The study, led by researchers including Edgar Cristancho-Mejía and Walter Schmidt, recruited healthy, non-smoking adults who had lived at their respective altitudes for at least two years. Participants came from six locations spanning 970 to 2600 meters, including regions around Bogotá in the Central Eastern Andes and the lower Western Andes near Tuluá and Cali. Crucially, none of the participants were endurance athletes, eliminating a major confounding factor that has plagued earlier comparisons between highland populations and sea-level dwellers. The cohort included 133 men and 148 women, with ancestry distributions reflecting the regional population mix of African, European, and Native American heritage.

Measuring hemoglobin mass directly is technically demanding, and the team used the optimized carbon monoxide rebreathing method, widely regarded as the gold standard. Participants inhaled a precisely calculated dose of carbon monoxide over two minutes, and the researchers tracked how much of it bound to hemoglobin in the blood, allowing an accurate calculation of total hemoglobin mass. From this, together with venous hemoglobin concentration and hematocrit, the team derived red cell volume, plasma volume, and total blood volume. Blood samples drawn from a cubital vein provided hemoglobin concentration, hematocrit, and oxygen saturation values for each participant.

The results revealed a clear and unexpected divergence in thresholds. Hemoglobin mass rose significantly only in residents living at 2180 meters or higher, with the effect becoming firmly established at 2600 meters. Plasma volume, by contrast, was already significantly reduced at 1730 meters—roughly 450 meters lower than the erythropoietic threshold. Between the lowest and highest study locations, hemoglobin concentration increased by about 1.0 grams per deciliter in men and 1.3 grams per deciliter in women, while hematocrit rose by roughly 3.2 to 3.7 percentage points. Relative to lean body mass, hemoglobin mass was 5.8 percent higher in men and 8.9 percent higher in women at 2600 meters compared with 970 meters, while plasma volume per kilogram of lean mass fell by about 6 percent in both sexes.

The physiological logic behind these two thresholds traces back to the shape of the oxygen dissociation curve. This sigmoid curve allows arterial blood to remain nearly fully saturated with oxygen up to altitudes of roughly 2000 meters, meaning the kidneys—the primary sensors of oxygen availability—detect little reason to stimulate red cell production. Below that ceiling, the flat upper portion of the curve buffers the modest drop in ambient oxygen pressure. Once saturation begins to fall more steeply, however, the kidney responds by releasing erythropoietin, triggering the bone marrow to expand hemoglobin mass. Plasma volume regulation operates on a different logic entirely: it responds to the overall oxygen-carrying deficit rather than to a specific saturation threshold, and it does so within days rather than weeks.

When the researchers combined their data with earlier measurements taken at 420 and 3800 meters in Bolivia using the same methodology, a striking pattern emerged. Hemoglobin mass increased slowly and modestly up to about 2600 meters, then climbed sharply at higher elevations, reaching values 27 percent above near-sea-level levels at 3800 meters. Notably, within the moderate-altitude range of the current study, there was no significant relationship between hemoglobin mass and oxygen saturation—suggesting that up to roughly 2600 meters, altitude exerts only a gentle influence on erythropoiesis. Above that point, the diminishing oxygen supply becomes the dominant driver, most likely registered by oxygen-sensing mechanisms in the kidneys.

To quantify how much each component contributed to the observed rise in hemoglobin concentration, the team performed multiple regression analyses. The calculations showed that every 100-gram increase in hemoglobin mass raised concentration by 1.59 grams per deciliter, while every liter of plasma volume loss raised it by 3.07 grams per deciliter. Applying these coefficients to the altitude-related changes, the researchers found that of the 1.2 grams per deciliter elevation in hemoglobin concentration at 2600 meters, approximately 0.56 grams per deciliter was attributable to increased hemoglobin mass and 0.61 grams per deciliter to reduced plasma volume—a near-perfect split. When the team applied an alternative correction for the Fåhraeus effect, which accounts for differences between central and peripheral hematocrit, the plasma volume contribution grew even larger.

The findings carry immediate clinical weight. Hemoglobin concentration is one of the most commonly ordered diagnostic measurements in medicine, and anemia thresholds are already adjusted for altitude. Yet the hemoconcentration caused by plasma volume contraction at moderate elevations has never been factored into those thresholds. If a substantial fraction of the altitude-related rise in hemoglobin reflects a shrunken liquid blood volume rather than genuinely enhanced oxygen-carrying capacity, then current anemia criteria may misclassify residents of towns like Bogotá at 2600 meters. The study also resonates with recent work on high-altitude populations worldwide: data from Sherpa communities in the Himalayas suggest their lower hemoglobin concentrations compared with Andean highlanders stem largely from larger plasma volumes rather than smaller hemoglobin masses, underscoring that the ratio of red cells to plasma, not red cell count alone, defines the blood’s oxygen transport profile.

