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COAGULATION AND TRANSFUSION MEDICINE
Original Article
D e t e c t i o n
Development
of
of the Factor V
a Testing
and
Algorithm
L e i d e n
Combining
Molecular
M u t a t i o n
a Coagulation
Assay
Diagnosis
LINDA M. WASSERMAN, MD, 1 J. ROGER EDSON, MD, 2 NIGEL S. KEY, MB, MRCR 3 RAJNICHIBBAR, MD, 2
AND RONALD C. McGLENNEN, MD 2
The Coagulation and Molecular Diagnostic laboratories at the
University of Minnesota Medical School (Minneapolis) have
collaborated to develop a diagnostic algorithm to identify all
factor VLeiden mutation carriers without performing unnecessary
and expensive genetic testing. The algorithm uses a coagulation
assay for activated protein C resistance (APCR) to determine the
need for genetic testing. We report the results of our experience
validating this program. We compared the sensitivity, specificity, and positive and negative predictive values of two measures of APCR, the APCR ratio and the normalized ratio. We
found that the normalized ratio was the more sensitive but less
Resistance to activated protein C was first described
by Dahlback et al 1 and Griffin et al 2 in patients
younger than 50 years with a history of v e n o u s
thrombosis but no other known thrombotic risk factors. Activated protein C resistance (APCR) in these
patients was defined as a relative failure to prolong
their activated partial thromboplastin times (APTTs)
by addition of activated protein C.
Plasma-mixing experiments for patients with APCR
could be corrected by all deficient plasmas except
those missing factor V.3 Identification of a G to A transition mutation at codon 506 in the factor V gene
quickly followed.4 The mutation acts as an autosomal
dominant and renders factor Va approximately 10
times less sensitive to proteolysis by activated protein
specific parameter to determine the need for genetic testing. By
using the normalized ratio as the standard by which to refer
patients to the Molecular Diagnostics Laboratory, all mutation
carriers were identified. We found a large overlap in both measures of APCR between symptomatic patients with normal
genotype and mutation carriers. Furthermore, we demonstrated
that increased factor VIII levels with a normal genotype are
associated with apparent APCR. In this article we also review
other correlates of apparent APCR. (Key words: Factor VLeiden
mutation; Activated protein C resistance; Molecular diagnosis)
Am J Clin Pathol 1997;108:427-433.
C. This mutation is found in as many as 96% of
thrombophilic families with hereditary APCR.6 Results
of several European studies suggest that the prevalence of the mutation in white populations may be as
high as 5 % / Consequently, the factor V Leiden mutation
is considered the most common inherited hypercoagulability defect.
In the laboratory examination of the patient with
thrombophilia, the diagnosis of APCR due to the
presence of the factor V Leiden mutation can be made
by combining coagulation assays with genetic testing.
Because the coagulation assay is relatively inexpensive to perform, it is the more cost-effective test to be
ordered first to evaluate APCR. Genetic testing is
then indicated if an abnormal coagulation result is
obtained. Appropriate parameters for defining an
abnormal coagulation result must be established for
theSan
coagulation test. These parameters must be sensiFrom the ^Department of Medicine, University of California,
tive
enough to identify all patients likely to possess
Diego, La jolla, California,
and
the
Departments
of
^-Laboratory
Medicine
and Pathology and 3Medicine, University of Minnesota Medicalthe
School,
mutation but specific enough to limit unnecessary
Minneapolis, Minnesota.
genetic testing.
Since October 1994, the Coagulation Laboratory
Manuscript received November 14, 1996; revision accepted
February 21,1997.
and the Molecular Diagnostics Laboratory in the
Address reprint requests to Dr Wasserman: Molecular Genetics
Department of Laboratory Medicine and Pathology
Laboratory, Division of Medical Genetics, Department of Medicine,
at the University of Minnesota (Minneapolis) have
University of California, San Diego, La Jolla, CA 92093-0639.
