Low Platelet Counts: Diagnosis Using Flow Cytometry and Anti Platelet Antibody.

In our service of hematology 9,1 % of all consults are related to low platelet number finding in the CBC. Pseudothormbocytopenia is a rather unusual phenomenon, induced by agglutination of platelets by auto-antibodies acting on EDTA collected blood.

Objective: Classify, diagnose and analyze low platelet counts as obtained by automatic blood counters.

Methods: From January 1997 to May 2005; 16761 patients were attended in our service. Among them, 1174 cases (7,0%), 429 males and 745 females (1 to 75 years), were there because of low platelet counts alone. Platelet counts were done by Coulter T-890, with blood collected in EDTA K3 and sodium citrate. Counts were repeated after letting blood at 4^o C for 24 hours; also Fonio technique and Neubauer camera counts were done. Normal value was defined as platelet count between 150,000 to 450,000. Bone marrow was obtained when necessary and stained by conventional techniques. Serological and biochemical tests were done in a Cobas Core (Roche) machine. Immuno-phenotyping was done in 115 cases, to identify anti-platelet-antibody (CD41 PE Immunotech and anti human IgG FTIC conjugate Sigma)-direct method, by flow citometry (Coulter Epics XL-MCL).

Results: Among the 1158 cases, platelets counts between 120 000 to 150 000, were found in 15.8 %; laboratory error in 6,3 % and spurious low platelet counts in 1.3 %. In addition, the follow up for four years of those patients with “lower normal” platelet count (120 000 to 150 000) did not show any morbid process. Immunologycal idiopatic purpura (ITP) was diagnosed in 49,1 % of the cases, being as expected, more frequent in females, children and young adults. Hepatitis C as cause of low platelet counts was responsible for 6,9 % and HIV infection for 1.3 % of the cases Anti-platelet antibodies were present in 76.9 % of ITP cases and was not found in 88.3 % of normal platelet values.

Conclusions: In an hematological reference service should be considered a lower range for normal platelet values (120 000 to 450 000). Additionally, low platelet counts as diagnosed by automated machines should be confirmed by slide exam and anti platelet antibody is useful for asserting ITP diagnosis and to rule out spurious thrombocytopenia or normal values.