Extreme Reactive Thrombocytosis Caused by Obstructive Nephrolithiasis and Pyelonephritis.

Platelet counts in reactive thrombocytosis rarely exceed 1000 × 10 9 /L. We present the case of a male patient, aged 80 years, with quiescent rheumatoid arthritis who was found to have a platelet count of 1011 × 10 9 /L on routine laboratory testing. The patient was initially asymptomatic but developed leukocytosis to 23.1 × 10 9 /L on hospital day 2. Diagnostic work-up revealed obstructive nephrolithiasis and pyelonephritis, and the thrombocytosis and leukocytosis gradually resolved with empiric antibiotic treatment and ureteral stent placement. Tests for myeloproliferative disorders, including JAK-2V617F mutation, BCR-ABL for chronic myeloid leukemia and acute lymphocytic leukemia, and myeloproliferative neoplasms (MPL/CALR), were negative. Physicians should be aware that in rare cases reactive thrombocytosis can exceed 1000 × 10 9 /L, and that markedly elevated platelet counts in the setting of urinary tract infections may be an early sign of obstructive uropathy.