427
COAGULATION AND TRANSFUSION MEDICINE
Article
428
jointly p r o v i d e d evaluation of APCR d u e to the
presence of the factor V L e i d e n mutation. The goal of
this program has been to establish a testing algorithm in which evaluation of APCR by coagulation
assay precedes and determines the need for genetic
testing. We report the results of our experience,
including the sensitivity and specificity of a coagulation assay as an indicant of the presence of the
factor V L e i d e n mutation, a comparison of two methods to calculate APCR based on the coagulation
assay results, and an analysis of correlates of apparent or a c q u i r e d APCR in a d d i t i o n to the factor
V
Leiden mutation.
Characteristics
Reference ranges for APCR were established from
v a l u e s d e r i v e d from v e n o u s b l o o d collected in
buffered citrate vacuum tubes from 59 healthy men
and women volunteers younger than 40 years. After
the exclusion of nine outliers, reference ranges for
APCR, calculated as the APCR ratio and the normalized ratio, were determined.
Validation of the molecular assay was based on 22
men and w o m e n , some healthy volunteers, some
patients with thrombophilia, for whom APCR values
based on the coagulation assay had been determined.
The validation sample for the molecular diagnostic
assay was chosen to have a range of normal and
abnormal APCR values.
Continuing validation of the coagulation assay and
determination of its sensitivity and specificity was
based on an additional sample of 125 individuals for
whom molecular diagnostic testing and coagulation
a s s a y s for APCR w e r e c o m p l e t e d . This g r o u p
i n c l u d e d a c o m b i n a t i o n of h e a l t h y v o l u n t e e r s ,
patients with thrombophilia, and relatives of known
factor V L e i d e n mutation carriers.
Coagulation Assay for
Protein C Resistance
APCR ratio = APTT + Activated Protein C
APTT
Normalized Ratio = APCR Ratio of Patient
APCR Ratio Standard
The APCR ratio standard was determined by evaluating the APTT with and without the addition of
activated protein C from aliquots of 100 mL of pooled
venous blood collected from 40 volunteers with norm a l g e n o t y p e s , y o u n g e r t h a n 40 y e a r s , e q u a l l y
divided between men and nonpregnant women, and
not receiving medication.
Factor VIII Assay
METHODS
Sample
Calculation of APCR was performed in two ways:
Factor VIII assays were performed by a one-stage
assay on an (Elecktra-700, M e d i c a l L a b o r a t o r y
Automation, Pleasantville, NY) using i m m u n o d e pleted factor VHI-deficient plasma (manufactured
by Diagnostica Stago, Asnieres, France; purchased
from American Bioproducts, Parsippany, NJ). The
partial thromboplastin time reagent was prepared in
the Coagulation Laboratory as described in the preceding section. The test specimens a n d s t a n d a r d
plasma were each tested at dilutions of 1:10, 1:20,
1:40, and 1:80. Factor VIII activity of the unknown
compared with the standard was calculated using a
Microsoft Excel (Microsoft, R e d m o n d , Wash)
spreadsheet. Standard plasma was a pool containing
equal volumes of plasma from healthy donors with
n o r m a l genotypes, 25 m e n a n d 25 w o m e n , n o n e
older than 40 years. None of the women were taking
oral contraceptives. Factor VIII activities higher than
100% in the experimental specimens were obtained
by a d d i n g v a r i o u s a m o u n t s of an i n t e r m e d i a t e
purity factor VIII concentrate (Humate-P, manufactured by Behring Werke, AG, Marburg, Germany;
d i s t r i b u t e d in t h e U n i t e d S t a t e s b y A r m o u r
Pharmaceutical, Kankakee, 111).
DNA
Activated
Resistance to activated protein C was tested essentially as described by Griffin et al, 2 except that the
partial thromboplastin time reagent used was prepared in the Coagulation Laboratory. It consisted of
an equal-part mixture of the classic Bell and Alton 8
phospholipid reagent and a 0.075% suspension of a
finely divided aluminum silicate powder obtained
from Sigma Chemical(St Louis, Mo).
