JAK2-Negative Erythrocytosis: A Negative Test That Doesn't Settle Anything
A single patient, found incidentally with a striking hematocrit. The disagreement isn't whether he needs treatment now — it's how hard to chase a diagnosis before treating the number in front of us.
Nathaniel P. sleeps through daylight hours in a bedroom he's blacked out with heavy curtains, a routine built around nine years working the overnight shift maintaining a regional data center. His primary care doctor noticed his ruddy complexion at a visit scheduled for an unrelated knee complaint and ordered a CBC that came back with a hemoglobin of 18.2 and a hematocrit of 56 percent — numbers high enough to prompt an immediate hematology referral for suspected polycythemia vera. He also reports headaches most mornings and occasional itching after a hot shower, both of which he'd attributed to poor sleep rather than his blood count.
His JAK2 V617F testing came back negative, which narrows but does not close the question. Two explanations remain genuinely live: a JAK2 exon 12 mutation — the lesion Scott and colleagues described in the New England Journal of Medicine in 2007 in V617F-negative patients whose presentation was isolated erythrocytosis rather than the trilineage picture classic V617F-positive disease shows, which is exactly the shape of his counts — or a secondary erythrocytosis driven by chronic hypoxia. His snoring, reported by his partner and never followed up, is the untested half of that second explanation: obstructive sleep apnea is a well-established cause of secondary erythrocytosis through chronic intermittent nocturnal hypoxia, and no one has yet asked whether he has it.
His serum erythropoietin level came back in the high-normal range, which doesn't clearly favor either explanation on its own — a suppressed EPO would point toward primary disease, since an autonomously proliferating marrow needs less erythropoietin drive to keep producing red cells, and a frankly elevated one toward a hypoxic or renal driver, but a borderline value genuinely leaves both explanations on the table rather than settling the question.
A negative JAK2 test that doesn't settle anything
A high-normal EPO doesn't rule out either explanation, and I don't think we should reach for phlebotomy on symptom grounds alone until we've actually looked for JAK2 exon 12, which real polycythemia vera can carry even when V617F is negative, and for the specific secondary cause his shift-work history is practically pointing us toward. A polysomnogram is the obvious next step given the snoring history nobody had asked about before this referral.
This isn't reflexive over-testing — a missed exon 12 mutation would mean treating a real myeloproliferative process as benign, and a missed OSA diagnosis would leave the actual driver of his erythrocytosis untreated even if we phlebotomize him in the meantime.
I agree with the sleep study, that's a cheap, high-yield test given his history. Where I'd push back is on treating exon 12 testing as equally urgent right now — it's a rare cause even among V617F-negative cases, and starting low-dose aspirin plus symptom-triggered phlebotomy while the sleep study is pending isn't unreasonable given his hematocrit is genuinely high enough to carry near-term thrombotic risk regardless of the eventual diagnosis.
I'm not arguing against ever checking exon 12 — only against making it a prerequisite before addressing a hematocrit this elevated, when a treatable secondary cause is sitting right in front of us.
Agreed: sleep study and JAK2 exon 12 testing both sent this visit, with interim symptom-triggered phlebotomy and low-dose aspirin started rather than waiting on results to address the immediate thrombotic risk.
Not agreed: whether a positive sleep study would end the workup or simply add a treatable contributor alongside a still-open question about his marrow — left unresolved pending actual results rather than pre-decided.