Thrombocytosis on the Ward - What to Investigate

Definition

• Thrombocytosis = platelets >450 x10⁹/L
• Secondary (reactive) thrombocytosis accounts for >85% of cases, even with very high counts
• Primary causes (myeloproliferative neoplasms) are much less common

Step 1: Clinical Assessment

• Recent surgery, trauma, or acute blood loss
• Known inflammatory disease (RA, IBD, vasculitis, connective tissue disorders)
• Recent or active infection
• Prior splenectomy or functional hyposplenism (sickle cell, coeliac)
• Chronic blood loss or iron deficiency symptoms
• Recent chemotherapy (rebound thrombocytosis)
• History of or symptoms suggestive of malignancy
• Thrombotic events (DVT, PE, TIA, stroke) or bleeding symptoms

• Splenomegaly or absence of spleen
• Signs of infection, active inflammation, or malignancy
• Signs of thrombosis or microvascular symptoms (erythromelalgia - burning/redness in extremities)

Step 2: First-Line Investigations (do on the ward)

Note on U&E: Spurious hyperkalaemia is common with high platelet counts - potassium leaks from platelets during sample storage. This is a lab artefact, not a true result. Confirm with a lithium heparin (plasma) sample if K+ is raised.

Step 3: Second-Line / Directed Investigations

• Blood cultures (if febrile)
• Urine MC&S
• Chest X-ray
• Autoantibody screen, rheumatoid factor (if connective tissue disease suspected)

• CT chest/abdomen/pelvis
• PSA (men), CA-125 (women if indicated)
• Stool for occult blood / OGD / colonoscopy if GI malignancy suspected
• The North & East Devon NHS guidance identifies high-risk cancer groups: women aged 60-69 with platelets ≥550, women ≥70 with ≥500, men 60-69 with ≥450, men ≥70 with ≥400 - these warrant 2-week wait cancer referral consideration.

• JAK2 V617F mutation - first-line MPN driver mutation test (order via ICE, document "thrombocytosis" in clinical details)
• If JAK2 negative: CALR and MPL mutations will typically be checked reflexively by haematology labs
• BCR::ABL (Philadelphia chromosome) - if WBC also raised, to rule out CML

Step 4: Criteria for Haematology Referral / Escalation

• No reactive cause identified after initial workup
• JAK2/CALR/MPL positive
• Platelets persistently >1000 x10⁹/L (very high risk of bleeding via acquired von Willebrand disease)
• Associated thrombosis or bleeding
• Other cell lines also raised (polycythaemia, leukocytosis) suggesting MPN
• Rising platelet count despite treating the suspected cause

• Active arterial or venous thrombosis
• Bleeding (platelets >1500 x10⁹/L carry significant haemorrhagic risk)
• Features of blast transformation

Key Points to Remember

1. Reactive causes first - infection, inflammation, iron deficiency, post-surgical, malignancy account for the vast majority inpatient.
2. CRP is your friend - elevated CRP strongly supports a reactive cause; a normal CRP with high platelets raises suspicion for primary MPN.
3. Blood film is underused - can quickly suggest iron deficiency, leukoerythroblastic change, or giant platelets pointing to MPN.
4. Don't panic over very high counts - even platelets >1000 x10⁹/L are often reactive (post-splenectomy, severe infection). Thrombotic risk in reactive thrombocytosis is low.
5. Spurious hyperkalaemia - always check if K+ is high with high platelets; use a plasma (heparin) sample to confirm.

• Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 908
• Harrison's Principles of Internal Medicine 22E (2025), p. 969
• NHS NW Thrombocytosis Advice Leaflet (NNUH, reviewed 2027)
• NHS Scotland Thrombocytosis Guidelines (reviewed June 2026)
• BSH Guidelines - Thrombocytosis without JAK2/CALR/MPL mutations

Iron Deficiency and Thrombocytosis

Mechanism

• Thrombopoietin (TPO) cross-stimulation - iron deficiency causes erythroid progenitor cells to fail to differentiate normally. These stalled erythroid precursors share signalling pathways with megakaryocytes. Excess TPO (produced in response to anaemia) also stimulates megakaryocyte proliferation, increasing platelet output.
• Shared precursor stimulation - erythroid and megakaryocyte lineages share a common progenitor (the MEP - megakaryocyte-erythroid progenitor). When erythropoiesis is frustrated by iron lack, the balance tips toward platelet production.
• Reduced platelet clearance may also contribute in some cases.