Extraction
High-molecular-weight DNA was extracted from
peripheral blood lymphocytes anticoagulated with
buffered citrate (acid-citrate-dextrose) or EDTA using
the Puregene DNA Isolation Kit (Gentra Systems,
Research Triangle Park, NC) according to the manufacturer's directions. Quantification of the purified
DNA was determined by measuring the optical density at 260 um with a spectrophotometer (Beckman
DU-64, Allendale, NJ).
AJCP> lober 1997
WASSERMAN ET AL
Testing for Activated Protein C Resistance
429
values was obtained were referred to the Molecular
Diagnostic Laboratory for genetic testing.
Polymerase Chain Reaction
A 100-uL mixture including 500 ng genomic DNA,
1.5 m m o l / L m a g n e s i u m c h l o r i d e , 200 u m o l / L
deoxynucleoside triphosphate, 600 nmol/L oligonucleotide p r i m e r s , a n d 1 U A m p l i t a q (Promega,
Madison, Wis) was used to amplify a 267-base pair
fragment according to the methods of Bertina et al. 4
The parameters for thermocycling include the following: 1 minute at 94°C, then 35 cycles of 94°C for 1
minute, 56°C for 1 minute, and 72°C for 2 minutes. The
primer sequences were 5'-TGCCCAGTGCTTAACAAGACCA-3' and 5'-TGTTATCACACTGGTGCTAA-3'.
Molecular Diagnosis of the Factor V Mutation
The presence or absence of the G to A mutation at
codon 506 of the factor V gene was evaluated by a
combination of polymerase chain reaction and slot
blot hybridization with 3 2 P-labeled allele-specific
TABLE 1. APCR RATIOS AND NORMALIZED RATIOS FOR
REFERENCE AND VALIDATION SAMPLES
Slot Blot
Normalized
Ratio
APCR Ratio
For slot blot hybridization, 40 uL of denatured
polymerase chain reaction product was filtered onto
nitrocellulose (Gibco BRL, Grand Island, NY), crosslinked with 1200 uj of UV light, and hybridized for 1
h o u r at 42°C w i t h 3 2 P-labeled sequence-specific
oligonucleotides (5'-TGGACAGGCgAGGAATAC-3'
for the normal allele, 5'-TGGACAGGCaAGGAATAC3' for the mutated allele). After hybridization, blots
were washed at room temperature for 1 to 2 minutes
and then for 10 minutes in 2x SSPE/0.1% sodium
dodecyl sulfate at 52°C for the normal allele and 49°C
for the mutated allele. Overnight autoradiography
with intensifying screens was performed with medical x-ray film (Fuji, Greenwood, SC).
Sample
Reference
Validation
G/G
G/A
A/A
Number
Mean
SD
Mean
SD
50
2.06
0.24
1.09
0.12
63
58
4
1.74
1.36
1.02
0.26
0.14
0.28
0.90
0.70
0.52
0.14
0.065
0.02
APCR = activated protein C resistance.
G/A
G/G
RESULTS
G/G
Establishment of Reference Ranges ofAPCR
G/G
The reference ranges for the two m e a s u r e s of
APCR, the APCR ratio and the normalized ratio, were
established from APTT values o b t a i n e d using a
reagent made in the Coagulation Laboratory. When
using this in-house APTT reagent, the normal range
of the APTT, the response to increasing concentrations
of heparin, and the prolongation of the APTT associated with a number of inherited deficiencies are similar to APTT values obtained when using the Organon
Teknika (Durham, NC) and Stago APTT reagents.
The reference ranges of the APCR ratio and the
normalized ratio are given in Table 1. An abnormal
value for each parameter was defined as the mean
minus 2 SD. For the APCR ratio, this value was 1.57;
for the normalized ratio, 0.85. Patients examined in
the Coagulation Laboratory for APCR in whom an
APCR ratio or a normalized ratio near or below these
G/G
G/G
G/A
« !
