Hematology & Family Medicine & Internal Medicine
Hematocrit (Hct)
Hematocrit (Ht), also known as packed cell volume (PCV), expresses the volume fraction occupied by red blood cells (erythrocytes) in a given volume of whole blood. It is expressed as a percentage (%) or as a decimal fraction (L/L). Measured on modern hematology machines by indirect calculation (Ht = RBC × VGM / 10), or historically by capillary centrifugation (microhematocrit), it is one of the most widely used blood count parameters in clinical practice. Normal adult hematocrit values are 41 to 53 % in men and 36 to 46 % in women, with significant physiological variations depending on age (infant: 33-55 %, newborn: up to 65 %), pregnancy (physiological hemodilution → Ht may drop to 32-34 %) and altitude (hypoxia-adaptation polycythemia). Hematocrit is closely related to hemoglobin (Hb) by the approximate relationship: Hb (g/dL) ≈ Ht (%) / 3. A low hematocrit indicates anemia (reduced erythrocyte mass or plasma dilution), while a high hematocrit suggests polycythaemia (true or relative due to hemoconcentration). Interpretation of the hematocrit is always contextual - it is integrated into the complete blood count with the mean corpuscular volume (MCV), mean corpuscular hemoglobin concentration (MCHC), reticulocytosis and erythrocyte indices to guide the etiological diagnosis.
Physiology, Reference Values, and Relationships with Other Erythrocytic Parameters
- Definition, measurement methods, and calculation: definition: hematocrit represents the ratio of red blood cell volume to total blood volume → Ht = RBC volume / total blood volume × 100 → in practice on an automated system: Ht (%) = RBC (×10¹²/L) × GMV (fL) / 10 → this formula gives the calculated hematocrit (slightly higher than measured by centrifugation due to residual plasma trapped between RBCs - «trapped plasma» - approximately 1-3 % difference); historical method (microhematocrit): capillary or venous blood in a heparinized capillary tube → centrifugation 10,000 g × 5 min → direct reading of the height of the RBC column in relation to the total volume → reference method (Gold Standard) for detecting certain morphological abnormalities in RBCs (visible buffy coat) → microhematocrit method still used in pediatrics + in emergency (arterial blood gases → co-oxymeter); hematology automata (Sysmex XN + Beckman-Coulter DxH): hematocrit is calculated from RBC counting by electrical impedance (Coulter method) and VGM measurement → more reproducible result than microhematocrit + available in seconds + hematocrit-hemoglobin-VGM relationship: fundamental formulas: Ht ≈ Hb × 3 (rule of thirds - clinical approximation) + VGM = Ht / GR × 10 (fL) + CCMH = Hb / Ht × 100 (g/dL) + TCMH = Hb / GR × 10 (pg) → example: Ht 36 % + GR 3.8 × 10¹²/L + Hb 12 g/dL → VGM = 36 / 3.8 × 10 = 94.7 fL (normocytic) + CCMH = 12 / 0.36 = 33.3 g/dL (normochromic)
- Normal values by age, sex, and physiological conditions: adult reference values (Canadian laboratories - venous plasma - calibrated automatons): adult male (18-65 years): 41-53 % + adult female (18-65 years): 36-46 % → male-female difference: due to androgens (testosterone stimulates erythropoiesis → GR + Hb + Ht higher in men) + menstrual iron loss (Ht lower in women of childbearing age); age variations: newborn (birth): 44-64 % → physiological polyglobulia (intense fetal erythropoiesis + predominant fetal Hb HbF) → physiological neonatal hemolysis → drops to 30-40 % at 2-3 months (physiological nadir) + infant 6 months-2 years: 33-40 % + child 2-12 years: 35-45 % + adolescent: gradually towards adult values (M/F divergence at puberty) + elderly (>70 years): slight physiological decline accepted (Ht >36 % man + >33 % woman) - NB: anemia should not be attributed to aging alone without investigation + altitude: hypoxia → increased EPO → stimulation of erythropoiesis → higher Ht → at 3,000 m: Ht can reach 55-60 % + at 4,500 m (Himalayas + Andes): 60-65 % + pregnancy: physiological hemodilution (increase in plasma volume of 40-50 % + lesser increase in erythrocyte mass of 20-30 %) → drop in Ht to 32-34 % in the 2nd-3rd trimester → non-pathological physiological anemia → WHO threshold for anemia in pregnancy: Hb <110 g/L (Ht <33 %) in 1st and 3rd trimester + Hb <105 g/L (Ht <32 %) in 2nd trimester + smoking: slight rise in Ht (smoking polyglobulia - carboxyhemoglobin → relative tissue hypoxia → EPO stimulation) + intense, chronic physical training (endurance athletes): proportionally greater increase in plasma volume → «sportsman's anemia» (lowered Ht despite normal erythrocyte mass)
