Hematology Calculations

The following are common calculations used in the hematology discipline.

RBC Indices

There are 3 RBC indices that are calculated from measured RBC parameters: MCH, MCV, MCHC.

Mean Cell Hemoglobin MCH (g/L)

Average amount of hemoglobin present per red blood cell.

$$MCH=\frac{HGB}{RBC}$$

Mean Cell Volume MCV (fL)

Average size (volume) of red blood cell.

$$MCV=\frac{HCT*1000}{RBC}$$

Mean Cell Hemoglobin Concentration MCHC (g/L)

$$MCHC=\frac{HGB}{HCT}$$

Rule of Three

In normal RBCs, the HCT value should be approximately 3x the hemoglobin. Used as a quick check of the CBC results for interfering substances or other issues with the blood.

$$\frac{HGB*3}{1000}=HCT\pm 0.03$$

Citrate Adjustment for High Hematocrit

Used when hematocrit > 0.55 L/L as high hematocrit means that there is less plasma present (therefore, the plasma-anticoagulant ratio is incorrect). This can result in excess sodium citrate, which can falsely prolong the clotting time.

$$C=(1.85*10^{-3})(100-HCT)(V)$$

• C = citrate volume that needs to be remaining in tube
• HCT = hematocrit
• V = volume of blood drawn

Then, use this value to determine that volume of citrate that needs to be removed.

$$R=V_c-C$$

• R = volume to be removed from tube
• V_c = volume of citrate present in tube
• C = citrate volume remaining

Reticulocyte Count (%)

For 1000 RBCs

$$\text{Relative reticulocyte count}=\frac{\text{Retics per 1000 RBCs}}{10}$$

For >1000 RBCs

$$\text{Relative reticulocyte count}=\frac{Retics*1000}{\text{RBC count}}$$

Corrected Reticulocyte Count (%)

The retic count should be corrected if there is a low hematocrit (less RBCs present), as this will falsely increase the reticulocyte count.

$$\text{Corrected retic count}=\frac{\mathrm{\%}\text{ Relative Retics * Patient HCT}}{\text{Average Normal HCT}}$$

• Average normal HCT (male): 0.45 L/L
• Average normal HCT (female): 0.42 L/L

Absolute Reticulocyte Count (x10⁹/L)

To determine the actual number of retics present in the blood.

$$\text{Abs Retic}=\frac{\mathrm{\%}\text{ Relative Retics * RBC}}{100}$$

Corrected WBC Count for NRBC

Used to correct the WBC count for NRBCs as they may be mistaken for WBCs by the analyzer, giving a falsely elevated result.

$$\text{Corrected WBC}=\frac{\text{100 * WBC Count}}{100+NRBC}$$

• NRBC = # NRBCs counted per 100 WBC

International Normalized Ratio (INR)

$$INR={\left(\frac{\text{Patient PT}}{\text{Control PT}}\right)}^{ISI}$$

Neubauer Chamber Total Cell Counts

To determine cell counts based on the amount of cells counted in the Neubauer Chamber. Values change depending on the fluid and cell being counted (fluid may be diluted, and the amount of squares counted depends on the cell type)

Neubauer counts calculation for various fluids.

• Average cell count = average (taken using both sides of the chamber)
• DF = dilution factor (reciprocal of dilution)

Fluid	Cell Type	Squares Counted	Dilution	DF
CSF	All cells! (WBC & RBC)	9 (all)	Clear: none	1
			Bloody WBC: 1:2	2
			Bloody: 1:10-1:200	10-200
Whole Blood	WBCs	4 (outer corners)	1:20	20
	Platelets	1 (large central square)	1:100	100
Synovial Fluid