•
A/A
H,0
FIG 1. Slot blot hybridization of normal and mutated factor V alleles. A 267-base pair amplicon was slot blotted onto nitrocellulose,
hybridized for 1 hour at 42°C with 32P-labeled oligonucleotides
specific for the normal or mutated sequence. Blots were washed in
2x SSPE at 49°C for the oligonucleotide bearing the G to A mutation and at 52°C for the oligonucleotide with the normal sequence
and autoradiographed overnight.
Vol. 108 • No. 4
430
COAGULATION AND TRANSFUSION MEDICINE
Original Article
oligonucleotides. The results of a representative slot
blot assay are s h o w n in Figure 1. Allele-specific
oligonucleotide hybridization was achieved by careful
monitoring of the stringency of the hybridization and
wash conditions. Overnight autoradiography was sufficient in most cases.
Comparison of the APCR Ratio and the Normalized
Ratio as Indicators of Factor V Genotype
Quantification of APCR, as measured by the coagulation assay, and determination of factor V L e i d e n mutation status, as evaluated by molecular testing, was
obtained in an additional cohort of 125 men and
women. Most of the cohort members were examined
because of a history of thrombosis. A minority of the
cohort members were asymptomatic volunteers or relatives of known factor V L e i d e n mutation carriers. The
mean APCR ratios and normalized ratios for each
genotype are shown in Table 1. Because this additional cohort included many patients with thrombophilia, the mean APCR ratio and the mean normalized ratio of the sample as a whole and of those with a
normal factor V genotype were lower than the values
obtained from the original reference group.
The relationship between genotype, the APCR
ratio, and the normalized ratio are shown in Figures 2
and 3. Both figures demonstrate a gene dosage effect
on the range of APCR ratio and normalized ratio values. There is also a large region of overlap, indicated
by s h a d i n g in each figure, b e t w e e n the v a l u e s
obtained by factor V L e i d e n mutation heterozygotes and
symptomatic patients with a normal factor V genotype who have apparent APCR.
The relative sensitivity, specificity, and positive and
negative predictive values of the APCR ratio and the
normalized ratio are given in Table 2. Based on the
reference ranges established by our original group of
healthy adults, the normalized ratio proved to be a
more sensitive indicator of the need for genetic testing, although it had lower specificity. None of the factor V L e J d e n mutation carriers were misclassified as
having a normal genotype when using the normalized ratio. In contrast, four heterozygotes would have
been misclassified as having a normal genotype using
the APCR ratio as the standard. Of the 63 patients
•••
•• •
1.57 —-i-JT
FIG 2. Relationship between activated protein C resistance (APCR)
ratios and genotype for the validation sample. APCR ratios and
factor V mutation status were determined for the validation sample. An APCR ratio less than or equal to 1.57 was considered
abnormal. Shading indicates the region of overlap between normal
genotypes (G/G) and mutation carriers (G/A).
G/G
G/A
FIG 3. Relationship between normalized ratios and genotype for the
validation sample. Values less than or equal to 0.85 were considered abnormal. Shading indicates the region of overlap between
normal genotypes (G/G) and mutation carriers (G/A).
AJCP • October 1997
WASSERMAN ET AL
Testing for Activated Protein C Resistance
with a normal genotype in the validation sample, 32
(51%) h a d a p p a r e n t APCR based on n o r m a l i z e d
ratios. In contrast, 13 patients with a normal genotype
were misclassified using the APCR ratio.
The diagnostic algorithm jointly established by the
Coagulation and Molecular Diagnostic Laboratories is
depicted in Figure 4. In this schema, a patient with
thrombophilia who is not receiving anticoagulant
t h e r a p y is referred to the Molecular Diagnostic
Laboratory for genetic testing only if the normalized
ratio is 0.85 or less. Depending on the patient's family
and personal history of thrombosis and the referring
physician's requests, evaluation of other heritable
causes of thrombophilia, including deficiencies of proteins C and S and antithrombin III, may proceed
while the patient's factor V L e i d e n genotype is determined by the Molecular Diagnostic Laboratory.
