Methods for measuring a 6 log10 white cell depletion in red cells

Real-time amplification of glyceraldehyde-3-phosphate dehydrogenase gene for quality control of leukopoor platelets

BACKGROUND

Leukoreduction of blood products is crucial to prevent white blood cell (WBC)-associated complications during transfusion. Of the widely accepted methods for quantifying WBCs in blood components, Nageotte hemocytometry is time-consuming and laborious whereas a specialized instrument is required for flow cytometry. A reliable and affordable method to assess WBC count in blood products is of particular interest.

STUDY DESIGN AND METHODS

Real-time polymerase chain reaction (PCR) of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene was developed for quantifying WBCs in leukopoor platelets (LPPs). After normalization by the cell-free prefiltrated and postfiltrated plasma DNA, the relative copy number of GAPDH gene in the platelet (PLT) concentrate and its corresponding LPPs was calculated according to the equation of 2(-ΔΔCt) of which Ct is defined as the threshold cycle. The percentage and the number of WBCs that remained in LPPs were consequently determined. This method was compared to Nageotte hemocytometry and was validated by using serially diluted PLT concentrate and 10 pairs of PLT concentrate-LPP samples.

RESULTS

Consistent with the removal of WBCs after filtration, the Ct values for the LPP samples were increased when compared to their corresponding PLT concentrate. As revealed by real-time PCR of GAPDH gene, there is a correlation between the calculated and theoretical WBC count in the serially diluted PLT concentrate (correlation coefficient, 0.9532). The WBC counts for the 10 LPP samples were comparable between Nageotte and real-time PCR method and were all below 3.3 × 10(6) WBCs/L.

CONCLUSION

The real-time PCR method we report in this study is applicable for routine quality assurance during leukoreduction process.

Quantitation of residual WBCs in filtered blood components by high-throughput, real-time kinetic PCR

BACKGROUND

The effort to eliminate transfusion complications associated with WBCs has led to the widespread use of filters able to reduce WBC concentrations to <or=0.1 WBC per microL blood. This has necessitated sensitive QC methods to quantitate residual WBCs in filtered units. One fast, effective method is DNA amplification using real-time kinetic PCR (kPCR).

STUDY DESIGN AND METHODS

Two methods of preparation of standards were compared and used for the optimization of quantitative kPCR. The first involved spiking genomic DNA cell lysate into a diluent, followed by a series of 1 in 10 dilutions. The second involved spiking serial 1 in 10 dilutions of WBCs into twice-filtered fresh whole blood. Two hundred fifty filtered frozen whole-blood samples were amplified in duplicate to show the kPCR assay's reproducibility. Another 359 filtered frozen whole blood samples were used to compare data from kPCR with data from a standard PCR protocol using (32)P-labeled probe and autoradiography. All specimens were amplified for conserved HLA DQ(alpha) sequences.

RESULTS

Standards prepared by both methods gave reproducible and equivalent results. Quantitation of standards representing a dynamic range of 8 x 10(o) to 8 x 10(5) WBCs per mL, yielded standard deviations ranging from 0.59 cycle to 1.04 cycles (a one-cycle increase is equivalent to a twofold increase in WBC concentration). The scatter graph of the 250 samples tested in duplicate by kPCR generated a slope of 1.0122 and an R(2) value of 0.9265. The comparison of kPCR and (32)P-probe hybridization results on 359 clinical samples gave a scatter-graph slope of 0.9428 and an R(2) value of 0.8718, indicating excellent agreement of the methods over a 4-log dynamic range.

CONCLUSION

kPCR is a high-throughput, sensitive assay that could prove useful in routine quality assurance of the WBC reduction process.

A general method for concentrating blood samples in preparation for counting very low numbers of white cells

BACKGROUND

To count extremely low levels of white cells (WBCs) in WBC-reduced blood components, a larger volume of sample must be processed. The goal was to develop an all-purpose method for concentrating the samples obtained from WBC-reduced red cells or platelets. The method was designed to be compatible with a variety of counting techniques.

STUDY DESIGN AND METHODS

Coded samples of red cell concentrates with an expected WBC concentration of 200, 100, 50, and 10 per mL and of the diluent (undetectable WBCs/mL) were sent to three sites on five occasions and counted by the use of the concentration method, crystal violet stain, and a Nageotte counting chamber. Additional samples were tested by flow cytometry, polymerase chain reaction, and volumetric capillary cytometry.

RESULTS

The results from the three test sites showed good linearity, with an overall r2 = 0.9994. The lower limit of accurate detection of the assay was 10 WBCs per mL. The results were biased toward underestimation, particularly at one of the test sites (Site A). There were no significantly different results in Sites B and C. The intra-assay CV was acceptable. Precision (reproducibility) at the three test sites varied.

