LEC 3 - Laboratory Evaluation of Platelets

150-450 x 109 per liter

platelets reference range

thrombocytopenia

A decreased count is known as

thrombocytosis

An increased count is known as

flow cytometry

electrical impedance

Most frequently used automated methods in determining platelet quantity

Electrical Impedance

Automated method that utilizes the Coulter principle

Electrical Impedance

A stream of cells in a suspension (blood) passes through a small

aperture across which an electrical current is applied and thus be differentiated based on the size (amplitude

generated).

False (decrease)

T or F: A disadvantage of electrical impedance is when giant platelets are present in the sample, because of their

large size, they can be mistaken for RBCs or WBCs and that can

falsely increase the platelet count.

False (electrical impedance)

T or F: One disadvantage of flow cytometry is when severely anemic patient with many microcytic RBCs (small

RBCs) in the sample can be mistaken for platelets, and that can falsely elevate the platelet count.

Flow Cytometry

Automated system where the suspension of cells or particles is aspirated into a channel surrounded by a narrow fluid system. The cells pass through one at a time through a focused laser beam

Forward angle

this angle scatter helps assess the size of passing cells, providing information about their dimensions.

Side Scatter

this angle scatter assesses the internal complexity of cells, including the presence of granules or certain organelles.

8-20

how many platelets estimate are there per oil immersion oil field in a properly prepared smear

at least 10

how many different OIF's should be carefully examined in a PBS where the erythrocytes barely touch or just overlap

20,000

The average number in 10 fields can be multiplied by a factor of __________ to arrive at an approximation of the quantitative platelet concentration

Platelet Satellitism

Platelet Clumping

Giant Platelets

what are the instances/cases when an examination of the blood film is necessary even after automated counting:

Platelet Satellitism

this is when platelets appear to encircle the neutrophils almost entirely and can result in inaccurate automated platelet counts

Platelet Satellitism

this usually appears in a pbs when EDTA is used as anticoagulant

Commonly seen from blood taken from difficult extractions where the blood was in the hub of the

syringe too long and they started to aggregate

False (low)

T or F: Platelet clumping Leads to falsely high platelet counts on machines.

Giant Platelets

Seen in certain disorders such as immune thrombocytopenic purpura and Bernard- Soulier syndrome and leads to falsely low platelet counts and falsely high RBC counts in automated counters.

Re-smear using sodium citrate anticoagulated blood.

In cases where platelet satellitism is observed what possible remedy can a medical technologist can do?

Re-draw / re-extraction

In cases where platelet clumping is observed what possible remedy can a medical technologist can do?

BRECKER-CRONKITE MANUAL METHOD OF PLATELET COUNTING

manual platelet count principle where whole blood is diluted with 1% ammonium oxalate, which completely hemolyzes erythrocytes. Then platelets can then be counted using a phase contrast microscope.

True

T or F: in Brecker-Cronkite method EDTA-anticoagulated whole blood is preferred but capillary blood may be used if venous blood is not available.

False (performed within 5 hours)

T or F: When using Brecker-Cronkite method in counting platelets the assay must be performed within 6 hours of the time that the blood specimen is collected or within 24 hours, if the specimen is refrigerated

True

T or F: 1% ammonium oxalate must be stored refrigerated and filtered prior to use; discarded if

turbid

5

how many squares are to be counted on each side of the hemacytometer in Brecker-Cronkite method

True

In cases of thrombocytopenia, the dilution may have to be adjusted

to 1:100 in an RBC pipette or 1:20 in a WBC pipette; the new dilution

- Specimen age

- Platelet clumping

- Debris in the diluting fluid

- Incorrect dilution

- Platelet adherence to glass

what are the sources of error in manual platelet counting

True

T or F: diluting fluid must be filtered prior to every use and any gross turbidity is already enough reason to dispose of it

Bleeding time

This is an in vivo measurement of platelet adhesion on locally injured vascular sub-endothelium which provides an estimate of the integrity of the platelet plug and thereby measures the interaction between the capillaries and platelet.

False (stop bleeding)

T or F: Bleeding time reflects platelet function by timing the length of time two standardized punctures of the ventral forearm take to initiate bleeding while a blood pressure cuff inflated to about 40 mmHg is in place on the upper arm

True

T or F: Thrombocytopenia can affect bleeding time by prolonging the test

3-8 minutes

Reference time interval for bleeding time

True

T or F: As the platelet count drops below 100 x 109/L, the bleeding time increases progressively from a normal time of 3-8 minutes to more than 30 minutes.

Von Willebrand Disease (VWD)

Bernard-Soulier syndrome

What platelet disorders can clinically prolong bleeding time

Simplate or Surgicutt method

This procedure of bleeding time is similar to the Ivy method with the difference being the use of a Simplate bleeding device (shown) instead of a lancet.

Duke’s method

Mielke’s method

Obsolete Methods in bleeding time that are deemed outdated and difficult to standardize or reproduce

clot retraction

When platelets are activated, the process can be visibly observed, and is seen by the separation of a clot from the sides of a test tube

Hematocrit

What parameter can be used to help in the degree of clot retraction observation

True

T or F: The smaller the hematocrit, the greater the degree of clot retraction

True

T or F: The degree of clot retraction is directly proportional to the number of platelets and inversely proportional to the hematocrit and the level of fibrinogen

active fibrinolysis

clot dissolution

What are some conditions that clot retraction time can be unreliable

False (begin within 1 hour)

T or F: At 37°C, normal clot retraction should begin within 6 hours and be complete by 24 hours

ADP

collagen

epinephrine

snake venom

arachidonic acid

thrombin

ristocetin

Examples of agonists that can be used to aggregate platelets in vitro

Platelet Aggregation / Aggregometry

The principle of this test is that platelet-rich plasma is treated with a known aggregating agent (agonists) and if aggregated, the degree of light transmittance in the form of varying levels of cloudiness or turbidity can be measured using a spectrophotometer.

True

T or F: Platelet aggregometry is performed on a turbidimetric aggregometer and the occurrence of platelet aggregation will result in increased light transmittance

Immune thrombocytopenia

Antibodies against platelets may appear in the plasma of patients in certain conditions such as

True

T or F: Antiplatelet antibodies have also been implicated in platelet refractoriness