Felis ISSN 2398-2950

Hematology: platelet count

Synonym(s): Thrombocyte (most often used in non-mammalian species)

Contributor(s): Karen L Gerber, Andy Torrance, Roger Powell

Overview

  • Assessing platelet (PLT) numbers, automated but often with manual smear checking. 

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. 

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 VetMed Resource 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 PubMed
  • Tasker S, Cripps P J & Mackin A J (2001) Evaluation of methods of platelet counting in the cat.JSAP42, 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 doi.org
  • 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 


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