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Hematology: platelet count

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Synonym(s): Thrombocyte

Overview

  • Thrombocytes of reptiles are small (generally smaller than erythrocytes), elliptical to fusiform cells with a centrally located oval nucleus containing dense nuclear chromatin that stains purple.
  • Some nuclei may contain a pale line that extends across the width of the nucleus.
  • The cytoplasm is typically colorless to pale blue in color and may contain a few azurophilic granules.
  • Activated thrombocytes are common and appear as aggregated clusters of cells with decreased cytoplasmic volume, irregular cytoplasmic margins, and vacuoles.
  • Indistinct cytoplasmic margins may be noted on some individual thrombocytes and are probably a product of age, artifact, or function.
  • Reptilian thrombocytes function similarly to avian thrombocytes and mammalian platelets and play a significant role in thrombus formation.
  • Activated reptilian thrombocytes exhibit ultrastructural features such as pseudopodia with fine granular material and many fibrin-like filaments that radiate between and around the cells.

Uses

Alone

  • Assessment of platelet numbers can give an indication of disease and coagulation problems.

In combination

  • With prothrombin time (PT), activated partial thromboplastin time (APTT), and fibrin degradation product (FDP) in diffuse intravascular coagulation (DIC).

Other points

  • Kidney involved in manufacture of thrombopoietin/thrombocytosis stimulating factor (TSF).
  • Platelets live longer than WBCs, therefore in bone marrow suppression leukopenia is usually present before thrombocytopenia.
  • Rarely, thrombocytosis can be seen in the following non-pathological conditions: excitement/exercise (splenic contraction), following splenectomy (loss of storage pool) and during late pregnancy.

Sampling

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Tests

Methodologies

Manual count

  • With commercial blood dilution system + standard Neubauer chamber gives accurate platelet count.
  • Most automatic counts are also checked manually on a smear.
  • On Romanowsky or supravitally stained smear:
    • Count platelets and WBCs over same oil immersion field - normal is around 1-5 platelets per oil immersion field (x1000).
    • 6-7 platelets per field = ~100 x 109/L (=~100x103/µL).
    • Check edges of smear for platelet clumping which leads to artificially low platelet counts.
  • Typically 25-350 platelets seen per 100 leukocytes on a reptile blood smear.
  • Estimates the count only, with considerable errors, but can be clinically useful to note if reduced, normal or increased numbers.

Automated blood counter

  • Automated cell counters tend to be ineffective in reptiles due to nucleated RBCs.

Technique intrinsic limitations

  • Platelets may be confused with small- to medium-sized lymphocytes.

Technician extrinsic limitations

  • Clumping can occur when blood collection has not been smooth, due to damage to vascular walls and initiation of coagulation cascade. This can also result in some thrombocytosis (by increased production of thrombocytes) or thrombocytopenia (by inability to count clumped platelets).
  • Clumping also occurs when the process of blood collection takes a longer time, particularly from small veins. In these situations, coagulation starts inside the syringe unless we pre-heparinize the syringe prior to blood collection.This problem is less likely to occur when the blood sample is collected from a larger vein.

Result Data

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Hernández Marrero J D, Castro P, Saavedra-Santana P, Ramirez P & Orós J (2017) Seasonal variations in haematological parameters in yellow-bellied slider turtles (Trachemys scripta scripta). Vet Medicina 62 (7), 394-400 VetMedResource.

Other sources of information

  • Campbell T (2015) Peripheral Blood of Reptiles. In: Exotic Animal Hematology and Cytology. 4th edn. Wiley-Blackwell, USA. pp 85.