If the normalized ratio is greater than 0.85, the
p a t i e n t does not have APCR, a n d a coagulation
workup for other causes of thrombophilia continues.
This w o r k u p includes platelet sizing, a D-dimer
enzyme-linked immunosorbent assay (ELISA), a prothrombin fragment 1.2 ELISA, evaluation of the levels
of Clauss fibrinogen, fibrinogen antigen, protein C,
protein S, and plasminogen, as well as evaluation of
the presence of a lupus inhibitor, fluorescent antinuclear antibody, and anticardiolipin antibody. In about
one third of cases, a methionine loading test for
hyperhomocysteinemia is included in the thrombophilia workup.
DNA testing for the factor V L e i d e n mutation proceeds immediately in the patient receiving anticoagulation therapy who has a history of thrombosis. This
algorithm is preferred to other methods of determining APCR in heparinized patients. Evaluating the
APCR ratio by first absorbing plasma on an AT-3 column to remove heparin is more expensive than performing the DNA test. Use of an AT-3 column would
likely introduce dilutional and other artifacts, making
the APCR ratio difficult to interpret.
431
TABLE 2. COMPARISON OF THE APCR RATIO AND THE
NORMALIZED RATIO AS INDICATORS OF THE PRESENCE
OF THE FACTOR VLE|DEN MUTATION
APCR Ratio
(<1.57)
Normalized Ratio
(<.85)
94
79
100
51
82
93
67
100
Sensitivity (%)
Specificity (%)
Predictive value
Positive (%)
Negative (%)
APCR = activated protein C resistance.
Thrombophilia patient
/
\
Anticoagulated
Refer for factor
Leider,genotyping.
v
•
v
Not anticoagulated
If normalized ratio •
<0.85, refer for factor
VLeiden genotyping.
Continue evaluation
of other heritable
causes
cause of thrombosis.
If normalized ratio >0.85,
continue coagulation workup
for other causes of thrombosis.
Patient does not have APCR.
\
If patient is G/G,
continue coagulation
workup for other
causes of thrombophilia,
If patient is G/A or A/A, provide
counseling re: risk factors for other
thrombotic episodes; advise
genetic testing for at-risk family
members.
FIG 4. Diagnostic algorithm for the evaluation of activated protein
C resistance (APCR) in patients with thrombophilia, combining a
coagulation assay with genetic testing.
between increasing factor VLTJ levels and APCR as measured by the APCR ratio and the normalized ratio is
shown in Table 3. As factor VIII levels increase, the
APCR ratio and the normalized ratio approach values
indicating APCR. Thus, patients with normal factor V
genotypes but with elevated factor VIII levels could be
misclassified as having APCR.
Effect of Factor VIII Levels on APCR
Coagulation assays for APCR reflect the combined
effect of complex interactions among proteins in the
coagulation and anticoagulation cascades, including
factors V and VIII, thrombin, antithrombin III, plasminogen, and proteins C and S. To evaluate other correlates of APCR in addition to the factor V ^ ^ mutation,
we measured the effect of increasing factor VIII levels
on the APCR ratio and the normalized ratio on a sample with a normal factor V genotype. The relationship
DISCUSSION
The Coagulation Laboratory and the Molecular
Diagnostic Laboratory at the University of Minnesota
Medical School have collaboratively established a testing algorithm for evaluation of APCR as a cause of
thrombophilia. The schema evaluates the diagnostic utility of using the less expensive and less time-consuming
coagulation assay to determine the need for genetic testing for presence of the factor V ^ ^ mutation in patients
Vol. 108 • No. 4
432
COAGULATION AND TRANSFUSION MEDICINE
Original Article
with thrombophilia who are not receiving anticoagulation therapy.