The authors acknowledge limitations. Sample sizes at individual altitudes were modest, and the high interindividual variability in hemoglobin mass—driven largely by differences in lean body mass—means the altitude effect, though statistically robust, is just one of several factors shaping any person’s blood profile. Ambient temperature may also play a role, since the lowland study sites were considerably warmer, and heat acclimatization is known to expand plasma volume. Plasma volume itself was calculated indirectly rather than measured directly, and the Fåhraeus correction introduces some uncertainty, though the researchers showed that an altitude-adjusted correction strengthened rather than weakened their conclusions. Nevertheless, the study provides the first systematic evidence that the blood’s response to moderate altitude unfolds in two distinct stages: plasma contracts first, at around 1730 meters, and the erythropoietic machinery follows at roughly 2180 meters. For the roughly 5 percent of humanity living above 1500 meters, that distinction may soon matter at the doctor’s office as much as on the mountain.

Subject of Research: Altitude thresholds for hemoglobin mass and plasma volume regulation and their effects on hemoglobin concentration in Andean residents

Article Title: Altitude thresholds for hemoglobin and plasma volume regulation in adult residents of different altitudes and their impact on hemoglobin concentration

Article References: Cristancho‐Mejía, E., Trompetero‐Gonzalez, A. C., Benavides‐Pinzón, W. F., & Schmidt, W. F.-J. (2026). Altitude thresholds for hemoglobin and plasma volume regulation in adult residents of different altitudes and their impact on hemoglobin concentration. Physiological Reports, 14(19), Article e71108. https://doi.org/10.14814/phy2.71108

Image Credits: AI Generated

DOI: 10.14814/phy2.71108

Keywords: hemoglobin mass, plasma volume, altitude, hypoxia, erythropoiesis, Andes, Colombia, anemia thresholds, oxygen saturation, blood volume, CO rebreathing, physiology

The research team, led by investigators at the Pain & Fatigue Study Center in the Department of Neurology, drew participants from an NIH-funded parent study designed to examine the relationship between cardiopulmonary exercise testing and post-exertional malaise, the hallmark symptom of ME/CFS. Between August 2021 and July 2025, forty-nine adults aged 25 to 60 years who met the 1994 Fukuda case definition completed both assessments. To ensure a substantial current symptom burden, participants were required to report moderate-to-severe ratings on post-exertional malaise and on at least three of seven additional symptoms, including sore throat, headache, myalgia, and unrefreshing sleep. Individuals with medical conditions that could explain the fatigue, including anemia identified by routine laboratory testing, were excluded, as were those taking medications that blunt cardiovascular responses and those with certain psychiatric diagnoses.

The orthostatic assessment relied on the 10-minute lean test, a protocol originally developed by NASA to detect orthostatic intolerance in astronauts returning from spaceflight. After at least ten minutes of supine rest, baseline blood pressure, heart rate, respiratory rate, and end-tidal carbon dioxide were recorded in triplicate at one-minute intervals using a portable capnograph and pulse oximeter. Participants then stood with feet together roughly six to eight inches from a wall and leaned against it without moving for ten minutes, while the same parameters were captured every minute. This test captures several distinct physiological abnormalities: postural orthostatic tachycardia syndrome, defined as a heart rate rise of at least 30 beats per minute or an absolute heart rate of at least 120 beats per minute; supine hypocapnia, defined as any baseline end-tidal carbon dioxide below 34 mmHg; and the postural orthostatic syndrome of hypocapnia, in which carbon dioxide falls below that threshold only upon standing.

Blood volume was measured with the semiautomated Daxor BVA-100 system, a technique regarded as a reference standard for quantitative intravascular volume assessment. After a baseline blood sample was drawn, human serum albumin labeled with iodine-131 was injected intravenously and allowed to equilibrate. Five additional samples were collected at 15, 21, 27, 33, and 39 minutes to correct for the gradual leakage of albumin out of the vasculature, and plasma volume at time zero was extrapolated from at least three samples with a coefficient of variation of no more than 3.9 percent. Total blood volume was calculated as the sum of plasma volume and red blood cell volume, and all values were normalized to each participant’s ideal body size. Total blood volume was then categorized as hypovolemic if it deviated more than 8 percent below the predicted value, normovolemic within 8 percent, and hypervolemic above 8 percent, thresholds consistent with prior ME/CFS blood volume research.

The results painted a picture of remarkable physiological heterogeneity. Seventeen participants, or 35 percent, were classified as hypovolemic, twenty-one as normovolemic, and eleven as hypervolemic. Red blood cell volume told a similar story: nearly half of the cohort, twenty-four participants, had a red cell volume deficit more than 10 percent below ideal, a phenotype that, when routine hemoglobin and hematocrit values are normal, can be considered an occult red cell mass deficit rather than conventional anemia. Yet when the researchers cross-tabulated these volume categories against lean-test outcomes, no significant associations emerged. Fifty-five percent of participants exhibited at least one orthostatic abnormality, with hypocapnic responses proving at least as common as tachycardia: twenty participants showed the postural orthostatic syndrome of hypocapnia, six showed supine hypocapnia, six met criteria for POTS, and four had both POSH and POTS. The distribution of these abnormalities, however, was statistically indistinguishable across hypovolemic, normovolemic, and hypervolemic groups, and the same held true for red blood cell volume categories.

