Is automated platelet counting still a problem in thrombocytopenic blood?

Extended sample storage for platelet function testing in healthy dogs

BACKGROUND

Platelet function testing is important for monitoring the effects of antiplatelet therapy but is not readily used due to time constraints for testing and the need for specialized equipment.

OBJECTIVES

This study evaluated the effects of various storage methods on selected platelet function tests to determine if delayed platelet function testing is feasible in canine blood samples. Our hypotheses were that platelet function would not decline during storage and, thus, no differences in test results would be found over time.

METHODS

Thirteen healthy dogs were studied. Citrated blood samples were tested with a Platelet Function Analyzer-200 (PFA), which mimics high-shear conditions, using P2Y and CADP cartridges, after being held at room temperature for 2 h and refrigerated for 24 and 48 h. Plateletworks (PW), which measures aggregation based on platelet counting, was performed on an optical hematology analyzer using 10-min-old native samples, citrated samples held at room temperature for 3-4 h and refrigerated for 24 and 48 h, and samples stored in the preservative solution, AGGFix, up to 7 days.

RESULTS

PFA closure times increased with storage, especially with the P2Y cartridge. Median aggregation with fresh PW was 94%, and this was maintained at all time points (range of median values 88%-94%). Most samples showed decreased, yet still robust (>70%), aggregation with longer storage. Spontaneous aggregation in citrate was noted in most dogs. AGGFix stabilized platelet aggregates to allow for delayed testing.

CONCLUSIONS

Delayed platelet function testing is feasible, but ranges of expected values may differ from tests using fresh samples.

Reproducibility of Manual Platelet Estimation Following Automated Low Platelet Counts

OBJECTIVES

Manual platelet estimation is one of the methods used when automated platelet estimates are very low. However, the reproducibility of manual platelet estimation has not been adequately studied. We sought to assess the reproducibility of manual platelet estimation following automated low platelet counts and to evaluate the impact of the level of experience of the person counting on the reproducibility of manual platelet estimates.

METHODS

In this cross-sectional study, peripheral blood films of patients with platelet counts less than 100 × 10⁹/L were retrieved and given to four raters to perform manual platelet estimation independently using a predefined method (average of platelet counts in 10 fields using 100× objective multiplied by 20). Data were analyzed using intraclass correlation coefficient (ICC) as a method of reproducibility assessment.

RESULTS

The ICC across the four raters was 0.840, indicating excellent agreement. The median difference of the two most experienced raters was 0 (range: -64 to 78). The level of platelet estimate by the least-experienced rater predicted the disagreement (p = 0.037). When assessing the difference between pairs of raters, there was no significant difference in the ICC (p = 0.420).

CONCLUSIONS

The agreement between different raters using manual platelet estimation was excellent. Further confirmation is necessary, with a prospective study using a gold standard method of platelet counts.

Platelet factor 4 is a negative autocrine in vivo regulator of megakaryopoiesis: clinical and therapeutic implications

Platelet factor 4 (PF4) is a negative regulator of megakaryopoiesis in vitro. We have now examined whether PF4 regulates megakaryopoiesis in vivo by studying PF4 knockout mice and transgenic mice that overexpress human (h) PF4. Steady-state platelet count and thrombocrit in these animals was inversely related to platelet PF4 content. Growth of megakaryocyte colonies was also inversely related to platelet PF4 content. Function-blocking anti-PF4 antibody reversed this inhibition of megakaryocyte colony growth, indicating the importance of local PF4 released from developing megakaryocytes. The effect of megakaryocyte damage and release of PF4 on 5-fluorouracil-induced marrow failure was then examined. Severity of thrombocytopenia and time to recovery of platelet counts were inversely related to initial PF4 content. Recovery was faster and more extensive, especially in PF4-overexpressing mice, after treatment with anti-PF4 blocking antibodies, suggesting a means to limit the duration of such a chemotherapy-induced thrombocytopenia, especially in individuals with high endogenous levels of PF4. We found that approximately 8% of 250 healthy adults have elevated (> 2 times average) platelet PF4 content. These individuals with high levels of platelet PF4 may be especially sensitive to developing thrombocytopenia after bone marrow injury and may benefit from approaches that block the effects of released PF4.

Platelets in the paediatric population: the influence of age and the limitations of automation

Accurate and precise platelet counting is important for the clinical management of children with platelet disorders. Current automated technologies are often unable to discriminate platelets from non-platelet particles particularly in circumstances where platelet anisocytosis is common. This study compares manual methodology and the automated technologies; impedance, optical density and CD61 immunoplatelet method (available on the Cell Dyn 4000) with the reference method of flow cytometric analysis in a paediatric population. A total of 141 samples were analysed and divided into specific age related groups and groups with thrombocytopenia and thrombocytosis. Data analysis showed that the CD61 method compared best with the reference method and this was evident in all the specified groups. The mean platelet count obtained by optical and manual methods were lower, suggesting that these methods are less reliable. The impedance count method was accurate despite its limitations. Strong correlations were observed in the 2-14 year age group but there was greater variation in the <1 month group supporting the theory that there is a greater variation in platelet characteristics in neonates. The CD61 method is the automated method of choice and would be particularly useful in the problem groups (platelet counts <50 x 10(9)/l and neonates <1 month old).