Unit Converter
Hematocrit (HCT)
(Red Cell Volume Fraction – Core Marker of Anemia, Polycythemia & Hydration Status)
Synonyms
- Hematocrit
- Packed cell volume (PCV)
- Hct
- Erythrocyte volume fraction
- Red cell volume percentage
- Volume fraction of RBCs
Units of Measurement
- % (percentage)
- L/L
- Proportion of 1.0
- Volume fraction
Unit Conversions
1% = 0.01 L/L
1 L/L = 100%
Volume fraction = proportion (decimal form)
Example:
- 0.45 L/L = 45% = 0.45 volume fraction
Description
Hematocrit (HCT) represents the fraction of blood volume occupied by red blood cells.
It is one of the most essential components of the complete blood count (CBC).
HCT is affected by:
- RBC number
- RBC size (MCV)
- Plasma volume
Hematocrit is used to diagnose and classify anemia, polycythemia, dehydration, and many hematologic disorders.
Physiological Role
Hhematocrit directly reflects oxygen-carrying capacity because RBCs contain hemoglobin.
HCT influences:
- Blood viscosity
- Tissue oxygen delivery
- Volume status assessment
HCT rises when RBC mass increases or plasma volume decreases, and falls when RBC mass decreases or plasma volume expands.
Clinical Significance
LOW Hematocrit (Anemia)
Occurs when:
- RBC production decreases
- RBC loss occurs
- RBC destruction increases
- Plasma volume expands
Causes of Low HCT
1. Nutritional deficiency anemia
- Iron deficiency
- Vitamin B12 or folate deficiency
2. Hemolytic anemia
- Autoimmune hemolysis
- G6PD deficiency
- Sickle cell disease / thalassemia
3. Bone marrow disorders
- Aplastic anemia
- Leukemia
- Myelodysplasia
4. Chronic diseases
- CKD
- Chronic infection/inflammation
5. Acute or chronic blood loss
6. Pregnancy
- Physiologic hemodilution
7. Overhydration / IV fluids
- Dilutional anemia
HIGH Hematocrit (Polycythemia)
Occurs when RBC mass increases or plasma volume decreases.
Causes of High HCT
1. Polycythemia vera (primary polycythemia)
- Myeloproliferative neoplasm (JAK2 mutation)
2. Secondary erythrocytosis
- Chronic hypoxia (COPD, sleep apnea)
- High altitude
- Congenital heart disease
- Smoking
- EPO-secreting tumors (renal, hepatocellular)
- Exogenous EPO use
3. Relative/Euvulemic (Hemoconcentration)
- Dehydration
- Burns
- Severe vomiting/diarrhea
4. Athletic/Performance Enhancing Drugs
- Anabolic steroids
- Testosterone therapy
Reference Intervals
Adults
| Group | Hematocrit |
| Men | 0.41 – 0.53 L/L (41–53%) |
| Women | 0.36 – 0.46 L/L (36–46%) |
Children
- Newborns: 0.49 – 0.63 L/L
- Infants: 0.33 – 0.39 L/L
- Older children: similar to adults
Pregnancy
- Lower values due to hemodilution
- Hematocrit < 33% considered anemic (WHO)
Critical Values
- < 0.20 L/L (20%) – risk of ischemia
- > 0.60 L/L (60%) – hyperviscosity risk
Percentage (%)
45%=0.45 L/L=0.45 (fraction)45\% = 0.45\ L/L = 0.45\ (fraction)45%=0.45 L/L=0.45 (fraction)
L/L
L/L=HCT (%)100\text{L/L} = \frac{\text{HCT (\%)}}{100}L/L=100HCT (%)
Proportion of 1.0
0.40=40%0.40 = 40\%0.40=40%
Volume Fraction
Same as L/L or decimal proportion.
Diagnostic Uses
1. Diagnosis of Anemia
Low HCT indicates decreased RBC mass.
2. Classification of Anemia
Combined with MCV:
- Microcytic
- Normocytic
- Macrocytic
3. Diagnosis of Polycythemia
High HCT → consider PV or hypoxic states.
4. Volume Status Assessment
- High HCT → dehydration
- Low HCT → fluid overload
5. Monitoring
- Response to anemia treatment
- Blood loss in surgery/ICU
- Dialysis patients
- Hydration therapy
- Athletes on performance-enhancing drugs
6. Screening in Pregnancy
Helps identify maternal anemia.
Analytical Notes
- Derived from RBC count × MCV or directly measured after centrifugation
- Hemoconcentration/hemolysis affects results
- Plasma trapping may falsely elevate HCT with manual methods
- Automated analyzers more accurate
- IV fluids can rapidly dilute hematocrit
Clinical Pearls
- Hematocrit generally ≈ 3 × hemoglobin, but hydration status affects this rule.
- Dehydration raises HCT more dramatically than RBC mass changes.
- Newborns have physiologically high HCT; values drop after 8–12 weeks.
- Extremely high HCT (>65%) in neonates indicates polycythemia of newborn.
- Athletes at altitude often show moderately elevated HCT due to hypoxia-induced EPO.
Interesting Fact
The original hematocrit measurement (early 1900s) used Wintrobe tubes and manual centrifugation, visually reading the packed RBC layer.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Hematology.
- WHO Hemoglobin & Hematocrit Criteria for Anemia.
- AABB Hemolysis & Hematology Standards.
- BCSH Hematology Guidelines - Anemia & Polycythemia.
- Mayo Clinic Laboratories - Hematocrit.
- ARUP Consult - Erythrocytosis & Anemia Evaluation.
- MedlinePlus / NIH - Hematocrit Test.