Hematology Laboratory



Hematology Laboratory




RULE OF THREE


Hematology is the study of the cells within the blood. These include the white blood cells (WBCs), platelets, and the cells in the largest quantity, the red blood cells (RBCs). The RBCs carry oxygen from the lungs to the tissues and carbon dioxide from the tissues back to the lungs where it is exhaled. The oxygen is carried by hemoglobin molecules. The quantity of hemoglobin can be determined spectrophotometrically by automated hematology instruments. If the hemoglobin concentration is low, the oxygen-carrying capacity of the RBC is decreased as well. This will lead to the condition called anemia. The most common cause of anemia is iron deficiency. Iron is vital to the structure of the hemoglobin molecule, and if the iron concentration is low, the hemoglobin concentration will be adversely affected.


A second very common hematology test that is performed is the hematocrit. This is the ratio of the blood cells to plasma expressed as a percent. The reference range for hematocrit is 37% to 47% for adult women and 42% to 52% for adult men, meaning that approximately half of the whole blood in the body is made up of cellular components; the remainder is the liquid plasma portion. If the number of RBCs is decreased, the hematocrit will be decreased. If a person is anemic, the hemoglobin and hematocrit will both be decreased. The adult reference range for hemoglobin is 12-18 g/dL for men and 12-16 g/dL for women.


There is a relationship between the hematocrit and the hemoglobin values that is useful as a quality assurance tool. The hematocrit is usually three times the hemoglobin value plus or minus 3. This is known as the rule of three. If the hematocrit is known then the hemoglobin value can be estimated and vice versa. This is a good rule of thumb to keep in mind even when the hemoglobin or hematocrit is either below or above the normal range. When the rule of three is not found, it may indicate an error or an abnormality in the size of the RBC or the amount of hemoglobin in the RBC. This error may be an instrument malfunction of some sort, a mix up in patient identification, a problem in reporting the correct result, or a problem in the patient’s RBCs.






Example 9–1

A finger stick was performed on a young boy in a pediatrician’s office. The medical laboratory technician performed a spun hematocrit while the medical assistant performed the hemoglobin using a CLIA-waived hemoglobin instrument. The hematocrit result was 45% and the hemoglobin was 11 g/dL. Are these results consistent with the rule of three?


No, these results do not follow the rule of three. The hemoglobin should be 15 g/dL if the hematocrit is accurate, or the hematocrit should be 33% if the hemoglobin was accurate. The medical laboratory technician should investigate the discrepancy.





RED BLOOD CELL INDICES


The size of the RBC is important in the treatment of anemias. Some anemias cause the RBC to be smaller than normal; others cause it to be larger. The relationship among size, hemoglobin, and hematocrit was mathematically determined by Wintrobe in the 1920s. Three formulas were developed to describe RBC morphology and to aid in the classification of anemia.



Mean Corpuscular Volume


The first formula is the mean corpuscular volume (MCV). The MCV describes the average volume or size of the RBC in femtoliters (fL) and is calculated by the following formula:


MCV(fL)=Hematocrit(%)×10RBC count(1012/L)


image

The normal range for MCV is 80 to 100 fL. MCV values less than 80 fL suggest RBCs that are smaller in size, whereas MCV values greater than 100 fL suggest RBCs that are larger than normal.




Mean Corpuscular Hemoglobin


The second formula is the mean corpuscular hemoglobin (MCH). As the name implies, it represents the average amount of hemoglobin present in the individual RBC in units of picograms (pg), significant only to the nearest tenth. It is calculated by the following formula:


MCH(picograms)=Hemoglobin(g/dL)×10RBC count(1012/L)


image

The normal range for MCH is 27 to 31 pg.




Mean Corpuscular Hemoglobin Concentration


The third calculation is the mean corpuscular hemoglobin concentration (MCHC). This calculation is a ratio of the hemoglobin to the hematocrit and is expressed in terms of percent and significant only to the nearest tenth. The MCHC represents the average hemoglobin content in the patient’s RBC population.


MCHC=Hemoglobin(g/dL)×100Hematocrit(%)


image

The normal range for MCHC is 32% to 36%.





