Underestimation of platelet count due to increase of mean platelet volume at the end of pregnancy

We report the case of a pregnant woman for whom the platelet count (77 x 10(9)/L) was underestimated by Coulter((R)) STKS analyzer during the third trimester because of large platelets. The microscopic counting of platelets revealed an isolated thrombocytopenia (120 x 10(9)/L). When not pregnant, the patient has low but normal platelet count (155 x 10(9)/L) with high mean platelet volume (MPV > 12 fL). This case report recalls that concomittantly to the decrease in platelet count, the MPV significantly increases at the end of pregnancy. This poorly known phenomenon does not impair platelet count by blood cell analyzers in as much as the platelet volume is in the range of measurement but may be responsible for underestimation of the platelet count if the MPV is already high before pregnancy. We describe how to detect this anomaly and propose simple guidelines for thrombocytopenia in normal pregnancy.