Page 2173 - Hematology_ Basic Principles and Practice ( PDFDrive )

1926 Part XII Hemostasis and Thrombosis

normal plasma (i.e., in a 1 : 1 mix), or if an apparent factor deficiency three categories: (1) clot-based assays that detect the LAC; (2) immu-
is not consistent with the patient’s clinical history. Screening for noassays that detect aCL; and (3) immunoassays that detect aβ 2GPI.
inhibitors is accomplished by using a 1 : 1 mix in a clot-based assay. There is currently a lack of standardization of these assays, and their
If coagulation factor–deficient plasma is mixed with normal plasma, findings do not always correlate with clinical symptoms. However,
clotting will not be impaired, as the normal plasma will compensate laboratory identification of APLAs is a key component of diagnosing
for the deficient plasma and “correct” its prolonged clotting time. antiphospholipid syndrome, an autoimmune syndrome that increases
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However, if inhibitor-containing plasma is mixed with normal the risk of thrombotic and pregnancy complications. Current labo-
plasma, clotting will be impaired, as the inhibitor will decrease the ratory principles of detecting APLAs are well described in Chapters
activity of coagulation factors in the normal plasma as well. Specific 140 and 141. Highlights are outlined here.
inhibitors of coagulation proteins are generally time-dependent, that LAC screening comprises two tests based on different principles:
is, clotting may not be impaired immediately after the 1 : 1 mix is the dilute Russell viper venom time, and a sensitive APTT, using an
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performed. However, if it is incubated for 2 hours, the clotting time APTT reagent containing silica as the activator. Both tests are
will be prolonged. phospholipid-dependent. If at least one of the two tests suggests the
Inhibitors of specific coagulation proteins increase a patient’s risk presence of a LAC, a 1 : 1 mixing test using pooled normal plasma
for bleeding. The most common acquired inhibitor is an antibody to should be performed. Confirmatory testing must also be done with
factor VIII (see Chapters 135 and 136). It can occur as an alloanti- an assay that is less sensitive but more specific to LAC, by increasing
body in persons with hemophilia, or as an autoantibody in those with the phospholipid content. aCL and aβ 2GPI antibodies are detected
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previously normal hemostasis. Patients with an autoantibody-type using immunoassays. Traditionally, the enzyme-linked immunosor-
acquired factor VIII inhibitor present with a prolonged APTT and bent assay (ELISA) technique was used. However, newer automated
hemophilia-like bleeding. Both males and females can develop this assays have been introduced; these are expected to improve test
type of inhibitor. It is characteristically seen in older adult patients, reproducibility, specificity, and accuracy of detection. At this time,
patients with B-cell malignancies, patients with connective tissue aCL and aβ 2GPI testing is not well standardized, and there is a lack
disorders such as systemic lupus erythematosus, and in the postpar- of uniformity in reference material used for calibration. Reporting is
tum period. Management decisions in these patients are influenced also not standardized. aCL assays are expressed in GPL (units of aCL
by the severity of bleeding and the titer of the inhibitor. 10,11 Quantita- immunoglobulin [Ig] G antibodies, calculated against the original
tive measurement of factor VIII inhibitors is performed using the Harris standards) or MPL (units of aCL IgM antibodies, calculated
Bethesda method. This method is based on the observation that if a against the original Harris standards). aβ 2 GPI can be expressed in U/
coagulation factor is incubated with plasma containing its specific mL, ng/mL, and µg/mL among others.
