Progressive chromogenic anti-factor Xa assay and its use in the classification of antithrombin deficiencies

BACKGROUND

Antithrombin (AT) is a slow-acting progressive inhibitor of activated clotting factors, particularly thrombin and activated factor X (FXa). However, the presence of heparin or heparan sulfate accelerates its effect by several magnitudes. AT deficiency, a severe thrombophilia, is classified as type I (quantitative) and type II (qualitative) deficiency. In the latter case mutations may influence the reactive site, the heparin binding-site (HBS) and exert pleiotropic effect. Heterozygous type II-HBS deficiency is a less severe thrombophilia than other heterozygous subtypes. However, as opposed to other subtypes, it also exists in homozygous form which represents a very high risk of venous thromboembolism.

METHODS

A modified anti-FXa chromogenic AT assay was developed which determines both the progressive (p) and the heparin cofactor (hc) activities, in parallel. The method was evaluated and reference intervals were established. The usefulness of the assay in detecting type II-HBS AT deficiency was tested on 78 AT deficient patients including 51 type II-HBS heterozygotes and 18 homozygotes.

RESULTS

Both p-anti-FXa and hc-anti-FXa assays showed excellent reproducibility and were not influenced by high concentrations of triglyceride, bilirubin and hemoglobin. Reference intervals for p-anti-FXa and hc-anti-FXa AT activities were 84%-117% and 81%-117%, respectively. Type II-HBS deficient patients demonstrated low (heterozygotes) or very low (homozygotes) hc-anti-FXa activity with normal or slightly decreased p-anti-FXa activity. The p/hc ratio clearly distinguished wild type controls, type II-HBS heterozygotes and homozygotes.

CONCLUSIONS

Concomitant determination of p-anti-FXa and hc-anti-FXa activities provides a reliable, clinically important diagnosis of type II-HBS AT deficiency and distinguishes between homozygotes and heterozygotes.

Quantification of antithrombin isoform proportions in plasma samples of healthy subjects, sepsis patients, and in antithrombin concentrates

Antithrombin (AT) circulates in plasma in two isoforms, AT-alpha (90-95%) and AT-beta (5-10%). AT isoform proportions were measured in plasma samples of 17 healthy subjects and 26 posttraumatic or postoperative septic patients, as well as in 4 commercially available AT concentrates. Total AT was immune-purified from plasma and concentrates. Micellar electrokinetic chromatography was used to analytically separate and quantify the isoforms. Compared with plasma samples of healthy donors, septic plasmas revealed significantly reduced AT activity (p < 0.001) and beta-isoform content (p < 0.05). AT-beta correlated inversely with urea and creatinine serum concentrations (p < 0.01), indicating a relationship between better renal function and higher beta-isoform content. beta-Isoform neither correlated with age, gender, and 28-day mortality, nor with plasma concentrations of various inflammatory and organ function parameters. The commercial AT concentrate, which is equivalent to the current WHO standard, had an AT-beta content close to that found in plasma of healthy subjects. The availability of this novel quantitative AT isoform assay allows, for the first time, a closer look at the role of AT isoforms in hemostasis and sepsis pathophysiology.

High-titer screening PCR: a successful strategy for reducing the parvovirus B19 load in plasma pools for fractionation

BACKGROUND

Human parvovirus B19 (B19) is regarded as a potential risk factor for certain patient populations receiving plasma components.

STUDY DESIGN AND METHODS

The prevalence of B19 was determined in a limited plasma donor population. Conditions for high-titer screening PCR were designed to allow the removal of plasma donations in the acute phase of infection with virus loads >or=10(7) genome equivalents per milliliter before manufacturing. Antithrombin III lots originating from screened plasma were compared to lots originating from untested plasma with respect to their B19 DNA load by a sensitive PCR assay.

RESULTS

B19 was shown to have a prevalence of about 1 per 800 plasma donations. Only a minority (1/8000) of occurrences were in the acute phase of infection. Removing plasma units with high virus load as determined by high-titer screening PCR significantly decreased peak virus loads of plasma pools for fractionation. Together with a virus-removal capacity of 10.4 log(10) of the manufacturing process, this screening resulted in a final antithrombin III product that was nonreactive for B19 on PCR.

CONCLUSION

Combining the strategy of high-titer screening PCR with the virus reduction capacity of the manufacturing process considerably increased the margin of B19 virus safety of antithrombin III. This strategy should have positive impact on other plasma components as well.

