Hematology: platelet count (Thrombocyte)

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• Assessment of platelet numbers.

• In immune-mediated thrombocytopenia.
• With activated partial thromboplastin time (APTT) + fibrin degradation product (FDP) in diffuse intravascular coagulation.
   Platelet count can be normal, but platelet function can be abnormal. Can test 'in house' with buccal mucosal bleeding time .

Other Points

• Kidney involved in manufacture of thrombopoietin/thrombocytosis-stimulating factor (TSF).
• Platelets live longer than WBCs, therefore in bone marrow suppression leucopenia is usually seen 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 .

• Standard venepuncture .

• Collect into anti-coagulant (EDTA) or sodium citrate.
• Citrate better if sample to be posted.

Either Test within 1/2 hour (maximum 2 hours), to avoid agglutination of platelets in stored blood.
Or Prepare blood smear at time of sampling for manual counting method.

• Blood smear preferably.
• A reasonable estimate of platelet numbers can usually be made on an EDTA sample sent by post arriving within 24 hours of sampling.

• Quick but very unreliable.

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 Romansowsky or supravitally stained smear
• Count platelets and WBCs over same oil immersion field
• 6-7 platelets per field = ~ 100 x 10*9/l.
• Normal cat: 10-29 platelets/oil immersion field.
• Check edges of smear for platelet clumping which artificially low platelet counts.
   Estimates only, considerable errors, but clinically useful.

Automated blood counter

• Rapid but if platelet clumping occurs low readings.
• Size of cat platelet.

• 150-400 x 10*9/l.
• <100 x 10*9/l significant.
• Spontaneous hemorrhage is likely to occur with platelet counts <30-50 x 10*9/l.

• Associated with iron-deficiency anemia.
• Drug-induced, eg vincristine , glucocorticoids , adrenaline .
• Chronic infection/inflammation, eg bronchitis (rare) .
• Persistent or chronic hemorrhage.
• Myeloproliferative disorders, eg primary polycythemia and basophilic leukemia .

Decreased [platelet] (thrombocytopenia)

• Immune-mediated thrombocytopenia (IMTP) .
• Aplastic anemia - many possible causes including estrogens (thrombocytosis initially), phenylbutazone , thiazide diuretics , toxins and chemotherapeutic drugs .
• Myeloproliferative and lymphoproliferative disorders .
• Myelophthisic disease.
• 'Rebound' thrombocytopenia (following blood transfusion).
• Panlucopenia infection, eg leptospirosis, salmonellosis , histoplasmosis , bacteremia and systemic mycosis.
• Disseminated intravascular coagulation (DIC).
• Splenomegaly/hypersplenism.
• Anaphylaxis .
• Other immune-mediated disorders, eg systemic lupus erythematosus (SLE) .
• Massive hemorrhage.

• Platelet aggregation (clumping) occurs rapidly and produces false low values.
• Clotting.

• Platelet clumping
  • Exposure to EDTA.
  • Prolonged storage

• Recent references from 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 PJ & Mackin AJ (2001) Evaluation of methods of platelet counting in the cat. JSAP 42, 326-332.

• K L Gerber BVSc DACVP MRCVS & A G Torrance MA VetMB PhD DipACVIM MRCVS, Axiom Veterinary Laboratories, 5 George Street, Teignmouth, Devon TQ14 8AH, UK. Tel: 01626 778844.

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