inhibitor, the factor will be progressively neutralized. If the amount
of factor added and the duration of incubation are standardized, the
inhibitor strength can be measured according to how much of the PRACTICAL APPROACH TO LABORATORY
factor is inhibited. One Bethesda unit is defined as the amount of TESTING OF COAGULATION PROTEINS
factor VIII inhibitor that will neutralize 50% of 1 unit of factor VIII
in normal plasma after 2 hours of incubation at 37°C. Normal plasma The first step in interpreting laboratory tests of coagulation proteins
pool is considered to represent 1 unit of factor VIII. The Bethesda is synthesizing the results of the PT, APTT, and TCT. When all three
assay is performed by incubating patient plasma with normal plasma assays are performed simultaneously, they can give the clinician a high
for 2 hours at 37°C At the end of the incubation period, the residual level snapshot of the patient’s coagulation factors, and inform a
factor VIII is assayed and plotted against a standard graph of Bethesda practical differential diagnosis (Table 129.1.) An abnormality in one
units versus residual factor VIII activity. The factor VIII percentage test suggests an abnormality in one branch of the Ratnoff-Davie-
nearest to 50% (between 30% and 60%) is chosen for calculating the Macfarlane coagulation cascade, whereas abnormalities in more than
strength of inhibitor. At 50% inhibition, the test plasma contains, by one test suggest a problem in the common pathway of coagulation.
definition, 1 Bethesda inhibitor unit per mL. The Nijmegen modifi- Isolated prolongation of the APTT is caused by abnormalities
cation allows for more accurate determination of inhibitor titers at in the intrinsic pathway, only some of which are associated with an
low levels of factor VIII inhibition. Increases in pH and decreases in increased bleeding risk. It is essential to consider the APTT in the
protein concentrations can lead to increased inactivation of factor context of a thorough clinical bleeding assessment. If the prolonged
VIII, potentially leading to false-positive results using the standard APTT is associated with bleeding, then the differential diagnosis in
Bethesda assay. The Nijmegen modification buffers normal plasma decreasing likelihood of frequency is: factor VIII deficiency; factor
with 0.1 M imidazole buffer, and uses immunodepleted factor IX deficiency; or factor XI deficiency. Factor VIII and IX deficiencies
VIII–deficient plasma in the control mixture. are X-linked, and affect 1 : 5000–10,000 males and 1 : 25,000–30,000
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Inhibitors directed against other coagulation factors are rare. males respectively. Factor XI deficiency is an autosomal disorder that
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Autoantibodies against von Willebrand disease (vWD) can develop affects both males and females, and has a variable prevalence. It
in the context of a paraproteinemia, whereas autoantibodies against causes a clinically significant bleeding disorder in 1 : 1 million in the
prothrombin are a rare complication of systemic lupus erythematosus. general population, but has a rate of heterozygosity as high as 8% to
Inhibitors to other factors can be measured with modifications of the 9% in Ashkenazi Jews (see Chapters 135 and 137 for a full discus-
Bethesda method. sion of these factor deficiencies). If the isolated prolonged APTT is
not associated with bleeding symptoms, the most common cause is
Evaluation of Acquired Inhibitors of an APLA, often found in otherwise healthy individuals. Coagula-
tion protein deficiencies that prolong the APTT but do not cause
Coagulation Proteins bleeding include factor XII, prekallikrein, and high-molecular-weight
kininogen deficiency (see Table 129.1). It is important to consider
The most commonly detected acquired inhibitors are the antiphos- these three proteins in the evaluation of an isolated prolonged
pholipid antibodies (APLAs) (see Chapter 141). APLAs represent APTT, to preclude misguided attempts to “correct” the prolonged
antibodies directed to epitopes of proteins that bind phospholipids. APTT by giving patients unnecessary blood products or factor
They include the lupus anticoagulant (LAC), anticardiolipin anti- concentrates.
bodies (aCL) and anti-β 2 glycoprotein I antibodies (aβ 2GPI). LAC An isolated PT prolongation associated with bleeding usually
variably interfere with the APTT and PT, depending on the nature indicates factor VII deficiency (see Chapter 137); these deficiencies
of the commercial APTT or PT reagent. Despite their ability to are partial, as complete factor VII deficiency appears to be incompat-
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inhibit coagulation factors, LAC (like other APLAs) do not increase ible with life. Sometimes, mild defects in common pathway proteins
bleeding risk. APLAs may have no clinical significance at all, or they (e.g., factors II, V, and X) may initially appear as an isolated PT
may predispose to thrombosis. Laboratory assays of APLAs fall into prolongation, though as they progress they eventually prolong the