CORRECTION OF THE WBC COUNT FOR NUCLEATED RBCs


Nucleated RBCs (NRBCs) are immature RBCs that have not lost their nucleus. They are also called orthochromic normoblasts or metarubricytes. They are the last immature RBC that contains a nucleus, and when they further mature they become reticulocytes. Normally the NRBC is found only in the bone marrow. In times of severe anemic stress they may be found in the peripheral blood and they are also normally found in the peripheral blood of healthy newborns but disappear after a few days. Premature infants will have NRBCs present at birth and they may be present longer than a week.


Since NRBCs contain a nucleus, the WBC automated cell count should be adjusted downwards as the instrument may count the NRBC nucleus as a WBC. A correction calculation is usually performed when greater than 5 NRBCs per 100 WBCs are seen on a peripheral smear.


The formula to correct for NRBCs is:


Automated WBC count×100(NRBC per100WBCs)+100


image




CELL COUNTING BY THE HEMACYTOMETER METHOD


The majority of RBC, WBC, and platelet counts are performed by automated hematology analyzers. Occasionally, the count may be too low for the instrument to count the cells accurately. WBCs in body fluids are often counted manually. In these instances, cells are counted with the aid of a microscope using a counting chamber called a hemacytometer (Figure 9–1). The most common hemacytometer used in clinical hematology is the Neubauer hemacytometer. This hemacytometer consists of two identical counting chambers. Each chamber contains an etched grid constructed to have a total surface area of 9 mm2. The chambers are identical, so that a sample can be counted in duplicate using the same hemacytometer. The count from each chamber should be within 10% of each other to ensure an accurate count. A dilution of whole blood is used and a small amount of diluted sample is allowed to flow between the coverslip and hemacytometer chamber. The distance between the coverslip and the surface of the hemacytometer is 0.1 mm.



Figure 9–1 is a schematic of a Neubauer hemacytometer grid in which each of the large squares is numbered 1 to 9. WBCs are counted in the four corner squares (squares Nos. 1, 3, 7, and 9), whereas platelets and RBCs are counted in the central square (square No. 5). Each large square is 1-mm2 in area. The large squares are further divided into 16 smaller squares to facilitate ease of counting the WBCs. The center square is divided into 25 small squares. Each of the 25 small squares is further divided into 16 smaller squares for RBC and platelet counting. The small square A is shown enlarged in Figure 9–2.




White Blood Cell Count


When the WBC count is abnormally elevated or decreased or the quantity of WBCs in fluid is requested, the hemacytometer method may be used to quantify the total WBC count. Using a micropipette, a 120image dilution is usually used when counting the total WBC count. Occasionally, a 110image dilution is performed if the WBC count is abnormally decreased. WBCs are counted using high power or 400× magnification in each of the 16 smaller squares contained in each of the four corner squares. The area of each large square is 1 mm2, and the depth is 0.1 mm. Therefore, the volume in each large square is 0.1 mm3 (volume = area × depth).


The mathematical formula to calculate the number of cells per cubic millimeter is as follows:


No.cells/mm3=No.cellscounted×depthfactor×dilutionfactorTotal area counted


image

where:


Area counted = number of large squares counted


Depth factor = reciprocal of depth [1/(1/10)] = 10


Dilution factor = reciprocal of dilution


The WBC count reference range in adults is 4–11 (109/L)





Example 9–7

A WBC count was performed on a sample that was abnormally decreased below the linearity of the automated cell counter. A 110image dilution was performed, and all WBCs in each of the four corner squares of both grids of the hemacytometer were counted. A total of 75 WBCs were counted. What is the total WBC count?


In this problem, all eight squares were counted; therefore, the area counted would be 8. The dilution factor would be 10, as a 110image dilution was performed. Substituting into the basic formula for cell counting, the following equation is derived:


image

No. WBCs/mm3 = 938


For many WBC counts, the dilution performed is 120image, and all WBCs within the four corner squares are counted. Therefore, within the volume in one large square is the amount of WBCs per 1200image mm3 of blood (0.1mm3×120)image This amount is multiplied by the quantities of squares counted (four). Thus, the typical WBC count quantifies the amount of WBCs per 50 mm3 ((1200×4)image × 4) of blood. The factor of 50 can be used to quickly calculate the WBC count using a hemacytometer. Simply multiply the total number of cells counted in the four large squares times 50 to yield the total WBC count/mm3



The mathematical formula to calculate the factor is as follows:


Factor = 1/area × depth factor × dilution factor


Area counted = number of large squares counted


Depth factor = reciprocal of depth [1/(1/10)] = 10


Dilution factor = reciprocal of dilution = [1/(1/20)] = 20


Therefore the factor=14×10×20=50


image


Example 9–8

A WBC count was performed using a hemacytometer and a 120image dilution. There were 140 WBCs counted in the four large squares. Using the factor method, what is the total WBC count?