CONCLUSION

This method allows reliable determination of WBC concentrations as low as 0.01 per microL in blood. Despite the use of technologists trained in Nageotte chamber counting, validation testing demonstrated that one test site's performance was significantly different from that of the other two sites, because of both underestimation bias and variation in count results. The sample concentration method, when used in conjunction with an automated assay for WBC identification, should permit larger volume analysis with a greater degree of precision and a lower limit of detection than is found in assays that do not concentrate the sample before counting.

[Current data on the counting of weak leukocyte concentrations in labile blood products]

The precise measurement of low numbers of leukocyte below 0.1 WBC/microliter in filtered red cell or platelet suspensions meet both aims: to check the compliance with previously determined requirements and to evaluate the performances of novel filtering material (5 log depletion or more), justified by more and more important clinical use. The reliability of results, obtained with the chosen method, is ensured by applying of validation protocol, including training of technologist, assessment of the analytical range and the detection limit, assessment of precision and accuracy. The flow cytometry (FC) and Nageotte Chamber (NC) method are the both techniques which are currently used in routine Quality Control (QC) and validated by multicenter studies. Recent developments are made for increasing the sensibility of these counting methods, thanks to higher concentration or volume of the sample to be analysed. Among the experimental techniques, requiring more advances before implementing in QC program, quantitative PCR must become essential as reference method for evaluating the efficiency of filtration, in the future.

Reducing the infectivity of blood components--what we have learned

The safety of the nation's blood supply has improved over the last several years as a result of more intensive donor screening and viral testing. Concurrently, there has been more judicious use of blood components. Although the risk is small, transmission of blood borne viruses, bacteria and parasites can occur. Investigators have studied a myriad of processes for pathogen depletion and/or inactivation, including the use of chemicals, extended storage, filtration, heating, irradiation, photochemicals and washing. Pasteurization, methylene blue and solvent-detergent processes have been introduced in parts of Europe for improving the safety of plasma used for transfusion. The FDA is reviewing a license application for the solvent-detergent process. For red cells, use of highly efficient leukodepletion filters is believed to be equivalent to antibody testing for the prevention of CMV disease transmission. Otherwise, no successful treatments have yet been identified for red cells or platelets. Several photochemicals, which may be useful for treating these components, are being studied. However, there appear to be trade-offs between the extent of pathogen inactivation, platelet or red cell damage, and genotoxicity. These as well as other biological parameters and operational issues will need to be further evaluated before implementation can be considered.

Relative efficiency of leucocyte removal procedures for the production of leucocyte-poor red cell concentrates assessed by flow cytometry

Flow cytometry was used to: (1) determine residual leucocyte numbers in red cell suspensions following the range of leucocyte depletion procedures used in our organisation, and (2) to characterize phenotypically the leucocytes using direct immunofluorescence with monoclonal antibodies to cell surface receptors. Under the conditions used, a lower limit of detection of 2.5 leucocytes per microliter (equivalent to 3.43 log10 or 99.96% removal) could be achieved. Filtration through polyester filters was found to remove up to > 99.96% of the initial leucocytes; however, a significant differential efficacy was observed between filters from different manufacturers even when filters with similar costs were compared. The order of filter brands with respect to leucocyte removal found was Pall BPF4 = Erypur Optima G-O > Sepacell R500 > Pall RC50. Phenotyping revealed that increasing filtration efficacy was associated with a preferential removal of lymphocytes; conversely, a second filtration over one brand of filter allowed proportionately more lymphocytes to pass through compared with the first filtration. A saline wash following filtration removed a further 0.5% of the initial leucocyte content, and was associated with a preferential loss of granulocytes. Freeze-thawing the red cell suspension removed fewer leucocytes (96.3%) than did filtration (98.74% to > 99.6%) or filtration followed by washing (99.22%), and also led to preferential loss of granulocytes. Flow cytometry provides a reliable tool for the quality control of leuco-depleted red cells, and allows a qualitative assessment of the residual leucocytes. This information is of value in choosing procedures aimed at decreasing the risk of alloimmunisation and post-transfusion reactions.

[Description and validation of a flow cytometric method to estimate residual leukocytes in leukocyte-depleted red cell concentrates]

We describe a flow-cytometric method for estimating residual white blood cells (WBC) counts in WBC depleted blood components, namely units of packed red cells. The method uses fluorescent staining of nuclear ADN with ethidium bromide. WBC nuclei are discriminated from background events using fluorescence ratio (585 nm versus 650 nm). A facultative procedure of concentration is described, in order to get better sensitivity at very low WBC counts. Main steps of validation are dilution assays and correlation with hemocytometer counts. The method can be used to explore very low concentrations (less than 100 WBC/ml) and should be useful in quality control of blood products.

