Overview
Hematocrit, also known as packed cell volume (Hct/PCV), is the volume occupied by red cells after blood is centrifuged at high speed. It is one of the most commonly performed investigations in hematological laboratories for evaluation of patients with anemia.
Together with hemoglobin and red cell count, hematocrit is used to calculate three essential red cell indices — MCV, MCH and MCHC — which form the basis of the morphological classification of anemia.
Learning Objectives
After this lesson you will be able to…- Describe the methods of estimation of hematocrit
- Describe the normal value and interpretation of result
- Explain the calculation of red cell indices
- Classify anemia on morphological and kinetic grounds
Clinical Story
Why This MattersA 40-year-old man presents with pallor and fatigue. His hemoglobin is low, but the physician needs more information to determine whether this is due to iron deficiency, chronic disease, or another cause. The lab technologist measures the hematocrit and calculates red cell indices (MCV, MCH, MCHC) to morphologically classify the anemia — a critical first step that narrows down the differential diagnosis before further testing.
Core Concepts
Hematocrit (Hct/PCV) is the volume occupied by red cells after blood is centrifuged at high speed. It can be measured by two methods: the microhematocrit method (manual) and the automated method (used in modern hematology analysers). The macrohematocrit method used historically is no longer in routine use.
Three red cell indices are calculated from Hb, Hct and red cell count, and are essential for the morphological classification of anemia:
MCV (Mean Corpuscular Volume): Average volume of each red cell = (PCV % × 10) / Red cell count (millions/mm³), normal 90 ± 10 fl.
MCH (Mean Corpuscular Hemoglobin): Average amount of Hb per red cell = (Hb g/dl × 10) / Red cell count (millions/mm³), normal 29.5 ± 2.5 pg.
MCHC (Mean Corpuscular Hemoglobin Concentration): Average Hb concentration per volume of packed red cells = (Hb g/dl × 100) / PCV %, normal 33 ± 1.5 g/dl.
Morphological classification (based on MCV/MCH): Microcytic hypochromic anemia (MCV <80fl, MCH <27pg), Macrocytic anemia (MCV >100fl), Normocytic normochromic anemia (MCV 80–100fl, MCH 27–32pg).
Kinetic classification divides anemias into two groups: Impaired erythrocyte production (iron deficiency, chronic disease, hypoplastic, infiltrative, megaloblastic, thalassemia) and Increased erythrocyte destruction (hemolytic anemia, treated nutritional anemia).
Red cell indices are mean values and cannot express variation within a population of cells. If the number of abnormal cells is small, indices may still appear normal. Similarly, if two distinct cell populations are present (e.g., both microcytic and macrocytic cells), the indices can average out to a falsely normal value — making peripheral smear examination an essential complementary tool.
Laboratory Principle
In the microhematocrit method, anticoagulated whole blood is drawn into a narrow capillary tube and centrifuged at very high speed (12000g). Because red cells are denser than plasma, centrifugal force packs the red cells at the bottom of the tube, separating them from the buffy coat (white cells and platelets) and plasma above. The proportion of the tube occupied by packed red cells, read off using a microhematocrit reading device, gives the hematocrit value.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| EDTA blood sample | Standard venous EDTA anticoagulated blood | Sample for hematocrit estimation | Test within 6 hours of collection |
| Modeling clay | Sealant grade | Seals capillary tube end to prevent leakage | Room temperature |
Step-by-Step Procedure
Place the capillary tube in the venous EDTA blood sample, allowing blood to enter by capillary action. Leave the last 15mm unfilled.
Seal the tube with modeling clay, ensuring no air is trapped between the clay and the column of blood.
Place the tube in the microhematocrit centrifuge with the sealed end to the outer rim and centrifuge at 12000g for 5 minutes.
Use the microhematocrit reading device to determine the proportion of the tube occupied by packed red cells, expressed as a percentage or L/L.
Combine the hematocrit value with hemoglobin and red cell count to calculate MCV, MCH and MCHC for morphological classification of anemia if indicated.
Flow Diagram
Quality Control
Centrifuges must be checked at regular intervals for speed and accuracy to ensure consistent packing of red cells. Capillary tubes must be of defined specifications (75mm length, 1mm internal diameter) for results to be comparable across runs.
