PLATELET COUNT
PLATELETS
• Aka “THROMBOCYTES” or
“THROMBOPLASTIDS”
• Fragments of megakaryocyte, non-nucleated,
irregular in size
• 2-4 m in diameter
• Life span: 8-11 days in circulation (turnover of
35,000/L/day)
• Average count: 150-450 x 109/L
• Platelet distribution:
o 2/3 in the circulation
o 1/3 in the spleen
Function:
• Hemostasis
o Aggregation and formation of primary
platelet plug
o Thromboplastic activity – to initiate clotting
as the source of thromboplastin
o Clot retraction (thrombothenin)
o Maintain capillary integrity
• Increase platelet:
o Polcythemia vera
o Idiopathic thrombocythemia
o Chronic myelogenous leukemia
o Splenectomy
• Decrease platelet:
o Thrombocytopenia purpura
o Aplastic anemia
o Acute leukemia
o Gaucher’s disease
o Pernicious anemia
o Chemotherapy
o Radiation therapy
Platelets are difficult to count because:
• They easily disintegrate
• Small, colorless, refractile bodies
• Difficult to distinguish from debris
• Unevenly distributed in the blood
• Have a tendency to clump with each other
***EDTA helps to decrease platelet clumping, but the mean
platelet volume (MPV) will increase during the first hour in
the tube
***BEST to measure MPV – 1-3 hours after obtaining the
specimen
METHODS
1. INDIRECT METHOD – platelets are counted in relation
to 1000 RBCs in the smear
A. Dameshek method
B. Fonio’s method
C. Olef’s
D. Cramer and Bannerman
E. Modified Indirect Platelet Count
- count platelet in 10 consecutive OIO field and
multiply by 200
2. DIRECT METHOD – most accurate way of platelet
count uses RBC pipet, diluting fluid or Unopette for
platelet and WBC count
A. Rees – Ecker
B. Guy’s and Leake method
C. Phase Microscopy method (Brecher-Cronkite)
- recommended method
- 1% ammonium oxalate
D. Walker and Sweeney method
E. Unopette method – 1:100
F. Van Allen’s method
G. Nygard’s method
H. Tocantin’s method
I. Kristerson as modified by Leiupert
J. Feissly and Ludin method
AUTOMATION:
A. Impedance counting
B. Laser Light scattering
a. Coulter Counter
b. Technicon Hemolab
c. Fisher Autocytometer
d. MK-4 Platelet Counter
Common Artifacts in Automated Platelet Counting
• Non-technical factors may produce falsely low
platelet count in instruments but normal in smear
o Platelet cold agglutinins
o Abnormal amount of plasma protein in
various paraproteinemias
o Previous contacts of platelet with foreign
surface such as dialysis membrane
o Giant platelets
o Platelet satellitosis
o Lipemia
o EDTA-induced platelet clumping
• Platelet count is low on smear but normal in
instrument (analyzer)
o Patient receiving chemotherapy for acute
leukemia/lymphoma
o White cell cytoplasmic fragments are
counted as platelets
o RBC fragments are counted as platelets
• High platelet count may be due to:
o Microspherocytes
o Fragments of leukemic cells
o Pappenheimer bodies
Rees-Ecker
Sodium citrate – 3.8g
Brilliant Cresyl Blue – 0.1g
Neutral HCHO – 0.2mL
Distilled water – 100mL
NOTE:
CLUMPS – use 0.109 M Sodium Citrate as anticoagulant
and multiply by 1.1
50 platelet – 1:20 dilution
platelet – 1:200 dilution
REES-ECKER Method:
- count platelet in 25 intermediate squares
= platelet ct x 10 x 200 x 25 (1)
25 OR
- count platelet in 4 large corner squares
= platelet ct x 10 x 200
4 OR
- count platelet in 5 central squares
= platelet ct x 10 x 200 25 (5)
5 OR
- count platelet in each ruled area
= platelet ct x 10 x 200
2
COAGULATION
BLEEDING TIME
• Reflects both platelet number and platelet functional
integrity (vascular response to injury)
• “global test” – for the adequacy of primary
hemostasis
• If standardized, considered the best test