Problems with serum-free production of antithrombin III regarding proteolytic activity and product quality

Human antithrombin III (AT-III) was produced using a recombinant BHK-21 cell line with a microcarrier culture in spinner flasks. Cells were cultivated for the first 4 days in a medium containing 10% fetal calf serum (FCS). Afterwards, the medium was exchanged and production of AT-III occurred at high cell numbers in a serum-free medium. The product was determined by an immunoassay and further analysed after isolation from the culture medium. During cultivation, high proteolytic activity was detected which caused a considerable product decomposition. Furthermore, a higher level of non-glycosylated AT-III was found after serum-free production.

Conditions for radioiodination of antithrombin III retaining its biological properties

In order to obtain a radioiodinated antithrombin III (AT III) with a good labelling yield and optimal biological properties towards heparin, thrombin and its anti-AT III monoclonal antibodies, we compared the classical labelling methods and found them wanting. Thus, we perfected a new labelling procedure which fulfils the above requirements.

Preservation of antithrombin III activity in stored whole blood

Antithrombin III (AT-III) is the major inhibitor of thrombin, Factor Xa, and other coagulation enzymes. Congenital and acquired deficiencies of AT-III are thought to contribute to thrombosis and disseminated intravascular coagulation. Because a recent report suggested reduced AT III in stored blood, we evaluated blood bank storage effects. Serial samples were taken from 6 units of whole blood drawn into citrate-phosphate-dextrose-adenine over 42 days, and assays for AT-III functional activity were performed on the same day. The values (mean +/- SD) were as follows: day 0,91.8 +/- 10.7 percent; day 2, 101.9 +/- 10.7 percent; day 8, 107.3 +/- 7.4 percent; day 15, 118.9 +/- 11.1 percent; day 22, 105.4 +/- 9.8 percent; day 35, 93.4 +/- 8.8 percent; and day 42, 97.4 +/- 7.5 percent. The rise from day 0 to day 15 was significant but presumably secondary to interassay variation because analysis of frozen aliquots showed no significant change when all samples from each unit were assayed in one batch. Immunoassay of AT-III also showed no change with storage. The results indicate AT-III retains functional activity in whole blood stored at 2 to 6 degrees C for 42 days, and AT-III replacement does not require fresh blood or fresh-frozen plasma. Low values may reflect individual donor differences or dilution of plasma by anticoagulant.

Studies of the heterogeneity of antithrombin III concentrates

The biological activities of antithrombin III (At III) concentrates, prepared by several manufacturers for clinical use, have been compared by three assay methods, and their heparin-binding properties studied by crossed immunoelectrophoresis and heparin affinity chromatography. Concentrates from two of the four manufacturers showed discrepancies between assay methods, with concentrations by heparin co-factor assays significantly lower than those by immunological and progressive antithrombin methods. Heterogeneity was also found by heparin binding studies, with about half the total At III antigen in these concentrates being unable to bind to heparin. These results confirm previous findings of heterogeneity in At III concentrates and show that some concentrates contain substantial amounts of altered At III molecules in which the heparin-binding site has been denatured but the thrombin-neutralizing site left largely intact.

Diagnostic efficacy of newer synthetic-substrates methods for assessing coagulation variables: a critical overview

With the introduction of synthetic substrates, many newer methods have been developed for testing coagulation parameters. Some of these methods are used in clinical settings, and many have been evaluated for their diagnostic efficacy in various coagulation disorders. Clinical chemistry laboratory instrumentation has been used to accommodate these methods. Commercially available kits for antithrombin III, heparin, plasminogen, alpha 2-antiplasmin, and factor X are used by many laboratories. Individual reagents have also become available and are being used to develop assays for routine diagnostic use. The development of suitable standards is rapidly underway, and their availability will facilitate adoption of these methods by hospital laboratories. Many of the synthetic-substrate-based assays that have been introduced for routine diagnostic purposes have been published recently. Here, we provide an overview of the newer amidolytic methods for the measurement of certain coagulation variables.

An international collaborative study establishing a reference preparation for Antithrombin III

An international collaborative study involving 12 laboratories in 7 countries was carried out to establish a suitable reference preparation of antithrombin III (At III). The amount of At III present in two purified preparations, a freeze-dried normal plasma and local normal plasma pools was measured by clotting, immunological, and amidolytic assays. 120 assays were submitted of which 105 were accepted as valid for inclusion in subsequent analyses. Less laboratory to laboratory variation was found when At III was assayed in freeze-dried normal plasma, as compared to purified preparations of At III, and there was also less method to method variation when At III was measured in freeze-dried plasma. When measured as heparin co-factor activity, the two purified preparations contained only about half the level of At III found by immunoassay or progressive At III clotting assays. In contrast, the use of freeze-dried plasma provided results which showed excellent agreement between the various laboratories by all assays; accordingly, this material has been established by the World Health Organization as the International Reference Preparation for At III, with an assigned potency of 0.9 i.u. per ml.