis within 5 years. The relative risk (RR) for developing any solid cancer within 2 years was 2.67 (95% CI, 2.56-2.79). Associations were found between thrombocytosis and cancers of the ovary (RR, 7.11; 95% CI, 5.59-9.03), stomach (RR, 5.53; 95% CI, 4.12-7.41), colon (RR, 5.41; 95% CI, 4.80-6.10), lung (RR, 4.41; 95% CI, 4.02-4.85), kidney (RR, 3.64; 95% CI, 2.94-4.51), and esophagus (RR, 3.64; 95% CI, 2.46-5.40).
Conclusions and Relevance — In this cohort study, an increased platelet count was associated with an increased risk of cancer for at least 2 years. The results suggest that individuals with unexplained thrombocytosis should be offered screening for several cancers.
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