- Hematocrit and blood volume - distinction between true polycythemia and relative polycythemia: fundamental concept: hematocrit measures a fraction (RBC / total blood volume) → may be elevated either because total erythrocyte mass is increased (polyglobulia vera) or because plasma volume is reduced (relative polyglobulia by hemoconcentration) → essential distinction because treatment is radically different; relative polyglobulia (hemoconcentration) : causes: dehydration (gastroenteritis + heat + excessive sweating + vomiting + profuse diarrhea) + Gaisbock syndrome (relative polyglobulia of stressed hypertensive heavy smoker + no increase in erythrocyte mass) + diuretics + extensive burns (plasma loss) → high Ht + normal or reduced total blood volume + normal reticulocytes + normal EPO → treatment: rehydration → normalization of Ht; polyglobulia vera (increased erythrocyte mass): primary: polyglobulia of Vaquez (PV) → JAK2 V617F mutation (95-97 % of cases) or JAK2 exon 12 → autonomous increase in RBCs (± platelets ± leukocytes) → low or normal EPO + secondary: chronic hypoxia (COPD + sleep apnea + altitude) + cyanotic heart disease + high-affinity Hb + EPO-secreting tumor (hepatocarcinoma + renal cell carcinoma + cerebellar hemangioblastoma) → elevated EPO + increased erythrocyte mass → distinction: EPO assay + JAK2 V617F + isotopic erythrocyte mass (chromium-51 - reference method but not widely available) → in practice: if Ht >52 % in men or >48 % in women → investigation of polycythaemia + volume scintigraphy if necessary
Reduced, elevated hematocrit, and special situations
| Clinical situation | Interpretation and assessment | Conduct and follow-up |
|---|---|---|
| Reduced hematocrit — diagnostic approach to anemia VGM — reticulocytes — iron — B12 — folates — hemolysis |
A lowered hematocrit indicates anemia - the etiological diagnosis of which is based on systematic analysis of erythrocyte indices and complementary examinations; WHO definition of anemia: Hb <130 g/L (Ht <39 %) in adult men + Hb <120 g/L (Ht <36 %) in adult non-pregnant women + Hb <110 g/L (Ht <33 %) in pregnant women (1st + 3rd trimester) + Hb <105 g/L (Ht <32 %) in pregnant women (2nd trimester) + Hb <110 g/L (Ht <33 %) in children aged 6 months to 5 years; morphological classification by VGM (first orientation tool): microcytic anemia (VGM 100 fL) : vitamin B12 deficiency (gastric atrophy + Biermer's disease + malabsorption + vegans) + folate deficiency (pregnancy + alcoholism + MTX + carbamazepine) + alcoholism (direct macrocytosis independent of deficiency) + hypothyroidism + drugs (hydroxyurea + zidovudine + stavudine) + myelodysplastic syndromes (MDS) | 1st-line workup for anemia (low Ht): Complete CBC with blood smear (erythrocyte morphology) + reticulocytes (correct for anemia → reticulocyte production index - RPN = reticulocytes (%) × observed Ht / normal Ht × 1 / maturation factor → RPN >2: regenerative anemia → blood loss or hemolysis → RPN <2: aregenerative anemia → insufficient production) + serum ferritin (best marker of iron stores - lowered if deficient → specific if 100 µg/L does not exclude deficiency if inflammatory state) + CRP (ferritin is an acute-phase reactant → if elevated + microcytic anemia → deficiency possible despite normal ferritin-high → soluble transferrin receptor sTfR assay) + vitamin B12 + serum or erythrocyte folate (if macrocytosis) + TSH (hypothyroidism) + renal work-up (creatinine + GFR - anemia of renal LDH + haptoglobin + indirect bilirubin + Coombs direct (DAT) if hemolysis suspected → elevated LDH + collapsed haptoglobin + elevated indirect bilirubin + reticulocytosis = hemolysis triad ; 2nd-line tests according to orientation: Hb electrophoresis (if microcytosis + numerous RBCs + Mediterranean/Asian/African origin → thalassemia or hemoglobinopathy) + G6PD erythrocyte assay (if hemolysis + triggering context) + bone marrow smear + myelogram (if pancytopenia + suspected MDS or aplasia) + digestive exploration if occult bleeding suspected (calprotectin + colonoscopy + FOGD) |
