Assessing platelet number and function when diagnosing acquired thrombocytopenia

How is acquired thrombocytopenia diagnosed?

For proper diagnosis, a thorough history and physical examination are needed, in addition to testing with laboratory assays to rule in or rule out the various causes. These assays include but are not limited to those for platelet number and function, a peripheral blood smear, and immature platelet fraction.

What methods are available for measuring platelet number?

Platelets can be counted manually using a microscope with a hemocytometer or in automated hematology analyzers using several methods such as impedance platelet counting, optical light scatter counting, and platelet-specific labels with flow cytometry counting.

The International Council for Standardization in Haematology and International Society for Laboratory Hematology recommend the red blood cell/ratio method for platelet number. With this method, platelets are labeled by fluorescein isothiocyanate-conjugated monoclonal antibodies against two epitopes (CD 41 and CD 61). This is followed by flow cytometry analysis and calculation of the platelet count from the red blood cell/fluorescent platelet ratio.

Most automated systems also measure the mean platelet volume, which is a measure of the average size of platelets. This may be higher in immune thrombocytopenia (ITP) because of the giant (more immature) platelets that may be present in peripheral blood. Other parameters that can be measured as part of the complete blood count include the platelet distribution width, which is a measure of the platelet anisocytosis, and the “plateletcrit,” which reflects the volume of circulating platelets in a unit volume of blood.

What methods are available for assessing platelet function?

The most common platelet function tests available for routine use in clinical labs include:

• Platelet adhesion assays, which measure the closure time for platelet plug formation when exposed to an in vitro mimic of endothelial damage due to high shear blood flow.
• Aggregometry-based tests, which assess the ability of platelets to aggregate in the presence of agonists, as well as the response of platelets to antiplatelet therapy.
• Flow cytometry, which involves laser-based detection of labeled platelets.

Visco-elastic assays, which are global hemostasis tests that assess the rate of clot formation based on agonist effect in the presence of low shear induction.

What should you look for in a peripheral blood smear when diagnosing acquired thrombocytopenia?

Platelet clumping, platelet satellitism, and platelet phagocytosis are just a few of the many abnormalities associated with acquired thrombocytopenia in general.

As for abnormalities associated with particular types of acquired thrombocytopenia, giant platelets can be seen in ITP. Red blood cell schistocytes are seen in microangiopathic hemolytic anemia (e.g., thrombotic thrombocytopenic purpura). Hypersegmented or hyposegmented neutrophils are seen in primary hematologic disorders such as myelodysplasia and myeloproliferative disorders.

How can immature platelet fraction (IPF) be used when diagnosing acquired thrombocytopenia?

Also called reticulated platelets, immature platelets are those that the bone marrow has recently released. They have a high RNA content and are usually larger in size compared to normal platelets. The percent IPF indicates the ratio of immature platelets to the total number of platelets in peripheral blood. It reflects the rate of thrombopoiesis and bone marrow platelet production, and is useful in differentiating between peripheral platelet destruction (associated with ITP and heparin induced thrombocytopenia) and decreased bone marrow production (associated with myelodysplastic syndrome and acute myeloid leukemia).

Olajumoke Oladipo, MD, DABCC, FADLM, is an associate professor of pathology at Pennsylvania State University and medical director of hematology and coagulation at Penn State Hershey Medical Center in Hershey, Pennsylvania. +Email: [email protected]

Read the full November/December 2024 CLN issue here.