Summary
Light transmission platelet aggregation tests are important for diagnosing platelet
function defects. However, uncertainties exist about the best procedures to determine
aggregation reference intervals. We investigated methods for determining reference
intervals for light transmission aggregation tests, using the % maximal aggregation
values for prospectively collected data on healthy control samples. Reference intervals
for samples tested at 250 x 109 platelets/l were determined by mean ± 2 standard deviations and non-parametric analyses.
To establish reference intervals for tests on thrombocytopenic subjects, regression
analyses were used to estimate 95% confidence limits for % maximal aggregation, according
to sample platelet counts, using data for control samples diluted to match the platelet
count of undiluted thrombocytopenic patient platelet-rich plasma samples. For samples
tested at 250 x 109 platelets/l, non-parametric analyses described 95% of data for healthy control samples
better than mean ± 2 standard deviations. For samples tested at lower counts, to match
thrombocytopenic samples, the % maximal aggregation was influenced by platelet count
and derived limits were wider at very low platelet counts for almost all agonists.
With ristocetin, it proved feasible to test samples with very low platelet counts
to exclude Bernard-Soulier syndrome and type 2B von Willebrand disease. Non-parametric
analyses should be the preferred method to establish light transmission aggregation
reference intervals for samples tested at normal platelet counts. The derived limits
for thrombocytopenic samples provide guidance for evaluating thrombocytopenic platelet
function disorders, including which agonists to test, based on the sample platelet
count.
Keywords Platelet function - platelet aggregation - maximal aggregation - reference intervals
- thrombocytopenia
