Dear Editors,
In a recent study published in Thrombosis and Haemostasis, we investigated the effects
of the novel bispecific antibody emicizumab on a variety of in vitro laboratory coagulation
tests.1 Emicizumab provides effective bleed prevention in persons with hemophilia
A (PwHA),2 but due to its mechanism of action, we reasoned that it may affect tests
commonly used to monitor coagulation. The goal of our study was to inform the selection
and interpretation of coagulation assays for PwHA receiving emicizumab prophylaxis.1
We showed that emicizumab has a strong interference effect on assays based on the
activated partial thromboplastin time (aPTT), such as one‐stage assays for FVIII,
protein C, and protein S activity; a weak effect on the prothrombin time and derived
fibrinogen assays; and no effect on various chromogenic and immunologic assays.1 However,
there are no data currently available on the effect of emicizumab on functional assays
to detect lupus anticoagulants (LAs). LAs are immunoglobulins that inhibit phospholipid‐dependent
coagulation tests by binding to phospholipid cofactor proteins. While rare, the presence
of LAs in PwHA is significant in that they can interfere with the ability to detect
and monitor FVIII inhibitors by Bethesda assays based on aPTT, and in a few instances
even with chromogenic Bethesda assays (which are recommended for management of emicizumab
patients).3 Therefore, it is valuable to explore the effect of emicizumab on several
functional assays for LA detection based on different assay principles. The intent
of this letter is to supplement the findings of our previous study1 by investigating
the effect of emicizumab on the results of LA detection assays.
To this end, we used patient plasma samples to assess the effect of emicizumab therapy
on three different LA detection assays. Frozen plasma samples were sourced as follows:
two from persons with severe hemophilia A without FVIII inhibitors (Precision BioLogic
Inc), two samples positive for LAs (George King Bio‐Medical Inc and Precision BioLogic
Inc), and CRYOcheck™ pooled normal plasma (Precision Biologic Inc). In addition, individual
samples from 12 healthy plasma donors were generated (menal GmbH) via blood collection
into plastic tubes containing 0.109 mol/L sodium citrate as an anticoagulant, centrifuged
(2000 × g, 15 minutes at room temperature) and the resulting plasma was transferred
to another plastic tube for a second spin. Plasma was then aliquoted, immediately
frozen (−70°C), and thawed in a water bath at 37°C immediately before testing. Analyses
were performed in duplicate using the commercially available diagnostic kits according
to manufacturer instructions; results presented are means of the two determinations.
In each experiment, emicizumab was spiked into the samples to produce final plasma
concentrations of 0, 50, 100, and 150 µg/mL.
Three LA detection assays were evaluated: the aPTT‐based STA‐Staclot LA assay (Stago);
the Dilute Russel Viper Venom Time (DRVVT; STA‐Staclot DRVV Screen and Confirm LA
assays, Stago); and the Taipan venom time (TVT; Diagnostic Reagents Ltd).
The aPTT assay is based on hexagonal phase phospholipid neutralization of LAs.4 An
LA‐sensitive aPTT is performed with and without the addition of hexagonal phase phospholipids,
and the difference in clotting time is calculated. LAs interfere with thrombin generation
in the aPTT assay by disrupting the formation of two phospholipid‐dependent coagulation
factor complexes (tenase and prothrombinase). Shortening of the detected clotting
time by ≥8 seconds (as per manufacturer recommendations) with the addition of hexagonal
phase phospholipids was indicative of the presence of LAs. However, the assay cannot
be used with plasma samples containing anti‐factor antibodies (inhibitors), as these
will prolong the clotting time regardless of the presence of LAs or addition of hexagonal
phase phospholipids.
