Page 202 - Basic Monitoring in Canine and Feline Emergency Patients

platelets unable to aggregate. In addition, light
Box 9.1. Preanalytical variables that transmission aggregometry can only be performed
VetBooks.ir may result in erroneous results of removed from the sample so as to not interfere
on platelet-rich plasma because all red cells must be
commonly used coagulation tests.
Falsely lowered platelet count: with the light transmission methodology. The need
to create platelet-rich plasma in turn leads to
Platelet clumping
Breed-specific differences in platelet count increased processing of samples before testing,
Citrate tube for CBC analysis which increases the risk of activating platelets sim-
Falsely prolonged BMBT: ply by sample handling, potentially also influencing
Anemia the results of aggregation testing.
Low serum fibrinogen concentration
Thrombocytopenia
Falsely prolonged PT/aPTT: Secondary hemostasis
Inappropriate tube volume Appropriate filling of the 3.2% citrate tube is very
Elevated hematocrit (hemoconcentration) important as the 1:9 citrate to blood ratio is crucial
Hemolysis for accuracy of tests that require the addition of
Lipemia
Prolonged (unfrozen) storage of plasma calcium to the sample to initiate clotting (PT, aPTT,
Contamination with anticoagulants viscoelastic testing). Underfilling of tubes will result
Clotting in sample in a higher concentration of citrate in the plasma. In
Falsely shortened PT/aPTT: this situation, the standardized amount of calcium
Severe anemia added to the sample may not be enough and clot-
Contamination with Ca ting times may be erroneously prolonged. A tube
aPTT, activated partial thromboplastin time; BMBT, filled to <90% of the recommended volume should
buccal mucosal bleeding time; Ca, calcium; CBC, be considered underfilled. Samples with elevated
complete blood count; PT, prothrombin time. hematocrits have less plasma per unit volume, and
therefore result in a decreased plasma:citrate ratio
in the tube. This results in an artificial prolongation
of clotting times that increases exponentially as
hematocrit rises above 55%. In humans, these arti-
Platelet function tests
facts have been shown to be clinically relevant and
While the BMBT is used as a screening test for adjustment of citrate levels in samples with hemato-
thrombocytopathias, it is neither sensitive nor spe- crit (Hct) >55% is recommended (see Marlar et al.,
cific. While it will obviously be prolonged in 2006). A simplified method in humans is to remove
thrombocytopenic patients regardless of platelet 0.1 mL of sodium citrate from a 5 mL 3.2% tube
function, low fibrinogen levels and significant ane- prior to sample collection if the Hct is >55%. To the
mia can also prolong bleeding times. author’s knowledge, this has not been validated in
Platelet aggregation testing can be influenced by veterinary species.
many pre-analytical factors. In humans (and likely Conversely, overfilling of tubes or severe anemia
in animals), things like the time of day, diet, exer- (greater amount of plasma present per same vol-
cise, and supplements have all been shown to ume of whole blood) will result in a lower concen-
affect platelet aggregation. In addition, interpreta- tration of citrate in the plasma, which may falsely
tion of platelet aggregation results as normal or shorten clotting times. However, one study in
abnormal is difficult. A study by Blois et al. (2015) humans (Siegel et al., 1998) did not find clinically
found high variation in platelet function testing for relevant shortening of clotting times in patients
healthy dogs, implying that interpreting trends with hematocrits as low as 14%. Contamination
within one individual is likely more appropriate of the sample with additional calcium may also
than comparing a patient’s results to ‘normal’ ref- falsely shorten clotting times, and contamination
erence intervals. with other anticoagulants (such as EDTA or
Samples anticoagulated with EDTA cannot be heparin) may falsely prolong clotting times.
used for platelet aggregation testing as EDTA irre- Therefore, blue-top tubes should be filled first or
versibly disassociates the GPIIb/IIIa receptor from only after non-additive (red-top) tubes to prevent
platelets and prevents fibrinogen binding, making cross-contamination.

194 E.J. Thomovsky and A.C. Brooks