What hemoglobin level is too high?

We could not find an official danger line for hemoglobin in healthy people, but there is a diagnostic one. In its 2016 classification of myeloid neoplasms, the World Health Organization set the first major criterion for polycythemia vera, one of the myeloproliferative neoplasms in that classification, at a hemoglobin above 16.5 g/dL in men or above 16.0 g/dL in women. In g/L, the unit WHO's anemia guideline uses, that is 165 and 160 (our arithmetic: multiply g/dL by 10). A 2025 American Society of Hematology education review by Szuber, Tefferi and Gangat, of which we read the abstract, states that JAK2 wild-type erythrocytosis, meaning raised red cells without the mutation behind most polycythemia vera, is conventionally defined by the same hemoglobin and hematocrit thresholds as polycythemia vera.

The table's footnote adds one exception: the bone marrow biopsy may not be required with sustained hemoglobin above 18.5 g/dL in men (hematocrit 55.5%) or above 16.5 g/dL in women (hematocrit 49.5%) if the JAK2 mutation and the low erythropoietin are both present. Even then, the authors note that early marrow scarring, present in up to 20 percent of patients, can only be found by biopsy.

The 2016 revision lowered the hemoglobin thresholds, in part to separate an early, masked form of the disease from JAK2-mutated essential thrombocythemia, and the same Barbui review records the objection: critics argued the lower lines would send a large segment of the healthy population into costly investigation. The WHO's fifth edition in 2022 kept the criteria of previous editions with minor refinements and dropped red cell mass as a criterion, because the radioactive-label test behind it has become uncommon. Its summary does not restate the numbers.

Clearing 16.5 or 16.0 meets one of three major criteria. Under the WHO rule, polycythemia vera also needs the marrow finding plus either the mutation or a low erythropoietin. The JAK2 V617F mutation is present in about 95 percent of polycythemia vera, and JAK2 exon 12 changes in about 3 to 5 percent of those without it, and Barbui and colleagues write that the modern diagnostic approach requires knowledge of mutation status. The 2025 ASH review calls ruling out polycythemia vera through JAK2 mutation screening the most clinically relevant step in the workup.

The same review lists what has to be considered beside it: relative polycythemia, which on our reading means a high concentration from less plasma rather than more red cells; people who are simply normal outliers; erythropoietic drugs; and other illnesses. It also says that telling a long-standing high result from a newly acquired one streamlines the workup. On our reading, that makes old results useful evidence.

In secondary erythrocytosis, a 2025 systematic review in Blood Advances explains, hemoglobin rises in response to raised erythropoietin rather than a fault in the marrow itself. The ASH review groups the acquired causes as hypoxia-driven (heart and lung disease, altitude, renal artery stenosis), other sources of excess erythropoietin such as erythropoietin-secreting tumors, hypersensitivity to erythropoietin, mechanisms that do not depend on erythropoietin, and drugs. The British Society for Haematology's 2019 guideline and the Blood Advances review put numbers on several.

The BSH guideline also describes idiopathic erythrocytosis, an absolute erythrocytosis with no identifiable cause, as a diagnosis made only after primary and secondary causes have been excluded.

WHO's 2024 anemia cutoff guideline states that hemoglobin concentrations increase in response to hypoxia, such as from elevation above sea level or smoking, and publishes adjustments for diagnosing anemia: amounts subtracted from a reading or added to the anemia cutoff. They are an anemia tool. WHO does not apply them to high results or to the polycythemia vera line, and we show them here only to indicate how far these two causes can move hemoglobin.

In these rows the adjustments run from 0.3 to 1.4 g/dL. WHO applies them only to deciding whether someone has anemia, by subtracting them from a reading or adding them to the anemia cutoff; it does not use them to discount a high result. WHO itself notes that at the individual level it is unclear whether the smoking difference is physiologically relevant. Our no anemia band page applies these adjustments, below 2,500 m, where they belong: to a low result.

Testosterone has its own threshold, set in hematocrit rather than hemoglobin. The Endocrine Society's 2018 guideline defines treatment-induced erythrocytosis as hematocrit above 54%. Its monitoring table reads: check hematocrit at baseline, 3 to 6 months after starting treatment, and then annually; if hematocrit is above 54%, stop therapy until hematocrit decreases to a safe level, evaluate the patient for hypoxia and sleep apnea, and reinitiate therapy with a reduced dose. Its recommendation 2.2 recommends against testosterone therapy in men with an elevated hematocrit, and its table of conditions with a moderate to high risk of adverse outcomes gives that as hematocrit above 48% (above 50% for men living at high altitude).

The guideline is also candid about the evidence: the hematocrit level at which the risk of neuro-occlusive or cardiovascular events increases is not known. Those are instructions to the prescribing clinician; a change to therapy is a decision to make with them. Our guide to low testosterone, levels and treatment covers the rest of the monitoring.

The clearest evidence that a red cell count can be too high comes from a randomized trial of targets. CYTO-PV, published in the New England Journal of Medicine in 2013, assigned 365 adults with JAK2-positive polycythemia vera, already treated with phlebotomy, hydroxyurea or both, to a hematocrit target under 45% or a target of 45 to 50%. After a median 31 months, death from cardiovascular causes or a major blood clot occurred in 2.7 percent of the lower-target group and 9.8 percent of the higher-target group, a hazard ratio of 3.91 for the higher target.

Barbui and colleagues and the BSH guideline both give a hematocrit target under 45% in polycythemia vera. It is a finding in people who have the disease. The BSH says there is no evidence on which to set a target for idiopathic erythrocytosis and that the relevance of the polycythemia vera data to it is unclear, and for hypoxic lung disease it warns that a hematocrit lower than 45% is detrimental and associated with poorer outcome. The right number depends on the cause, which is the argument for finding it.

The WHO table's footnote speaks of sustained values and the ASH review separates a long-standing high result from a new one, so a repeat count and your previous results are the first evidence to gather. If hemoglobin stays above 16.5 g/dL (men) or 16.0 g/dL (women), or hematocrit above 49% or 48%, those are the WHO lines at which the questions in this guide apply: is there a JAK2 mutation, what is the erythropoietin level, and is there a lung, sleep, medication or altitude explanation. Bring a list of every medicine, including testosterone and diabetes drugs, and your smoking history.

Our hemoglobin reference page carries the WHO cutoffs with their source and effective date, and the lab result lookup will place a value against them. The low end of the scale is covered in our guide to normal hemoglobin levels, and the panel worth repeating each year is in blood tests worth getting every year. For the screenings that apply at your age, build your personal screening plan.