The DRVVT assay is based on the activation of FX and FV using the snake venom of Daboia
russelii (Russell's viper) with the addition of a low (DRVV Screen) versus high (DRVV
Confirm) concentration of phospholipids. The assay detects LAs by their ability to
interfere with thrombin activation by the prothrombinase complex. By activating coagulation
downstream of factors VIII and IX, the test is not subject to being affected by their
deficiencies or specific inhibitors. Per the test manufacturer's recommendation, a
DRVVT screen ratio of >1.2 (test sample ÷ normal plasma pool) was indicative of the
presence of LAs.
The TVT assay is based on the activation of prothrombin to thrombin by the venom of
Oxyuranus scutellatus (Taipan snake), which is dependent on the presence of phospholipids
and free calcium ions5; LAs prolong the TVT. TVT was performed by mixing 100 µL of
sample with 100 µL of Bell and Alton phospholipid (Diagnostic Reagents, reconstituted
with 5 mL of water, diluted 1:6 with imidazole buffer) and finally adding 200 µL of
Taipan venom reagent (Diagnostic Reagents). The TVT clotting times were divided by
the mean clotting time of 10 determinations of the normal plasma pool. A ratio >1.12
was indicative of the presence of LAs.6
The aPTT‐based and DRVVT LA assays were performed on the STA‐R Evolution analyzer
(Stago), and the TVT assay was performed on the MC10 coagulometer (ABW Medizin und
Technik GmbH). Investigations were performed at menal GmbH.
Our results showed that emicizumab substantially shortened clotting times of the aPTT‐based
LA assay, both with and without the presence of hexagonal phase phospholipids (Figure
1). The presence of emicizumab also affected the difference between the clotting times,
and thus could alter the assignment of samples as LA positive or negative.
Figure 1
Effect of emicizumab on the aPTT‐based hexagonal (II) phase phospholipid clotting
assay (Staclot LA). Two samples from healthy individuals (A and B), two samples from
PwHA (C and D), and two samples positive for LAs were analyzed (E and F). Each panel
(A–F) shows the dose response for emicizumab of an LA‐sensitive aPTT assay (diamonds)
and the same aPTT assay with the addition of hexagonal (II) phase phospholipids, which
can neutralize LAs (circles). The difference of the two aPTTs is marked with triangles
(blue dashed line, secondary y axis) and is considered positive if it exceeds 8 s.
Note that according to manufacturer's instructions, the test plasma is mixed with
an equal volume of healthy donor plasma prior to measurement, resulting in similar
baseline aPTT values for healthy donor samples (A and B) and hemophilia A samples
(C and D). Footnotes: *Sample with addition of hexagonal (II) phase phospholipids.
aPTT, activated partial thromboplastin time; HV, healthy volunteer (samples); LAs,
lupus anticoagulant (samples); PwHA, persons with hemophilia A
Emicizumab triggered a weak but detectable concentration‐dependent prolongation of
DRVVT clotting times (Figure 2A), and accordingly a slight increase in DRVVT ratios
(Figure 2B). The mean increase in DRVVT with 50 µg/mL emicizumab (which corresponds
to the clinical median trough plasma concentration)2 was 1.5 seconds (range, 1.3‐1.8 seconds)
for non‐LAs samples and 7.0 seconds (range, 6.6‐7.3 seconds) for LAs samples (Table
S1). The mean increase in the ratio was 0.04 (0.03‐0.05) and 0.18 (0.17‐0.18), respectively.
The use of the normalized ratio (DRVVT screen ratio ÷ DRVVT confirm ratio) halved
the effect of emicizumab. Emicizumab had no effect, however, on the prothrombin‐activator
based TVT assay (Figure 3).
Figure 2
Effect of emicizumab on the DRVVT assay (STA‐Staclot DRVV Screen) expressed in seconds
(A) and as ratio of the results from a normal plasma pool provided in the kit (B).
Two samples from healthy individuals (black open symbols, triangle and square), two
samples from persons with hemophilia A (blue open symbols, circle and diamond), and
two samples positive for LAs were analyzed (green closed symbols, circle and diamond).