[Assessment of filtration performance]

The technique of filtration which has constantly been improved over past years makes it possible to obtain highly pure blood products (the rate of leukocyte depletion can reach 3 to 5 log). Several study groups (BEST, PSL) and international committees of experts have defined a number of standards with which leukocyte depleted blood products must comply. Assessment of filtration procedures is thus made necessary, it applies to the different steps of the filtration procedure: preparation techniques of red cell or platelet suspension, priming and rinsing of the filter. Likewise a number of parameters have to be checked: filtration time, temperature, age of suspensions. Moreover quality control procedures must be implemented, using adapted and validated measuring methods (Nageotte hemacytometer, for instance). The parameters to be monitored include the number of residual leukocytes, (mean value: 1 x 10(6), i.e. about 4 WBCs/microliters), and the rate of hemoglobin or platelet recovery. Any new filtering equipment or material must satisfy strict requirements and standards including clinically acceptable limits as part of the same quality approach. Validation, in this case, consists in determining the maximal leukocyte content which the filter can absorb (capacity) and the average rate of leukocyte removal (efficacy). The constant monitoring of filtration performance contributes to improving the quality of red blood cell or platelet suspensions and thus meet clinicians' requirements for their patients.

[Filtration of platelet concentrates. Technical aspects, quantitative results. Group PSL]

This work summarizes a multicentric study (20 Blood Transfusion Centers) concerning the use of different filters applied to platelet collection. The results are good with the different filters and platelets concentrates evaluation. Nevertheless, we have seen a great heterogeneity of platelet concentrates. This characteristic could explain the variability in filter performances. Maybe we have to adapt some type of filter with specific platelet concentrates. In conclusion, it is important to continue the evaluation of new platelet concentrate filters, in order to increase the performances and the filtration quality. Clinical trials seem to be useful in the future to appreciate the effects of filtered platelets on alloimmunization and CMV transmission.

Stabilization of von Willebrand factor in banked blood by leucocyte depletion

The von Willebrand factor (vWf) activity, as measured by the ristocetin co-factor (vWf:RCo) and collagen binding (vWf:CBA) assays, declined progressively in standard blood units stored at 4 degrees C after a 2-day storage period. This loss of activity was accompanied by a loss and degradation of high molecular weight (HMW) vWf multimers. In studies using a paired design, filtration of blood with a high efficiency leucocyte-removal filter, prior to storage at 4 degrees C, led to significantly improved maintenance of vWf:RCo and vWf:CBA compared with unfiltered units (P < 0.01 after 8 days). Loss and degradation of HMW vWf decreased when blood was filtered prior to 4 degrees C storage. Filtration had no effect on vWf-associated activities when blood was stored at 22 degrees C for 10 days. These results indicate that part of the storage lesion of vWf in banked blood is due to leucocyte-mediated removal and degradation of HMW vWf. This has implications when specifying plasma for the production of vWF concentrates and may also play a role in the haemostatic lesion associated with massive transfusion of stored blood.

Viral inactivation and reduction in cellular blood products

Even though the risks associated with the transfusion of blood products are lower than ever before, considerable efforts are being employed to improve the safety of the blood supply. Based upon available data, a six log (99.9999%) reduction in virus level from screened and tested blood components should significantly reduce or eliminate the risk of post-transfusion infection. The objective has been to identify "generic" methods, that is, one that would be applicable to all virus. For red cells, physical and chemical approaches have been studied; for platelets, the approaches have been limited to chemical. The physical methods include depletion of leukocytes by filtration, removal of plasma by washing, and viral inactivation by heat. Among the chemicals investigated to inactivate or help displace virus are ozone, detergents, and hypochlorous acid. Several photochemicals have also received intensive investigation: merocyanine 540, a benzoporphyrin derivative, aluminum phthalocyanine, and methylene blue. For platelets, photochemical inactivation methods using merocyanine 540, and two psoralen derivatives, 8-methoxypsoralen (8-MOP) and aminomethyl trimethyl psoralen (AMT), have also been studied. Approaches which include washing are not suitable. For the most part, either viral removal or inactivation has been insufficient, or red cell or platelet damage unacceptable. However, there are a few indications that at least inactivation of a specific virus, such as HIV, may be possible without major cell damage. These studies are in their early stages and significant work remains. If feasibility is clearly shown in vitro, it is likely that in vivo primate studies to demonstrate safety and efficacy will be required.