How useful is thrombocytosis in predicting an underlying cancer in primary care? a systematic review

Although the association between raised platelet count (thrombocytosis) and cancer has been reported in primary and secondary care studies, UK GPs are unaware of it, and it is insufficiently evidenced for laboratories to identify and warn of it. This systematic review aimed to identify and collate evidence from studies that have investigated thrombocytosis as an early marker of cancer in primary care.

EMBASE (OvidSP), Medline (Ovid), Web of Science and The Cochrane Library were searched for relevant studies. Eligible studies had reported estimates of the association between thrombocytosis and cancer, in adults aged ≥40 in a primary care setting. Raw data from included studies were used to calculate positive predictive values and likelihood ratios (LRs) for cancer.

Nine case-control studies were identified. Study quality was judged to be high. Included studies reported on the following cancer sites: colorectal, lung, ovary, bladder, kidney, pancreas, oesophago-gastric, uterus and breast. LRs indicated that thrombocytosis was a predictor of cancer in all sites except breast. In a consulting population, thrombocytosis is most highly predictive of lung and colorectal cancer.

These results suggest that patients with thrombocytosis in primary care have an increased risk of cancer, and that some, but not all, cancers have raised platelets as an early marker. This finding is expected to be of use in primary care, for GPs receiving blood test results unexpectedly showing high platelet counts. Further research is needed to identify the cancers that are most strongly associated with thrombocytosis.