The red dashed line in panel B (ratio > 1.2) depicts the cutoff for a positive DRVVT
assay, indicative of the presence of LAs. Footnotes: DRVVT, dilute Russel viper venom
time; LA, lupus anticoagulant
Figure 3
Effect of emicizumab on the Taipan venom time assay expressed in seconds (A) and as
ratio of the results from the mean of 10 plasma samples from healthy volunteers (B).
Two samples from healthy individuals (black open symbols, triangle and square), two
samples from persons with hemophilia A (blue open symbols, circle and diamond), and
two samples positive for LAs were analyzed (green closed symbols, circle and diamond).
The red dashed line in panel B (ratio > 1.12) depicts the cutoff for a positive Taipan
venom time assay, indicative of the presence of LA. Footnotes: LAs, lupus anticoagulant
Taken together, all three assays correctly discriminated the LA and non‐LA samples
in the absence of emicizumab. However, emicizumab interfered with the Staclot LA assay
to the extent that an LA‐positive sample would have been incorrectly classified as
LA‐negative; this is not surprising given that emicizumab has a documented very strong
shortening effect on the aPTT.1 Emicizumab had a weak effect on DRVVT. This effect
of emicizumab on DRVVT is much smaller than the effect of direct oral anticoagulants
on this assay.7 The mechanism of DRVVT prolongation is likely due to a weak steric
interference with coagulation reactions in which FXa is generated, due to the binding
of FX by emicizumab.8 The lack of effect of emicizumab on TVT was expected, as emicizumab
acts further upstream in the coagulation cascade.
Current guidelines for the detection of LAs recommend DRVVT followed by an LA‐sensitive
aPTT‐based test9; however, results of such work‐up are hard to interpret in the presence
of emicizumab, as its interference could lead to incorrect assay results. Interference
observed with aPTT‐based assays may yield false‐negative results, whereas interference
with DRVVT assays may result in false positives. The TVT may be an alternative option
for evaluating LA status in emicizumab‐containing samples. The assay has also been
proposed as an alternative to the DRVVT for samples containing direct oral anticoagulants.6,
10 However, the assay is not widely used, nor fully standardized, and it does not
have 100% sensitivity for LAs.10, 11
This exploratory study has a number of limitations: Plasma samples were derived from
a small number of individual donors spiked with emicizumab, and not ex vivo samples
from individuals receiving emicizumab therapy. Assay cutoffs from the literature or
from manufacturers’ instructions were applied to the interpretation of the results,
rather than locally generated reference ranges as recommended for clinical use.12
One reagent was tested for each assay; however, a number of different reagents are
commercially available for DRVVT‐ and aPTT‐based LA assays, and emicizumab could have
differential effects on assay outcomes using these different systems. Additionally,
further assays exist for the assessment of LAs than those evaluated in our study;
the dilute prothrombin time in particular may warrant investigation, as our previous
work found a very small reduction in standard prothrombin time in the presence of
emicizumab.1
In conclusion, when analyzing samples from individuals receiving emicizumab therapy,
all aPTT‐based assays should be avoided, including those for LAs, although it is acknowledged
that a few patients who express their LA activity exclusively via aPTT‐based LA tests
could be missed. Further studies on DRVVT and TVT in the presence of emicizumab are
desirable to confirm the safe and accurate use and interpretation of these assays
when testing for LAs in samples containing emicizumab.
CONFLICT OF INTEREST
JIA is an employee of Genentech, Inc, and holds stock with F. Hoffmann‐La Roche Ltd.
IPP is an employee of Genentech, Inc AK is an employee of and holds stock with F.
Hoffmann‐La Roche Ltd.
AUTHOR CONTRIBUTIONS
JIA designed the study, analyzed and interpreted data, and prepared the manuscript.
AK and IPP interpreted data and edited the manuscript. All authors granted final approval
of the manuscript for submission.
Supporting information
Click here for additional data file.