Prophylactic platelet transfusion in acute leukaemia

Continuing developments with the automated platelet count1

The four main procedures for platelet counting are: manual phase contrast microscopy, impedance, optical light scatter/fluorescence and flow cytometry. Early methods to enumerate platelets were inaccurate and irreproducible. The manual count is still recognized as the gold standard or reference method, and until very recently the calibration of platelet counts by the manufacturers of automated cell counters and quality control material was performed by this method. However, it is time-consuming and results in high levels of imprecision. The introduction of automated full blood counters using impedance technology resulted in a dramatic improvement in precision. However, impedance counts still have limitations as cell size analysis cannot discriminate platelets from other similar-sized particles. More recently, light scatter or fluorescence methods have been introduced for automated platelet counting, but there are still occasional cases where an accurate platelet count remains a challenge. Thus, there has been interest in the development of an improved reference procedure to enable optimization of automated platelet counting. This method utilizes monoclonal antibodies to platelet cell surface antigens conjugated to a suitable fluorophore. This permits the possible implementation of a new reference method to calibrate cell counters, assign values to calibrators, and to obtain a direct platelet count on a variety of pathological samples. In future, analysers may introduce additional platelet parameters; a reliable method to quantify immature or reticulated platelets would be useful.

An interlaboratory study of a candidate reference method for platelet counting

A multinational interlaboratory task force explored the important variables of platelet reference counting and developed a candidate flow cytometric reference method based on the RBC/platelet ratio. A multicenter comparison was performed to determine whether the method met the necessary criteria and was precise enough to be recommended as a new reference method. Each laboratory analyzed serial dilutions of normal specimens, stabilized material, and at least 60 patient specimens with a range of platelet counts from 1 to 400 x 10(3)/microL (1-400 x 10(9)/L). Pooled analysis of the serial dilutions showed that RBC-platelet and RBC-RBC coincidence events became negligible at sufficiently high dilutions (i.e., > 1:1,000). All laboratories demonstrated excellent intra-assay and acceptable interlaboratory precision. Two antibodies (CD61 and CD41) were used for identifying platelets and individually gave acceptable results, but in a minority of samples, staining differences were observed. The optimum method thus uses a double-labeling procedure with a final dilution factor of 1:1,000. The study demonstrated that this method meets the criteria for a reference platelet count.

Platelet transfusion for patients with cancer: clinical practice guidelines of the American Society of Clinical Oncology

OBJECTIVE

To determine the most effective, evidence-based approach to the use of platelet transfusions in patients with cancer.

OUTCOMES

Outcomes of interest included prevention of morbidity and mortality from hemorrhage, effects on survival, quality of life, toxicity reduction, and cost-effectiveness.

EVIDENCE

A complete MedLine search was performed of the past 20 years of the medical literature. Keywords included platelet transfusion, alloimmunization, hemorrhage, threshold and thrombocytopenia. The search was broadened by articles from the bibliographies of selected articles.

VALUES

Levels of evidence and guideline grades were rated by a standard process. More weight was given to studies that tested a hypothesis directly related to one of the primary outcomes in a randomized design. BENEFITS/HARMS/COST: The possible consequences of different approaches to the use of platelet transfusion were considered in evaluating a preference for one or another technique producing similar outcomes. Cost alone was not a determining factor.

RECOMMENDATIONS

Appendix A summarizes the recommendations concerning the choice of particular platelet preparations, the use of prophylactic platelet transfusions, indications for transfusion in selected clinical situations, and the diagnosis, prevention, and management of refractoriness to platelet transfusion.

VALIDATION

Five outside reviewers, the ASCO Health Services Research Committee, and the ASCO Board reviewed this document.

SPONSOR

American Society of Clinical Oncology

Current issues with platelet transfusion in patients with cancer

For the past 30 years, platelet transfusions have been used in the treatment of thrombocytopenia caused by decreased production, inadequate function, or increased destruction of platelets. The number of platelet transfusions has increased more than transfusions of other blood components, shifting from whole blood use for the platelet source to plateletpheresis. Hematology/oncology patients are among the largest group receiving platelet transfusions, primarily because the more aggressive chemotherapies produce more acute and prolonged thrombocytopenia. While platelet transfusions often rescue patients with very low platelet levels, they are associated with the risk of viral and bacterial infections, as well as alloimmunization. Platelet donor recruitment can also be difficult, and platelet transfusion can be very expensive depending on the source of platelets. As a result, prophylactic transfusions are less likely to be administered at higher platelet counts, reducing platelet use and cost of platelet transfusions. However, cancer patients receiving intensive chemotherapy or myeloablative regimens require multiple platelet transfusions. For these patients, alternate strategies are needed so that platelet transfusions can be significantly reduced or eliminated.