The factor=14×10×20=50


image

Multiply the amount of cells counted by the factor of 50:


image

Therefore, the WBC count is 7000/mm3. There are three ways that WBCs might be reported. As mm3 is equal to 1 μL, the count may be reported as 7000/μL. In addition, 1 fL is equal to 1000 μL, so many laboratories may report the count as 7.0 × 109/L.



Additional Examples




Since the four large squares were used, and the dilution was 120image, the factor method can be used.


image


Since the four large squares were used, and the dilution was 120image, the factor method can be used.


image


Red Blood Cell Count


As there are so many more RBCs than WBCs, the cells are diluted 1200image. The RBCs are counted in the four corner squares and the center square within the large central square. Each of these small squares is subdivided into 16 smaller squares to facilitate counting. Figure 9–2 is an enlargement of one of the center squares to aid in visualization of the 16 small squares. Therefore, there are a total of 80 squares in which the RBCs are counted. Each of the 25 small squares within the large central square is 0.04 mm2 (0.2 mm × 0.2 mm). Because five of these squares are counted, the total area counted is 0.04 × 5, or 0.2 mm2. Manual RBC counts in EDTA-anticoagulated blood are rarely performed in the United States but may still be used in educational settings as a method to teach the use of the hemacytometer, and in laboratories outside of the United States. Manual RBC counts in fluids such as spinal fluid are performed and are covered in the fluid section of this chapter. As in WBC counting, a factor can be calculated and used to easily quantify RBCs.


The formula to calculate the factor for RBC counts is as follows:


Factor = 1/area × depth factor × dilution factor


Area counted = area of small squares counted = 0.2 mm2


Depth factor = reciprocal of depth [1/(1/10)] = 10


Dilution factor = reciprocal of dilution = [1/(1/200)] = 200


Therefore the factor=100.2×10×200=10, 000


image



Platelet Count


Platelets are small fragments of cells found in the bone marrow called megakaryocytes. Platelets function in the initial phase of the coagulation process. The reference range for platelets is 150–400 (109/L). All 25 squares in the central square of the hemacytometer are used to count platelets. Remember, that within each of the 25 squares, there are 16 smaller squares, so the total amount of squares counted for platelets will be 400. As each of the 25 squares has an area of 0.04 mm2, and all 25 squares are counted, the total area counted is 1 mm2 (0.04 × 25). A 1100image dilution is performed on the whole blood for platelet counting. Just as for WBC and RBC counts, platelet counts can also have a calculated factor.


The mathematical formula for the factor for platelets is as follows:


Factor = 1/area × depth factor × dilution factor


Area counted = area of small squares counted


Depth factor = reciprocal of depth [1/(1/10)] = 10


Dilution factor = reciprocal of dilution = [1/(1/100)] = 100


Therefore the factor=11.0×10×100=1,000


image





Example 9–10

A platelet count was performed using a 1100image dilution and counting all platelets found in the 400 smallest squares of the central square in the hemacytometer. One hundred fifty platelets were counted. Using the factor method, what is the platelet count?


The factor for platelets is 1000. As 150 platelets were counted, the total platelet count is as follows:


image



BODY FLUIDS


RBC, WBC, and platelet counts are not the only cell counts performed in the hematology laboratory. Cells from CSF, transudates, and exudates, along with semen analysis may be performed. The hemacytometer is used to count both RBCs and WBCs in the different types of fluids and to provide both qualitative and quantitative sperm counts.




Cell Counts for Synovial, Pleural, Pericardial, and Peritoneal Fluid


Samples for these fluids are usually collected in a heparinized or an EDTA tube. Table 9–1 contains the body fluid normal results.


Nov 18, 2017 | Posted by in PHARMACY | Comments Off on Hematology Laboratory
Premium Wordpress Themes by UFO Themes
%d bloggers like this: