What formula indicates the correlation check between Hemoglobin (Hb) and Hematocrit (Hct) on an automated analyzer?

Formula for Correlation Check Between Hemoglobin and Hematocrit on Automated Analyzers

The formula that indicates the correlation check between hemoglobin (Hb) and hematocrit (Hct) on an automated analyzer is D. Hct = MCV × RBC. This is the calculation method used by automated analyzers to determine hematocrit values 1.

Understanding Automated Hematocrit Calculation

Automated hematology analyzers calculate hematocrit using the following process:

• Hematocrit is not directly measured but calculated from two other measured parameters
• The formula uses Mean Cell Volume (MCV) and Red Blood Cell count (RBC)
• The calculation is: Hct = MCV × RBC 1, 2

This differs from traditional manual methods where hematocrit is determined by centrifugation (microhematocrit method).

Why This Formula Is Important

Understanding this calculation is critical because:

1. Accuracy implications: Since Hct is calculated rather than directly measured, it's subject to errors in either MCV or RBC measurements
2. Storage effects: MCV is stable at room temperature for only 8 hours and refrigerated for only 24 hours, after which it increases, causing falsely elevated calculated Hct values 1
3. Hyperglycemia effects: In hyperglycemic states, MCV can be falsely elevated, resulting in falsely elevated calculated Hct 1
4. Analyzer variability: There is greater variability across automated analyzers in estimating RBC number and size than in measuring Hb directly 1

Correlation Between Hb and Hct

While not the formula used by analyzers, there is a general relationship between Hb and Hct that can be used as a quality check:

• In normal subjects, the Hct/Hb ratio is approximately 3:1 3
• This means Hct (%) is roughly three times the Hb value (g/dL)
• This relationship can be expressed as: Hct ≈ Hb × 3 4

However, this 3:1 ratio is not universal and can vary in certain conditions:

• In alpha-thalassemia, the Hct/Hb ratio is higher (approximately 3.5:1) 3
• In chronic kidney disease, the ratio remains close to 3:1 3
• The ratio may vary with extreme Hb values (<12 g/dL or ≥16 g/dL) 4

Clinical Implications

Understanding how automated analyzers calculate Hct is important because:

• Hemoglobin measurement is more stable and reliable than calculated hematocrit 1, 2
• Guidelines increasingly recommend using Hb rather than Hct for monitoring anemia 1
• When evaluating for blood loss, serial measurements of both parameters increase sensitivity 1
• A single normal value may mask early-phase bleeding 1

Common Pitfalls to Avoid

• Don't assume Hct values are directly measured by automated analyzers
• Be cautious when interpreting Hct values from samples stored for >8 hours at room temperature
• Consider the potential impact of hyperglycemia on calculated Hct values
• Remember that the Hct/Hb ratio of 3:1 is an approximation and may vary in certain conditions
• Don't rely on a single Hct measurement to rule out bleeding, as initial values may be normal despite significant blood loss 1