Platelet count (PLT) in cats and dogs

What it shows

Platelets are made in the bone marrow by large cells called megakaryocytes. At a site of bleeding they form the first plug and help coagulation proteins build a clot. The analyzer counts platelets by their size, separately from red and white cells.

Platelets are easily activated during blood collection and stick together. eClinPath is direct: platelet clumps lower the count obtained by any method, so an automated result is best read as a minimum. The lab checks the count on a stained blood smear — each platelet seen per oil-immersion field corresponds to roughly 15 × 10⁹/L — and looks at the feathered edge of the smear for clumps.

Higher than reference — what it may be a sign of

A high count (thrombocytosis) is usually reactive, and eClinPath notes that reactive increases are usually not of direct pathologic importance:

• Inflammation — inflammatory signals such as interleukin-6 and the hormone thrombopoietin stimulate production.
• Chronic blood loss with iron deficiency.
• Excess corticosteroids — Cushing’s disease (hyperadrenocorticism) or corticosteroid medicines.
• Primary bone marrow disease — rare.
• Artefact — on some analyzers fragments of burst red cells are counted as platelets.

Lower than reference

First, the vet rules out a false low (pseudothrombocytopenia): clumps on the smear, a difficult blood draw, a sample that waited too long, or very large platelets that the analyzer did not recognise. Once the low count is confirmed, the causes are:

• Destruction — immune-mediated thrombocytopenia; counts are usually below 30 × 10⁹/L and often below 10 × 10⁹/L at diagnosis.
• Consumption — heavy bleeding, clotting disorders (DIC), some tumours.
• Infections — tick-borne Ehrlichia and Anaplasma in dogs; FeLV, FIP, panleukopenia, toxoplasmosis in cats.
• Reduced production in the marrow — toxins, some medicines, marrow disease.
• Sequestration in the spleen, for example during sedation.

Spontaneous bleeding usually does not start until the count is below about 30 × 10⁹/L — that is one of the things the vet weighs.

Look at together with

• Blood smear — clumps, platelet size, an estimate of the real count.
• MPV — average platelet size, read together with the smear when platelets look large.
• White blood cells — on some analyzers platelet clumps are counted as white cells, so a low PLT next to a high WBC in a cat hints at clumping.
• Hematocrit and hemoglobin — signs of bleeding.

Units and conversion

Platelets are reported as 10⁹/L (the SI unit; ×10⁹/л on Russian-language forms) or as thousands per microlitre — 10³/µL, тыс/мкл. The factor is 1: 250 × 10⁹/L equals 250 × 10³/µL, which some texts write out as 250,000/µL. In the app each row keeps the unit printed on the form, and the chart converts everything to one unit.

Reference range

The reference range is on your lab’s form. It depends on species and analyzer. A platelet count is especially sensitive to the sample: clumping increases the longer blood waits, and counts fall with storage. A single low result, particularly in a cat, is first checked on a smear or on a fresh sample; a trend over several tests says more.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Platelet count
2. eClinPath (Cornell University) — Platelet number (hemostasis)
3. McQuinn E. Platelet Disorders in Animals. Merck Veterinary Manual
4. McQuinn E. Platelets in Animals. Merck Veterinary Manual
5. Whitbread TJ. Clinical Hematology. Merck Veterinary Manual