Primary Method for Determining Hematocrit on Automated Cell Counters
On large volume, automated cell counters, the primary method for determining hematocrit is calculated using directly measured RBC count and MCV data (option C).
How Modern Automated Cell Counters Calculate Hematocrit
Automated hematology analyzers determine hematocrit through an indirect calculation rather than direct measurement. The calculation follows this formula:
Hematocrit = RBC count × MCV
Where:
- RBC count is measured directly by the instrument
- MCV (Mean Corpuscular Volume) is also measured directly
Technical Details of the Process
Modern automated cell counters use one of two primary methods to directly measure the RBC count and MCV:
Impedance Method (Coulter Principle):
- Blood cells flowing through an electrically charged aperture cause measurable changes in electrical resistance
- The number of pulses corresponds to the RBC count
- The amplitude of each pulse is proportional to cell volume (MCV)
Optical Method (Light Scatter):
- Laser light is scattered by cells passing through a flow cell
- The number of scatter events corresponds to the RBC count
- The intensity and angle of scattered light correlate with cell volume (MCV)
Once these direct measurements are obtained, the hematocrit is calculated mathematically 1.
Why This Method is Preferred
The calculated hematocrit method has several advantages over other potential methods:
- Accuracy: Provides more consistent results than manual methods
- Efficiency: Allows for rapid processing of large sample volumes
- Stability: Less affected by sample storage conditions than direct hematocrit measurement
The National Kidney Foundation guidelines specifically state that "MCV (from which Hct is calculated: MCV × erythrocyte count = Hct)" is the standard calculation method used in automated analyzers 1.
Important Clinical Considerations
Potential Sources of Error
Sample Storage: MCV is stable at room temperature for only 8 hours and for 24 hours when refrigerated. Prolonged storage can cause MCV to increase, resulting in falsely elevated calculated hematocrit values by 2-4% 1.
Hyperglycemia: In patients with high blood glucose, MCV can be falsely elevated, leading to falsely elevated calculated hematocrit 1.
Instrument Variability: There is greater variability across automated analyzers in estimation of RBC number and size (and hence in calculation of Hct) than in direct measurement of hemoglobin 1.
Clinical Implications
Because of these potential sources of error, many guidelines now recommend using hemoglobin rather than hematocrit for monitoring anemia, particularly in chronic kidney disease patients. The coefficient of variation for hemoglobin measurement is approximately one-half to one-third that of hematocrit 1.
Alternative Methods (Not Primary)
The other options listed are either not the primary method or have significant limitations:
Derived from a histogram: While histograms are used to analyze cell distribution, they are not the primary method for calculating the final hematocrit value.
Measuring overall conductivity of whole blood: This is not a standard method for hematocrit determination in modern automated analyzers.
Multiplying Hemoglobin by 3: This is an approximation sometimes used clinically (Hct ≈ Hgb × 3), but is not the primary method used by automated cell counters.
For accurate clinical laboratory results, understanding that hematocrit is calculated from directly measured RBC count and MCV is essential for proper interpretation of complete blood count results.