How high do platelets go?

• Typically 450-700 x10⁹/L in iron deficiency, though counts >1000 x10⁹/L are occasionally seen
• Usually proportional to the severity of iron deficiency

Key clues on the blood results

Important ward point

1. Treat the iron deficiency (oral or IV iron depending on severity/urgency)
2. Recheck FBC after 4-8 weeks - the platelet count should normalise with iron repletion
3. If platelets do not fall after correcting iron deficiency, this raises suspicion for a co-existing primary cause (MPN) and warrants JAK2 mutation testing and haematology review

Why it matters

Patient Counselling: High Platelet Count (Thrombocytosis)

1. Explain What It Is (in plain language)

"Your blood test has shown that your platelet count is higher than normal. Platelets are tiny cells in your blood that help it clot when you get a cut or injury. Having too many of them is called thrombocytosis."

"This is actually a very common finding, and in most cases it is not a sign of something serious - your body is usually raising the platelet count as a reaction to something else going on, like an infection, inflammation, or low iron levels."

2. Explain the Two Main Types

"In most people, the high platelets are the body's response to something - an infection, surgery, anaemia from low iron, or inflammation. Once we find and treat that cause, the platelet count usually comes back down on its own."

"In a smaller number of people, the bone marrow is producing too many platelets by itself. We will do some blood tests to check whether this is the case for you."

3. Address What They Are Likely Worried About

"A raised platelet count does not mean you have cancer. The most common causes are everyday things like infection or low iron. We do some tests to make sure we are not missing anything, but please do not worry that a diagnosis of cancer has been made - it has not."

"If your high platelets are reactive - caused by something like infection or iron deficiency - the risk of a blood clot is not significantly higher than normal. The risk is mostly relevant if the bone marrow itself is overproducing platelets, which we will test for."

4. Explain the Tests You Are Doing

"We are going to run a few blood tests to understand why your platelets are high. These include:"

• A full blood count repeat and blood film (to look at the cells in detail)
• Iron levels and ferritin (to check for iron deficiency)
• CRP and ESR (inflammation markers)
• Depending on the results, possibly a special gene test called a JAK2 mutation test

"These are simple blood tests - there is nothing invasive at this stage."

5. Lifestyle Advice

• Stop smoking - smoking is an independent risk factor for thrombosis and worsens outcomes in myeloproliferative conditions
• Stay well hydrated - reduces thrombotic risk
• Keep active - avoid prolonged immobility
• Manage cardiovascular risk factors - blood pressure, cholesterol, diabetes if relevant
• Avoid combined oral contraceptive pill if thrombotic risk is a concern (discuss with GP)
• Limit alcohol - relevant if liver disease is a contributing factor

6. Symptoms to Watch For and Return If...

"If you notice any of these, do not wait - go to A&E or call 999 if severe."

7. Follow-Up Plan

"We will repeat your blood count to see if the platelet level is coming down. If we find a treatable cause like low iron, correcting that should bring the count back to normal. If the tests suggest a bone marrow problem, we will refer you to a blood specialist (haematologist) who will guide the next steps."

"You do not need to do anything differently right now - just take any treatment we prescribe for the underlying cause, attend your follow-up appointments, and come back if you get any of the warning symptoms we discussed."

8. Written Information / Resources

• Give the patient a leaflet if available (many NHS trusts have haematology patient information leaflets)
• Leukaemia UK - Essential Thrombocythaemia (if MPN is suspected)
• MPN Voice - excellent UK patient support charity for myeloproliferative neoplasms