These findings directly confront a body of earlier work. Multiple previous studies have reported reduced total or compartmental blood volumes, smaller cardiac chambers, and diminished stroke volume in subsets of ME/CFS patients, and one pilot study using a dual-isotope technique found lower blood volumes among patients whose day-to-day symptoms suggested orthostatic intolerance. That earlier study, however, relied on clinical history rather than contemporaneous physiological testing, leaving open the possibility that subjective symptom reports and objective orthostatic physiology diverge. By combining quantitative blood volume analysis with a standardized lean test incorporating capnography, the Mount Sinai team demonstrated that even objectively confirmed orthostatic abnormalities do not track with measured volume status. In other words, a patient can be quantitatively hypovolemic yet pass the lean test, or have entirely normal blood volume yet exhibit marked hypocapnia or tachycardia upon standing.

The null result does not mean blood volume is clinically irrelevant. Identifying hypovolemia in an individual patient may still matter, because volume expansion remains a logical therapeutic target for that subgroup. But the new data help explain why pharmacological volume expansion has disappointed in controlled trials: randomized studies of fludrocortisone, a mineralocorticoid that promotes sodium and water retention, showed limited benefit in ME/CFS, implying that correcting hypovolemia alone is often insufficient. The authors argue that orthostatic intolerance in this illness likely arises from multiple converging mechanisms that static volume measurements cannot capture. Among the candidates are impaired venous constriction and excessive venous capacitance, which promote gravitational pooling of blood in the legs; autonomic dysregulation, evidenced by reduced vagal modulation and altered heart rate variability at rest, during sleep, and during orthostatic challenge; and abnormalities of neurohumoral control, including the renin-aldosterone paradox described in POTS and small-fiber neuropathy identified in subsets of patients.

Cerebral perfusion adds another critical dimension. Previous controlled work has shown that upright posture can substantially reduce cerebral blood flow in ME/CFS patients even when heart rate and blood pressure remain within conventional limits, and that this cerebral blood flow reduction during tilt testing is independent of deconditioning as defined by exercise testing. Deconditioning itself, which can reduce plasma volume and ventricular mass over weeks of bed rest, is unlikely to account for the high prevalence of orthostatic intolerance in this population, since patients show greater cerebral blood flow reductions than healthy, sedentary controls. Taken together, these observations support a model in which orthostatic pathology in ME/CFS reflects dynamic failures of autonomic, vascular, respiratory, and cerebral hemodynamic regulation rather than a simple deficit in the amount of blood in the circulation.

The study has limitations that the authors acknowledge candidly. The sample of forty-nine participants, recruited from a single tertiary care center and limited to patients well enough to undergo testing, constrains statistical power and may underrepresent the most severely affected patients. The eight-day interval between the lean test and the blood volume assessment, together with the lack of standardization of acute hydration and salt intake, may have introduced temporal variability in both volume and hemodynamic measures. Anxiety-related hyperventilation and other breathing-pattern abnormalities could not be excluded as contributors to the hypocapnic findings, because state anxiety and tidal volume were not assessed. The 10-minute lean test also imposes milder orthostatic stress than head-up tilt, so hypovolemia-driven instability might conceivably emerge only under more prolonged or severe challenge. The absence of a healthy control group further precluded direct comparison of volume patterns between patients and volunteers.

Even with those caveats, the message for the field is clear and consequential. Orthostatic intolerance was highly prevalent in this cohort, yet neither total blood volume nor red blood cell volume category predicted who would exhibit supine hypocapnia, POSH, or POTS on standardized testing. The authors call for multicenter studies that incorporate dynamic measurements of cerebral blood flow, respiratory pattern, and carbon dioxide regulation during graded orthostatic stress, integrated with blood volume status, to build comprehensive pathophysiological models of the illness. Such phenotype-directed approaches, they argue, will be essential for developing rational therapies that address the actual mechanisms driving orthostatic symptoms, rather than assuming that a single measurable deficit, however intuitively appealing, holds the key to one of ME/CFS’s most disabling features.

Subject of Research: The relationship between measured blood volume, red blood cell volume, and orthostatic intolerance in adults with myalgic encephalomyelitis/chronic fatigue syndrome

Article Title: Total blood volume and red blood cell volume are not associated with orthostatic intolerance in adults with myalgic encephalomyelitis/chronic fatigue syndrome

Article References: Total blood volume and red blood cell volume are not associated with orthostatic intolerance in adults with myalgic encephalomyelitis/chronic fatigue syndrome. (n.d.). https://doi.org/10.14814/phy2.71087

Image Credits: AI Generated

DOI: 10.14814/phy2.71087

Keywords: ME/CFS, orthostatic intolerance, blood volume, red blood cell volume, POTS, hypocapnia, lean test, capnography, autonomic dysfunction, cerebral blood flow, hypovolemia, Physiological Reports