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Hematocrit (Hct), or packed cell volume (PCV), is the volume occupied by red cells after blood is centrifuged. In this article, we will learn about different methods used to calculate hematocrit levels.
◉ Measurement of hematocrit
Hematocrit (Hct) is the ratio of the volume of red blood cells to the total blood volume, and it is usually expressed as a percentage or a fraction.
The measurement of hematocrit is important for diagnosing and monitoring various diseases, including anemia, polycythemia, dehydration, and heart or lung disorders.
Hematocrit levels can be estimated from other blood parameters, measured using manual techniques or automated analyzers.
Regardless of the measurement method, interpreting hematocrit results necessitates consideration of other clinical parameters and the patient's medical history. Moreover, reference ranges may vary depending on the specific laboratory and population.
◉ Estimation of Hematocrit
Hematocrit can be estimated by considering certain other blood parameters:
- Haemoglobin (Hg) (g/dl) × 3 = Hematocrit ± 3.
- Hematocrit (%) = MCV (fl) × Number red blood cells (RBC) per L.
- Hematocrit (%) = Haemoglobin (g/dL) / MCHC (g/dL) × 100
◉ Manual Method for Hematocrit Measurement
The manual method of measuring hematocrit has proven to be a simple, easy-to-implement, and accurate technique despite any potential errors.
It involves drawing a small blood sample into a tube, sealing one end, and centrifuging it for a fixed time and speed. The length of the packed red blood cell column is then measured and divided by the total length of the blood column to obtain the hematocrit value.
There are two types of manual methods:
- Macro Hematocrit Method uses a 110-mm-long Wintrobe tube, graduated from 0 to 100 mm in both directions, and has been replaced by the "micro-hematocrit" method.
- Micro Hematocrit Method uses a smaller capillary tube. This alternative requires less blood and reduces testing time. This is the reference method recommended by the National Committee for Clinical Laboratory Standards (NCCLS).
◉ Materials needed for microhaematocrit methods
- The microhematocrit tube, also known as a capillary tube, is a specialized tube measuring 75mm in length and 1mm in diameter. It is typically graduated and used to collect and hold a blood sample during centrifugation.
- A microhematocrit centrifuge device is used to centrifuge the capillary tubes, separating blood components.
- Sealing compound (such as clay or wax).
- A Microhematocrit Reader is used for measuring and reading the hematocrit value after centrifugation.
◉ Procedures of micro haematocrit methods
- Fill a capillary tube with blood.
- Seal one end of the tube with a sealant (such as clay or wax).
- Place the tube in a microhematocrit centrifuge.
- Centrifuge at 11000 to 12000 rpm for 4.5 to 5 min.
- Take the tube from the centrifuge and place it in a microhematocrit reader.
- Measure the length of the packed red blood cell column and divide it by the total length of the blood column to obtain the hematocrit value.
- The measurement should be performed within 10 minutes to avoid merging of the layers.
Capillary tube for hematocrit measurement
◉ Hematocrit Measurement by Automated Analyzers
Automated analyzers are widely used in clinical laboratories for hematocrit measurement due to their accuracy, efficiency, and ability to process large numbers of samples. It provide rapid and precise results, reducing the potential for human error and increasing overall laboratory efficiency.
These analyzers use various principles to measure hematocrit, including electrical impedance, optical detection, or a combination of both.
- Electrical impedance-based analyzers pass a small electrical current through a blood sample and measure the impedance changes caused by the passage of RBCs.
- Optical-based analyzers use light absorption or scattering to estimate the hematocrit value.
◉ Materials needed
- Automated hematocrit analyzer: These instruments are designed specifically for measuring multiple blood parameters, including hematocrit levels. The specific analyzer used may vary depending on the laboratory or healthcare facility. These analyzers often require specific reagents or cartridges.
- Blood samples are typically collected in tubes containing 3-5 mL of EDTA-anticoagulated blood.
◉ Procedures
- Load the sample: A blood sample is loaded into the automated hematocrit analyzer according to the manufacturer's instructions. This can involve placing the sample in a specific sample container or cartridge.
- Initiate the analysis: The instrument is started, and the analysis is initiated.
- The automated analyzer performs the necessary steps to measure the hematocrit value
- Obtain the results: Once the analysis is complete, the hematocrit value is displayed on the analyzer's screen or printed out as a result report.
◉ Interpretation of hematocrit levels
Normal Hct levels vary depending on factors like age, sex, and altitude. In general, normal hematocrit ranges for
- Adults male: 41% to 50%.
- Adult female : 36% to 44%.
- Infants: 32% to 42%.
- Newborns: 45% to 61%.
Interpretation of high or low hematocrit levels is conducted by healthcare providers after a physical examination, consideration of symptoms, family history, and in conjunction with other laboratory test results.
◉ Conclusion
Diverse methods of measuring hematocrits play a pivotal role in clinical laboratory testing. These techniques furnish healthcare professionals with essential information regarding a patient's red blood cell volume, contributing significantly to the diagnosis and monitoring of various medical conditions.