Platelets are usually counted electronically by enumerating particles in the unlysed sample within a specified volume window, where volume may be measured by electrical impedance or light scatter.
Platelets counting difficulties
The platelet count was more difficult to automate than the red cell count because of the small size, tendency to aggregate, and potential overlap of platelets with more numerous smaller red cells and cellular debris.
Current instruments typically construct a platelet volume histogram based on platelet size within a reliably measured platelet volume window and mathematically extrapolate this histogram to account for platelets whose size overlaps with debris (smaller) or small red cells (larger). This works because platelets volumes in health or disease follow a log-normal distribution. Some analyzers compare platelets counts determined by different methods (e.g. impedance, light scatter or fluorescence) to improve accuracy, especially useful for low platelets counts. Based on analysis of volume distribution histograms of platelets and red cells and comparison of optical and impedance-based platelets counts, suspect samples are flagged for microscopic review. Automated platelets counting by current instrumentation is accurate and far more precise than manual methods. However; very low platelets counts in the range of 10 x 10 3 /µL still present some difficulties, with most modern analyzers overestimating by 10 to 30 percent in comparison to international standardized methods using monoclonal antibodies.
Falsely abnormal platelets count
Causes of falsely decreased platelets counts include incomplete anticoagulation of the sample (sometimes accompanied by small clots in the specimen or fibrin strands on the stained film), and platelet clumping or “satellitism” (adherence of platelets to neutrophils), caused by nonpathogenic antibodies recognizing platelet adhesion molecule epitopes exposed as a result of chelation of divalent cations in the anticoagulated sample. The latter condition occurs in approximately 0.1 percent of hospitalized patients. Classical causes of falsely elevated platelet count include severe microcytosis, cryoglobulins, and leukocyte fragmentation. Infrequently, it may be necessary to confirm automated results by a microscopic (phase contrast) platelet count or platelet estimate from the blood film, bearing in mind that these methods are imprecise.