Immunoplatelet counting: a proposed new reference procedure

Given the high degree of interoperator error and poor precision of manual platelet counting, it has recently been proposed that an immunoplatelet counting method could become the new reference procedure. Platelets are identified immunologically with a suitable monoclonal antibody, and the platelet count is derived from the ratio of fluorescent platelet events to collected red blood cell (RBC) events that are also counted by a reliable and calibrated standard impedance counter (RBC ratio). In this study, we have set up a rapid and simple method for immunoplatelet counting and simultaneously compared the RBC ratio with the bead ratio derived from two different preparations of commercial calibration beads (Trucount and FlowCount beads). Comparison of the level of imprecision of the RBC ratio with either the manual count or bead ratios revealed a superior coefficient of variation of < 5% even in samples with a platelet count < 20 x 10(9)/l. The RBC ratio correlated extremely well with the existing manual phase reference method (r2 = 0.93) and especially well with three different commercial impedance counters and a dual-angle optical counter (r2 = 0.98-0.99). However, at < 100 x 10(9)/l, the correlation of the RBC ratio with the dual-angle optical count (ADVIA 120) (r2 = 0.96) was superior to all impedance counters. This suggests that automated optical counting methods may be more accurate at determining platelet counts in thrombocytopenic samples. As the RBC ratio is rapid, cheap and relatively easy to perform, we propose that this method could replace the manual count as a new international reference method.

The threshold for prophylactic platelet transfusions in adults with acute myeloid leukemia. Gruppo Italiano Malattie Ematologiche Maligne dell'Adulto

BACKGROUND

Prophylactic platelet transfusions are usually administered to patients receiving myelotoxic chemotherapy when their platelet count falls below 20,000 per cubic millimeter. Some observations suggest that lower platelet counts can be appropriate in patients in stable condition, but the safety of lower thresholds is uncertain.

METHODS

We evaluated 255 adolescents and adults (age, 16 to 70 years) with newly diagnosed acute myeloid leukemia (but not acute promyelocytic leukemia), who were treated in 21 centers. One hundred thirty-five patients were randomly assigned to receive a transfusion when their platelet count fell below 10,000 per cubic millimeter (or 10,000 to 20,000 per cubic millimeter in those with a temperature above 38 degrees C, with active bleeding, or a need for invasive procedures), and 120 patients were assigned to receive a transfusion when their platelet count was less than 20,000 per cubic millimeter.

RESULTS

Patients in the group with a threshold of 10,000 platelets per cubic millimeter received 21.5 percent fewer platelet transfusions than the patients in the group with a threshold of 20,000 platelets per cubic millimeter (P=0.001). The numbers of red-cell units transfused were not significantly different between groups. Major bleeding (defined as any bleeding more than petechiae or mucosal or retinal bleeding) occurred in 21.5 and 20 percent of patients, respectively (P=0.41), and on 3.1 and 2.0 percent of the days of hospitalization. One episode of fatal cerebral hemorrhage occurred in the group with a threshold of 10,000 platelets per cubic millimeter; none occurred in the other group (P= 0.95). Actuarial estimates of survival during induction chemotherapy, actuarial estimates of the absence of major bleeding, and the length of hospital stay were not significantly different in the two groups.

CONCLUSIONS

The risk of major bleeding during induction chemotherapy in adolescents and adults with acute myeloid leukemia (except acute promyelocytic leukemia, which we did not study) was similar with platelet-transfusion thresholds of 20,000 per cubic millimeter and 10,000 per cubic millimeter (or 10,000 to 20,000 per cubic millimeter when body temperature exceeded 38 degrees C, there was active bleeding, or invasive procedures were needed). Use of the lower threshold reduced platelet use by 21.5 percent.