#00064878

A 37 y.o. male was admitted for profound thrombocytopenia (platelets < 7000/mcL). On hospital day (HD) 3 his ADAMST13 activity was < 3%, thrombotic thrombocytopenic purpura (TTP) was diagnosed and plasmapheresis instituted. His platelet count stabilized at > 150,000/mcL by HD 20 and he was discharged. 

 Panel A: Normal RBC histogram.  The solid vertical lines are the points at which 4% of the RBC population are outside each side of the central 92%.  The dotted lines are set at 60 fL (left) or 120 fL (right) to quantify the % microcytic (MCV < 60 fL) or macrocytic (> 120 fL) RBCs.  Arrows depict the derivations of Mean corpuscular volume (MCV) and RBC distribution width (RDW).

 Panel B:  Evolution of this patient’s RBC histogram over time. The first RBC histogram (HD#3) displays increased microcytosis - these are schistocytes.  A macrocytic population emerges on HD#5 and becomes prominent by HD#10 - these are reticulocytes.  

 The % reticulocytes ranged from 3.7% (HD#3) and peaked at 15% (HD #8).  The  RDWs and MCVs increased correspondingly (14.9-30% and 83.3 – 97.7 fL respectively).

The RBC histogram may be a useful tool for identifying and monitoring the course of TTP.