In a female smoker with elevated lactate dehydrogenase, markedly high vitamin B12, hemoglobin 16.4 g/dL, hematocrit 49.1 %, leukocytosis (white‑blood‑cell count 11.9 ×10⁹/L), thrombocytosis (platelet count 402 ×10⁹/L), elevated alkaline phosphatase, and normal tryptase, what is the likely diagnosis and recommended work‑up?

Polycythemia Vera (PV) is the Most Likely Diagnosis

This female smoker should undergo JAK2 V617F mutation testing immediately, followed by serum erythropoietin measurement and bone marrow biopsy to confirm polycythemia vera, as her elevated hemoglobin (16.4 g/dL), hematocrit (49.1%), leukocytosis, thrombocytosis, elevated LDH, and markedly elevated vitamin B12 collectively point toward a myeloproliferative neoplasm rather than smoking-related changes alone. 1

Why Polycythemia Vera is the Leading Diagnosis

Hemoglobin and Hematocrit Thresholds Met

• Her hemoglobin of 16.4 g/dL exceeds the WHO screening threshold of >16 g/dL for women, and her hematocrit of 49.1% exceeds the threshold of >48% for women, fulfilling primary screening criteria for PV 1
• While smoking does increase hemoglobin and hematocrit, the genetic analysis shows only modest increases of 0.26–0.34% per smoking-related allele, insufficient to explain her degree of elevation 2

Thrombocytosis and Leukocytosis Pattern

• Her platelet count of 402 ×10⁹/L approaches the thrombocytosis range (≥450 ×10⁹/L seen in 53% of PV patients), and her WBC of 11.9 ×10⁹/L meets the leukocytosis threshold (≥11 ×10⁹/L occurring in 49% of PV cases) 1
• Smoking causes increases in leukocytes (up to 19%) and thrombocytes (up to 4.7%), but the combination with elevated hematocrit and other findings makes a myeloproliferative process more likely 2, 3

Markedly Elevated Vitamin B12 as a Key Clue

• Her vitamin B12 of 1700 pg/mL is strikingly elevated and serves as an important diagnostic marker for myeloproliferative neoplasms, arising from increased transcobalamin I secretion by proliferating leukocytes 4
• In a study of 467 MPN patients, 14% of PV patients had elevated B12 levels, with mean levels of 747 pg/mL before treatment 4
• This degree of B12 elevation is not explained by smoking and strongly suggests clonal myeloproliferation 4

Elevated LDH Supporting Myeloproliferative Disease

• Her LDH of 250 U/L is elevated and represents a minor diagnostic criterion for primary myelofibrosis in the WHO classification, though it can also be seen in PV 5
• Altered LDH profiles are documented in essential thrombocythemia and other MPNs, reflecting increased cellular turnover 6

Normal Tryptase Excludes Mastocytosis

• Her normal tryptase of 4.1 ng/mL effectively rules out systemic mastocytosis, which can present with elevated blood counts 1

Recommended Diagnostic Work-Up Algorithm

First-Line Testing (Immediate)

• JAK2 V617F mutation testing: Detected in >95% of PV patients and constitutes a major WHO diagnostic criterion 1
• Serum erythropoietin level: A subnormal level is a minor diagnostic criterion for PV and helps differentiate from secondary erythrocytosis 1

Second-Line Testing (If JAK2 V617F Negative)

• JAK2 exon 12 mutation testing: Recommended when JAK2 V617F is negative but clinical suspicion remains high 1
• CALR and MPL mutation analysis: These mutations are more typical of essential thrombocythemia but help exclude other MPNs if JAK2 testing is negative 1

Confirmatory Testing

• Bone marrow biopsy: Should demonstrate age-adjusted hypercellularity with trilineage myeloproliferation and pleomorphic, mature megakaryocytes to fulfill a major WHO diagnostic criterion for PV 1
• Look specifically for the absence of extensive dense megakaryocyte clustering and minimal reticulin fibrosis to distinguish PV from pre-fibrotic primary myelofibrosis 5

Elevated Alkaline Phosphatase Consideration

• Her alkaline phosphatase of 155 U/L is mildly elevated and warrants liver function assessment, though this is likely secondary to the myeloproliferative process rather than a primary hepatic issue 5

Critical Pitfalls to Avoid

Do Not Attribute All Findings to Smoking Alone

• While smoking increases hemoglobin, hematocrit, and WBC counts, the magnitude of her elevations combined with thrombocytosis and markedly elevated B12 cannot be explained by smoking alone 2, 3
• Smoking causes long-term increases in white blood cells but only short-term increases in red blood cell indices, whereas her constellation suggests sustained clonal proliferation 2

Do Not Confuse with Vitamin B12 Deficiency

• Vitamin B12 deficiency causes pancytopenia with macrocytosis, elevated LDH, and low B12 levels—the opposite of this patient's presentation 7, 8
• Her elevated (not low) B12 with elevated (not decreased) blood counts points toward MPN, not megaloblastic anemia 4

Do Not Delay Molecular Testing

• Red cell mass measurement is no longer required for PV diagnosis when hemoglobin/hematocrit thresholds are met together with a positive JAK2 mutation 1
• Proceed directly to JAK2 testing rather than pursuing secondary causes of erythrocytosis given the complete clinical picture 1