Thrombocytopenia is a decrease in the number of platelets in the peripheral blood. Most laboratories use a reference range for platelets that is approximately 150 - 450 x 109/L. Thrombocytopenia occurs when the platelet count falls below the lower limit of this range.
The image shows how a normal platelet count would appear on a peripheral blood smear (Wright-Giemsa stain; 1000x original magnification). Representative platelets are indicated by arrows.