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Platelet Counts
Platelets
Platelets are the smallest formed elements in the blood, normally ranging in size from 2-4 microns. They are actually fragments of cytoplasm that have broken off platelet precursors. Platelets function in the coagulation of blood.
Platelets are difficult to count due to:
(1) small size - often hard to differentiate from bacteria and debris
(2) adhesiveness - affinity for adhering to glass
(3) aggregation - tendency to clump together
When collecting a specimen for a platelet count, EDTA is the anticoagulant of choice because it keeps the platelets from clumping together.
A normal platelet count is 150,000-350,000/mm3. A platelet count should always be checked on the smear to make sure it looks accurate. To estimate a platelet count from a blood smear you regard each platelet seen per oil power field as being equivalent to 20,000 platelets/mm3. By determining the average number of platelets seen per oil field and multiplying this number by 20,000 you can estimate the platelet count. For example, if you see 10 platelets per oil field your estimated count would be: 10 x 20,000 or 200,000/mm3. Usually 7-20 platelets per OIF represents a normal platelet count. Care must be taken to stay in an appropriate area of the slide. The RBC's should barely be touching one another in the area of the estimate.
UNOPETTE Platelet Counts
I. Materials needed:
WBC/Platelet unopette
20 l pipette (provided)
hemacytometer chamber
gauze squares
applicator sticks
timer
II. Procedure: PERFORM ALL COUNTS IN DUPLICATE
A. Dilute blood using Unopette system technique. The same unopette is used for WBC's and for Platelets. The dilution is 1 to 100. Make a separate dilution for each side of the hemacytometer.
1. Puncture diaphragm: Push the tip of the pipette shield firmly through the diaphragm in the neck of reservoir.
2. Remove shield from pipette assembly and touch pipette tip to blood. The pipette will fill by capillary action. Filling is complete and will stop automatically when blood reaches end of capillary bore in neck of pipette.
3. Wipe excess blood from outside of capillary pipette.
4. Squeeze reservoir slightly to force out some air, do not expel any liquid. Maintain pressure on reservoir.
5. Cover upper opening of pipette with index finger and seat pipette securely in reservoir neck.
6. Release pressure on reservoir and remove finger from pipette opening.
7. Squeeze reservoir gently two or three times to rinse capillary bore and upper part of pipette but do not squeeze out any liquid.
B. Mix thoroughly and let stand for ten (10) minutes to allow red cells to hemolyze.
C. After 10 minutes, mix solution gently and rinse pipette by delivering about 3 drops onto a gauze square.
D. Carefully charge hemacytometer with diluted blood by gently squeezing sides of reservoir until properly filled.
E. Place the hemacytometer in a moist chamber, e.g., covered Petri dish, bottom lined with moistened gauze squares, and allow to stand for 10 minutes for cells to settle.
F. Using bright light or phase microscope, place hemacytometer on microscope stage. With low power objective, bring ruled area into focus.
G. Switch to 43X objective, locate large center square, and bring into focus.
H. Count platelets in all 25 small squares within the large, center square. This represents an area of 1 square millimeter.
CALCULATIONS:
I. Multiply the number of cells counted by 1,000 to get the total platelet count.
Example: No. of cells counted = 250
250 x 1,000 = 250,000 platelets/mm3
II. Interpretation of Results:
Platelet counts below normal indicate thrombocytopenia and are found associated with aplastic anemia, pernicious anemia, acute leukemias and idiopathic thrombocytopenia purpura. Platelet counts above normal indicate thrombocytosis and are found associated with polycythemia vera, hemolytic anemia, chronic myeloproliferative disorders, and after splenectomy.
IV. Sources of Error:
A. Light adjustment is critical. If the condenser is not lowered it will fade out the platelets.
B. Bacteria and debris can be misinterpreted as platelets. This type of artifact is generally much more refractile than platelets.
C. Clumping of platelets sometimes occurs, and if so the specimen must be recollected. EDTA is the anticoagulant of choice for preventing platelet clumping.
D. General hemacytometer errors, i.e. overloading chamber, counting wrong borders, etc.
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موضوع: Platelet Counts 