| Low hematocrit - iron deficiency anemia Ferritin — iron IV — occult bleeding — pregnancy |
Iron deficiency anemia is the most common cause of anemia and low hematocrit worldwide - leading cause in women of childbearing age and children; pathophysiology: iron is essential for hemoglobin synthesis (heme nucleus - protoporphyrin IX + Fe²⁺) → iron deficiency → insufficient hemoglobin synthesis → microcytosis (low GMV) + hypochromia (low MHCC + low corpuscular Hb content) + deformed RBCs (elliptocytes + poikylocytes) → hypochromic microcytic anemia → stages of deficiency: stage 1 (depletion of reserves): low ferritin (<12 µg/L - or <30 µg/L if inflammatory state) + normal Hb + normal VGM + stage 2 (functional deficiency): low serum iron + low CST (<20 %) + high TIBC + hypochromic reticulocytes (CHr - Hb content of reticulocytes - <26 pg on Sysmex XE/XN automaton) → Hb still normal but RBCs begin to be microcytic + stage 3 (constituted iron deficiency anemia): low Hb + VGM <80 fL + collapsed ferritin + abundant hypochromic microcytic RBCs; main causes of martial deficiency: in women of childbearing age: menorrhagia (number 1 cause in North America) + pregnancy (increased needs - maternal iron for fetus + placenta + expansion of erythrocyte mass) + digestive losses: gastrointestinal bleeding (peptic ulcer + colorectal cancer + angiodysplasia + Crohn's disease + aspirin + NSAIDs + anticoagulants) → main cause in adult men and postmenopausal women + malabsorption: celiac disease (duodenal villous atrophy → iron poorly absorbed in duodenum) + atrophic gastritis + post-bariatric surgery (gastric bypass → excludes duodenum = site of iron absorption) + inadequate intakes (strict veganism + restrictive diets + infants on unenriched cow's milk) | Treatment of iron-deficiency anemia - practical approach: etiological treatment first: investigate and treat cause of bleeding (colonoscopy if >50 years or digestive symptoms + FOGD if ulcer suspected + gynecology if menorrhagia) → without etiological treatment: almost-certain relapse after discontinuation of supplementation; oral iron supplementation: ferrous iron (Fe²⁺) better absorbed than ferric iron (Fe³⁺) → ferrous sulfate 300 mg (= 60 mg elemental iron) × 2-3/d or ferrous fumarate (Palafer) or ferrous gluconate (fewer side effects but less elemental iron) → take on an empty stomach (maximum absorption) → ascorbic acid (vitamin C) 200 mg simultaneously → improves absorption (reduces Fe³⁺ to Fe²⁺) → avoid with PPIs + antacids + calcium + tea (tannins) + coffee (iron chelation) → side effects : nausea + constipation + black stools (benign) → if poorly tolerated: split dose + take with a meal (absorption reduced by 40 % but better tolerated) + recent data (Moretti 2015 - Blood): alternate-day administration optimal → reduces post-dose hepcidin → improves cumulative absorption vs daily dose → Tolkien 2015 trial (PLOS ONE): ferrous sulfate better tolerated in single morning dose vs split + monitoring: Ht + Hb + reticulocytes at 4 weeks (reticulocytosis = early sign of response at D7-10) → Hb normalization in 6-8 weeks → continue 3-6 months after normalization to replenish reserves (target ferritin >50 µg/L); intravenous (IV) iron: indications: severe oral iron intolerance + malabsorption (celiac + bypass + Crohn's) + significant deficit to be corrected rapidly (pre-op + pregnancy + dialyzed CKD + heart failure) + agents: carboxymaltose iron (Ferinject - up to 1,000 mg per infusion × 15 min) + low-molecular-weight dextran iron (INFeD) + sucrose iron (Venofer) + superior efficacy to oral iron (Anker 2009 - NEJM - heart failure) + low allergic risk with modern preparations (<0.1 % severe reactions) |
| High hematocrit - true polycythemia (Vaquez) and relative JAK2 - EPO - phlebotomy - hydroxyurea - thrombosis |
An elevated hematocrit above normal values makes it necessary to distinguish true polyglobulia (increased erythrocyte mass) from pseudopolyglobulia (hemoconcentration); thresholds for investigation (WHO 2016 criteria): Ht >49 % in men or >48 % in women → polycythemia workup → complete CBC + EPO + JAK2 V617F; Vaquez polycythemia (PV) - myeloproliferative neoplasm: WHO 2016 diagnostic criteria: major criteria: (1) Hb >165 g/L in men or >160 g/L in women (or erythrocyte mass >25 % above normal value) + (2) bone-medullary biopsy: panmyeloid hypercellularity (erythroid + granulocytic + megakaryocytic) + megakaryocyte pleomorphism + (3) JAK2 V617F or JAK2 exon 12 mutation → minor criterion: subnormal or low serum EPO → PV diagnosis if: 3 major criteria or 2 major criteria + 1 minor → JAK2 V617F positive in 95-97 % of PVs → detection by allele-specific PCR or NGS sequencing → JAK2 V617F allelic load correlates with severity (thrombosis + progression to myelofibrosis); complications of Vaquez polycythaemia: thromboses (major complication): Stroke + MI + deep vein thrombosis + pulmonary embolism + thrombosis of suprahepatic veins (Budd-Chiari syndrome - classic presentations of PV) + portal vein thrombosis → high hematocrit → hyperviscosity → slowed blood flow → thrombosis → thrombotic risk correlates with Ht (goal: maintain Ht <45 % in men and <42 % in women) + aquagenic pruritus (contact with water → release of histamine by basophils) + erythromelalgia (burning and redness of extremities relieved by aspirin) + transformation: post-PV myelofibrosis (15 % at 15 years) + acute myeloid leukemia (AML) (<5 %) | Treatment of Vaquez polyglobulia: objectives: maintain Ht <45 % (male) or <42 % (female) to reduce thrombotic risk → based on CYTO-PV trial (Marchioli 2013 - NEJM): Ht 60 years + history of thrombosis + thrombocytosis >1,500 × 10⁹/L + poor bleeding tolerance + dose: 500-2,000 mg/d PO → monitor CBC every 4-8 weeks → goal: normalized RBCs + platelets + leukocytes >3 × 10⁹/L + ruxolitinib (Jakafi - JAK1/2 inhibitor): 2nd line if hydroxyurea insufficient or poorly tolerated → approved in Canada for refractory PV → superior control of Ht and symptoms (pruritus + splenomegaly) - RESPONSE trial (Vannucchi 2015 - NEJM) + pegylated interferon alpha-2a: option in women of childbearing age (no documented teratogenicity + anticlonal activity) + hematology opinion mandatory for any confirmed PV |
| Hematocrit and anemia in specific situations IRC — pregnancy — sickle cell disease — sport — transfusion |
Hematocrit is interpreted differently depending on the clinical context - several situations deserve special attention; anemia of chronic renal failure (CKD): main mechanism: deficiency in erythropoietin (EPO) production by renal peritubular cells (EPO cells) → normocytic normochromic + aregenerative anemia (low reticulocytes) + Ht often between 25 and 35 % → aggravating factors: martial deficiency (dialysis losses + malabsorption) + chronic inflammation + shortened RBC lifespan (uremia) + severe hyperparathyroidism (bone marrow invasion by fibrosis) → treatment: erythropoiesis-stimulating agents (ESAs): epoetin alfa (Eprex) + darbepoetin alfa (Aranesp) + methoxy-polyethylene glycol-epoetin beta (Mircera) → KDIGO 2012 indication: initiate if Hb 200 µg/L + CST >20 %) + Hb target under ESA: 100-115 g/L (do not aim for Hb >130 g/L → increased cardiovascular risk - TREAT 2009 NEJM trial + CHOIR 2006 NEJM) + reimbursement of ESAs in Quebec via RAMQ for dialyzed and pre-dialyzed CKD according to registration criteria + patient blood management (PBM): reduce recourse to transfusion → ESA + IV iron (Ferinject) + peri-operative management; hemolytic anemia - recognize the picture: reticulocytosis (RPN >2) + elevated LDH + collapsed haptoglobin + elevated indirect bilirubin + smear: spherocytes (AHAI) + sickle cells + schizocytes (MAT) + sickle cell disease : baseline Ht 20-30 % (severe chronic anemia due to sickle-shaped RBC hemolysis) + vaso-occlusive crises → accelerated