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Hematology: platelet count
Synonym(s): Thrombocyte (most often used in non-mammalian species)
Overview
- Assessing platelet (PLT) numbers, automated but often with manual smear checking.
Uses
Alone
- Assessment of platelet numbers and morphology.
- In diagnosis of immune-mediated thrombocytopenia (IMT/IMTP) Immune-mediated thrombocytopenia for primary coagulopathic presentations (eg petechiation, ecchymosis).
In combination
- As part of routine hematological screening for systemic and bone marrow diseases.
- Alongside cruder platelet function testing (eg BMBT Buccal mucosal bleeding time) or more dynamic analysis (eg thromboelastography with platelet aggregometry Thromboelastography (TEG)). These are primary coagulopathic investigations.
- With activated partial thromboplastin time (APTT) Hematology: activated partial thromboplastin time and prothrombin time (PT) Hematology: prothrombin time, with fibrin degradation products (FDP) Fibrin degradation products in diagnosis of disseminated intravascular coagulation (DIC) Disseminated intravascular coagulation. These are secondary (and tertiary) coagulopathic investigations.
Other Points
- Other measured parameters can include Mean Platelet Volume (MPV) and resulting calculated values such as Platelet Distribution Width (PDW) and Plateletcrit (PCT).
- These may aid in differentiating thrombocytopenic causes but suffer from lack of robust reference data, due in part to in vitro factors (such as anti-coagulant effects, storage time) as well as different ways of measuring them.
- Platelets are anucleate cytoplasmic fragments derived from large multi-nucleated megakaryocytes (MKC). They function in hemorrhagic control-vascular repair primarily, but also inflammation and wound healing.
- Thrombopoiesis (platelet production) and megakaryopoiesis (MKC proliferation and maturation) are principally controlled by the hormone thrombopoietin (TPO) in a negative feedback system. TPO is made in the liver (endothelial cells), as well as the kidney (tubular epithelium) and bone marrow (stroma).
- As with other cell lines, circulating platelet numbers result from the relative rates of production, sequestration/storage, demand/consumption and destruction. Their life spans are species dependent but in the order of 5-10 days (shorter than red cells, longer though than WBCs (neutrophils). Therefore, in bone marrow suppression, leukopenia-neutropenia is usually seen before thrombocytopenia, anemia occurring last of the three cell lines measured.
Sampling
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Tests
Methodologies
Manual count (dilutional Neubauer) and QBC (autoread) systems
Generally replaced by impedance and laser flow-based analyzer measurements.- Automatic counts should always be checked via microscopic estimation from the smear, especially in cats as their RBCs and platelets overlap in size and their platelets are more prone to significant clumping.
- On a Romanowsky Staining techniques: Romanowsky-type stains (or supravitally stained) smear in the monolayer:
- Check edges of the smear for platelet clumping as this will artificially lower the platelet count. If present, the count or smear estimation should be regarded as a minimum
. - In the monolayer, platelets are counted over 10 oil immersion fields (100x lens) = n.
- Platelet estimate (x109 /L) = (n/10) x 20
- This is an estimate only, so has considerably inherent variation yet remains clinically useful.
- Check edges of the smear for platelet clumping as this will artificially lower the platelet count. If present, the count or smear estimation should be regarded as a minimum
Automated platelet counting
- Impedance:
- Similar to erythrocytes/RBCs, platelets are counted via the number and size of their associated resistance/impedance pulses within a defined volume interval (often 2-30 fL) to produce a histogram plot.
- As measurement/counting is only size based, discrimination of feline PLT from RBCs is often poorer because they each have a larger variation in size, resulting in overlap and miscounting
. - Feline platelets are more prone to clumping in vitro when anti-coagulated with EDTA too.
- Laser-optical flow:
- Platelets are instead hemodynamically focussed for laser beam counting and dot plot analysis via their side scatter (complexity) and forward (size) characteristics. This discrimination can be enhanced fluorescently via use of specific dyes (eg thiazole orange).
- Clumping can be detected through an altered scatter plot, but smear checking remains vital.
Availability
- Automated counting widely available both in practice and at commercial laboratories, but their methods and accuracies will vary.
- Manual smear estimates more widely available.
Validity
Sensitivity
- Variable depending on sampling, anti-coagulant, age of specimen, platelet clumping and analyzer method.
Specificity
- Variable depending on sampling, type of analyzer measurement, staining and interferents (eg lipemia).
Result Data
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Further Reading
Publications
Refereed papers
- Recent references from VetMedResource and PubMed.
- Rizzo F, Tappin S W & Tasker S (2007) Thrombocytosis in cats: a retrospective study of 51 cases (2000-2005). J Fel Med Surg 9, 319-325.
- Tasker S, Cripps P J & Mackin A J (2001) Evaluation of methods of platelet counting in the cat. JSAP 42, 326-332.
- Zelmanovic D & Hetherington E J (1998) Automated Analysis of Feline Platelets in Whole Blood, Including Platelet Count, Mean Platelet Volume, and Activation State. VCP 27,1 2-9 Wiley.
- Shelton G H, Linenberger M L, Grant C K, Abkowitz J L S (1990) Hematologic manifestations of feline immunodeficiency virus infection. Blood 76 (6),1104-1109 PubMed.
Other sources of information
- Weiss D J, Wardrop K J (2010) Schalm’s Veterinary Hematology. 6th edn. Wiley Blackwell.
- http://eclinpath.com/hemostasis/disorders/platelet-numbers/