Hematocrit can be used as a reference method for calibrating automated hematology analysers, making it an important cross-check parameter in laboratory quality programs.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| MCV < 80fl, MCH < 27pg | Microcytic hypochromic anemia (iron deficiency, thalassemia, chronic disease) | Order iron studies, hemoglobin electrophoresis if indicated |
| MCV 80–100fl, MCH 27–32pg | Normocytic normochromic anemia | Investigate for chronic disease, hemolysis, or marrow disorders |
| MCV > 100fl | Macrocytic anemia (megaloblastic, aplastic, chronic liver disease) | Order vitamin B12/folate levels, liver function tests |
Common Errors & How to Avoid Them
Cause: PCV increases with storage of the blood sample.
Prevention: Always measure hematocrit within 6 hours of sample collection.
Cause: Improper sealing of the capillary tube with modeling clay traps air bubbles, distorting the blood column reading.
Prevention: Carefully inspect the seal to ensure no air gap exists between clay and blood.
Cause: Two distinct red cell populations (e.g., microcytic and macrocytic cells together) can average out to a normal MCV.
Prevention: Always correlate red cell indices with peripheral smear morphology.
Laboratory Tips from the Bench
Always run capillary tubes in pairs (duplicate fills) when possible — this helps catch sample or technique errors before reporting a hematocrit value.
When reading the centrifuged tube, identify the buffy coat layer (white cells/platelets) clearly — it should not be included in the red cell column measurement.
Remember "VHC" for the three indices in order of calculation logic: Volume (MCV) → Hemoglobin content (MCH) → Concentration (MCHC).
Important Notes
Reduced hematocrit is seen in anemia, while increased hematocrit is seen in polycythemia. Always interpret hematocrit alongside hemoglobin and clinical context.
A useful rule of thumb: hematocrit (%) is approximately three times the hemoglobin value (g/dl) in most normal samples, providing a quick internal cross-check.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeMr. Patel reports progressive fatigue and pale conjunctiva noticed by his wife over the past two months. He denies any obvious blood loss but admits to a poor, mostly vegetarian diet with limited iron-rich foods.
The combination of low Hb, low Hct, low MCV (<80fl) and low MCH (<27pg) confirms a microcytic hypochromic anemia. Given his dietary history, iron deficiency anemia is the most likely cause, though chronic blood loss must be excluded.
- →Red cell indices narrow the differential before iron studies are even ordered
- →Dietary history is an important clue but occult blood loss must always be excluded in adults
- →Microcytic hypochromic anemia has multiple causes — iron studies and hemoglobin electrophoresis help differentiate
Frequently Asked Questions
The buffy coat is the thin layer of white cells and platelets that settles between the packed red cells and plasma after centrifugation. Since hematocrit specifically measures the red cell volume fraction, the buffy coat must be excluded from the reading to avoid overestimating the result.
Yes, hematocrit can be used as a screening test for anemia and provides a rough guide to hemoglobin (Hct ≈ 3 × Hb). However, hemoglobin remains the most frequently used parameter because it is easier to measure directly.
Red cell indices are mean (average) values. If only a small proportion of cells are abnormal, or if two distinct cell populations exist (e.g., dimorphic anemia with both microcytic and macrocytic cells), the average can mask the true picture. Peripheral smear examination is essential to detect this.
Quick Revision
10-Minute ReviewKey Takeaways
- Hematocrit is the percentage volume of red cells in whole blood after centrifugation
- The microhematocrit method is fast, reliable, and uses minimal sample volume
- MCV, MCH and MCHC are calculated from Hb, Hct and red cell count
- Red cell indices form the basis of morphological classification of anemia
- Morphological classes are microcytic, normocytic and macrocytic anemia
- Kinetic classification distinguishes impaired production from increased destruction
Competency Checklist
Track Your MasteryReferences
- Dacie JV, Lewis SM. Practical Haematology. 11th ed.
- Bain BJ. Blood Cells: A Practical Guide. 5th ed.
- NIOS Vocational Course — Hematology and Blood Bank Technique, Lesson 4.