Overview

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

Sampling

Tests

Methodologies

Manual count (dilutional Neubauer) and QBC (autoread) systems 

• Automatic counts should still be checked via microscopic estimation from the smear. 
• On a Romanowsky (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 15 
  • 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.  
  • Measurement/counting is only size based, so small RBCs may be miscounted or large platelets and platelet clumps not counted. 
  • Laser-optical flow: 
    • Platelets are instead hemodynamically focused 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

Further Reading

Publications

Refereed papers

• Recent references from VetMedResource and PubMed.
• Couto C G (1999) Managing thrombocytopenia in dogs and cats. Vet Med 5, 460-465.
• Miyamoto T, Hachimura H, Amimoto A (1996) A case of megakaryoblastic leukemia in a dog. J Vet Med Sci 58, 177-179. 
• Hammer A S (1991) Thrombocytosis in dogs and cats: A retrospective study. Comp Haematol Intl 1 , 181-186. 
• Evans R J , Jones D R E , Gruffydd - Jones T J (1982) Essential thrombocythaemia in the dog and cat: a report of four cases. J Small Anim Pract 23, 457-467. 

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/  
• Duncan J R, Prasse K W & Mahaffey E A (1994) Veterinary Laboratory Medicine Clinical Pathology. 3rd edn. Iowa University Press, USA.
• Jain N C (1993) Essentials of Veterinary Hematology.  Lea & Febiger, USA.