destruction of RBCs → sudden drop in Ht → urgent transfusion if >20 % drop from baseline + baseline Ht monitoring essential for sickle cell patient. | Hematocrit and sport - sports anemia and doping: «sports anemia» (pseudoanemia): endurance athletes (marathon + cycling + triathlon) → plasma volume expansion disproportionate to erythrocyte mass increase → lowered Ht (often 36-40 % in well-trained male athletes) + normal-low Hb → no true anemia → no treatment → exclude martial deficiency (ferritin) → adapt nutritional requirements (iron + B12 + folates) + doping with EPO (exogenous erythropoietin): illicit administration of recombinant EPO → increased Ht → improved O₂ transport → enhanced performance → major thrombotic risk (several documented deaths in professional cyclists - 1990s) → anti-doping control: biological passport test (ABP - Athlete Biological Passport) → longitudinal variations in Ht + reticulocytes → detects profiles suggestive of doping even without direct EPO testing; transfusion thresholds - when to transfuse? restrictive transfusion thresholds (AABB 2016 + TRICC 1999): stable patient without symptoms: transfuse only if Hb <70 g/L (Ht <21 %) + patient with active coronary artery disease (ACS): threshold <80 g/L (Ht <24 %) + post-operative orthopedic patient with symptoms: threshold <80 g/L + post-cardiac surgery: threshold <75-80 g/L → transfusion decision should not be based on Ht figure alone - always integrate symptoms (asthenia + dyspnea + angina + hypotension) + comorbidities + fall kinetics |
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The hematocrit should always be interpreted in conjunction with the patient's hydration status: A hematocrit of 50% % in a dehydrated patient does not mean the same thing as in a normovolemic patient. Similarly, a hematocrit of 30% % in a pregnant woman in her second trimester can be entirely physiological. The correct interpretation of hematocrit integrates MCV, MCHC, reticulocytes, clinical context, and volume status—never in isolation.
Situations requiring urgent medical assessment
Hematocrit <20 % (Hb <65–70 g/L) + dyspnea at rest + tachycardia + hypotension + angina or altered consciousness → Severe symptomatic anemia → indication for urgent transfusion → O- red blood cell units if blood type is unavailable → CBC + reticulocytes + urgent etiological workup.
Sudden drop in hematocrit + abdominal pain + hemodynamically unstable (tachycardia + hypotension) or rectorrhagia + abundant melena → Acute digestive bleeding → CBC + blood type + RPR + coagulation and emergency workup + endoscopy within 24 hours → transfusion if Hb <70 g/L or hemodynamic instability.
Hematocrit >55-60% % (very high Hct) + headache + blurred vision + facial redness + recent thrombosis (stroke + DVT) or aquagenic pruritus → probable polycythemia vera → JAK2 V617F + serum EPO urgently → immediate therapeutic phlebotomy (500 mL) if Hct >60% % → urgent hematology consult.
Sudden drop in hemoglobin in a sickle cell patient with fever, intense bone pain, and acute splenomegaly. Acute splenic sequestration (especially in children) or aplastic crisis (parvovirus B19) → pediatric emergency → urgent transfusion if Ht <20 % baseline + immediate hospitalization.
Consult at Clinique Omicron
The doctors at Clinique Omicron prescribe and interpret complete blood counts, including hematocrit, as part of general health check-ups, chronic disease monitoring, prenatal assessments, and evaluations for symptoms of anemia or polycythemia. The initial management of iron-deficiency anemia – oral supplementation or referral for intravenous iron – as well as the monitoring of anemia in kidney failure can be coordinated at one of our service points in Quebec or via teleconsultation. To make an appointment, visit cliniqueomicron.ca.
The content of this page is provided for informational purposes only and is not a substitute for medical or hematological advice. Any abnormality in hematocrit should be evaluated in its full clinical context before diagnosis or treatment is initiated.
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