What is the appropriate workup for thrombocytosis?

Workup of Thrombocytosis

Begin by confirming sustained thrombocytosis (platelets ≥450 × 10⁹/L) and immediately assess for secondary causes before pursuing molecular testing, as this approach reduces unnecessary investigation and cost while maintaining diagnostic accuracy 1.

Initial Clinical Assessment

Evaluate for these specific secondary causes that predict reactive thrombocytosis:

• Iron deficiency anemia (check ferritin)
• Active malignancy
• Chronic inflammatory disease (rheumatoid arthritis, inflammatory bowel disease)
• Recent splenectomy
• Recent surgery or infection
• Smoking history

Patients with these conditions are significantly more likely to have secondary thrombocytosis and may not require molecular testing 1.

Laboratory Evaluation

Complete Blood Count Analysis

Specific CBC parameters help differentiate essential thrombocythemia (ET) from secondary causes 1:

Findings favoring ET:

• Higher hemoglobin
• Higher mean corpuscular volume (MCV)
• Higher red cell distribution width (RDW)
• Higher mean platelet volume (MPV)

Findings favoring secondary thrombocytosis:

• Higher white blood cell count
• Higher absolute neutrophil count
• Lower hemoglobin (especially with iron deficiency)

Molecular Testing

Proceed with molecular testing only if secondary causes are excluded or clinical suspicion for ET remains high 1, 2.

The diagnostic yield of molecular testing is approximately 52%, with 92% of positive results being JAK2, CALR, or MPL mutations 1. Test for:

1. JAK2 V617F mutation (present in ~50% of ET cases) - Use assays with 1-3% sensitivity 2
2. CALR exon 9 mutations (present in 60-80% of JAK2/MPL-negative ET)
3. MPL exon 10 mutations (if JAK2 and CALR negative)

Use whole blood or purified granulocytes; the latter is preferred for low mutation burden cases 3.

Bone Marrow Biopsy

Bone marrow biopsy is mandatory for definitive diagnosis of ET, even with positive molecular markers, as mutations alone are not disease-defining 4, 5.

WHO Diagnostic Criteria for ET (all 4 required) 4:

1. Sustained platelet count ≥450 × 10⁹/L
2. Bone marrow biopsy showing:
   • Proliferation mainly of megakaryocytic lineage
   • Increased numbers of enlarged, mature megakaryocytes
   • No significant increase or left-shift of neutrophil granulopoiesis or erythropoiesis
3. Exclusion of other myeloid neoplasms:
   • Not meeting criteria for polycythemia vera (check hemoglobin/hematocrit)
   • Not primary myelofibrosis (no significant reticulin fibrosis)
   • Not BCR-ABL1–positive CML
   • Not myelodysplastic syndrome
4. Demonstration of JAK2 V617F or other clonal marker, OR in absence of clonal marker, no evidence of reactive thrombocytosis

Critical Pitfalls to Avoid

• Do not order molecular testing reflexively - First exclude secondary causes through clinical assessment and basic labs 1
• Do not diagnose ET based on molecular testing alone - Bone marrow biopsy remains essential to exclude prefibrotic myelofibrosis and other myeloid neoplasms 4, 5
• Do not assume extreme thrombocytosis (>1000 × 10⁹/L) indicates ET - Reactive thrombocytosis can reach these levels and has never been shown to cause thrombosis 6
• Ensure adequate assay sensitivity - JAK2 V617F testing should detect mutant allele burden as low as 1-3% 2

Additional Testing for Risk Stratification (if ET confirmed)

• History of thrombosis (arterial or venous)
• Age >60 years
• Karyotype analysis (abnormal in <10% of cases)
• Additional mutation screening (TET2, ASXL1, DNMT3A, SF3B1) may provide prognostic information 7