We compared the sensitivity and specificity of
two methods of quantifying APCR by the coagulation assay and found the normalized ratio, which
c o m p a r e s the APCR ratio of the p a t i e n t to the
APCR ratio of pooled reference plasma to be the
more sensitive measure. No mutation carriers were
misclassified as having a normal genotype by this
measure. We favor use of the normalized ratio as
the most sensitive parameter because of the lifetime risk for thrombotic episodes in mutation carriers and the need to educate them about prevention
of future episodes, as well as the i m p o r t a n c e of
identifying first-degree relatives w h o also may
carry the mutation. However, maximizing the sensitivity of the c o a g u l a t i o n assay led to a lower
specificity and positive predictive value. Using this
algorithm, 32 of 125 patients referred for genetic
testing h a d a n o r m a l g e n o t y p e a n d a p p a r e n t or
acquired, rather than hereditary, APCR. Zehnder
and Benson 9 reported a similar loss of specificity
when maximizing the sensitivity of a commercially
available APCR assay to identify factor V L e i d e n
TABLE 3. EFFECT OF INCREASING FACTOR VIII
LEVELS ON APCR
Factor V11I Level (%) APCR Ratio
94
155
370
620
1.70
1.70
1.56
1.42
Normalized Ratio
0.88
0.88
0.81
0.74
APCR = activated protein C resistance.
TABLE 4. CORRELATES OF APPARENT APCR
WITH A NORMAL FACTOR V GENOTYPE
Correlate
Elevated factor VIII (> 100%)
Use of oral contraceptives
Female sex
Age 35 years or older
Reduced protein S
Elevated protein C
Elevated plasminogen
Presence of lupus anticoagulant
Presence of P2-glycoprotein
I antigen-antibody complexes
APCR = activated protein C resistance.
Reference No.
Current study, 10
10,11,13
Current study, 10-13
12
Current study, 12
Current study, 11
Current study
16
17,18
m u t a t i o n carriers. When factor V L e i d e n m u t a t i o n
carriers are identified by genetic testing, they are
referred for genetic counseling. They are informed
a b o u t risk factors a s s o c i a t e d w i t h t h r o m b o t i c
episodes, and they are counseled about the need
for genetic testing of first-degree relatives, siblings,
and children, who have a 50% risk of also carrying
the mutation.
False-positive or acquired APCR in a patient with
a normal factor V genotype is known to be correlated
with a variety of factors, including abnormal concentrations of procoagulant and regulatory proteins, age,
female sex, oral contraceptive use, and presence of
lupus anticoagulant and anticardiolipin antibody. 10-16 Reviewing with clinicians the role of these
factors in causing apparent APCR in patients with a
normal factor V genotype is an important function of
the directors of the Coagulation and the Molecular
Diagnostic Laboratories.
We have found that increasing factor VIII levels
decreased the APCR ratio and the normalized ratio
to levels indicating APCR, a relationship noted by
others. 10 We have confirmed as well the observation
that an abnormal APCR ratio and normalized ratio
in a patient with thrombophilia and a normal factor
V genotype can be associated with abnormally elevated levels of protein C, antithrombin III, and plasminogen and with reduced levels of protein S (data
not shown). 11
In our sample, 78% (28 of 32) of those with a normal factor V g e n o t y p e a n d APCR w e r e w o m e n .
O t h e r s also h a v e n o t e d l o w e r APCR r a t i o s in
women. 10,12 Calkins et al, 12 in validating the coagulation assay for APCR for the S c r i p p s Reference
Laboratory (La Jolla, Calif), suggested use of separate
reference ranges for men and women because women
had, on average, lower APCR ratios. This group also
f o u n d an a s s o c i a t i o n w i t h a g e ; APCR r a t i o s
decreased progressively in both sexes after 35 years
of age. We did not observe this age effect in our sample. The group of patients with a normal factor V
genotype and apparent APCR ranged in age from 25
to 77 years.
Several groups have noted that apparent APCR in
women is compounded by use of oral contraceptive
pills and have suggested that this effect might be
mediated by the effect of hormone modulation on
procoagulant and regulatory protein levels. 1 0 ' 1 1 ' 1 3
Indeed, epidemiologic studies have shown that the
use of oral contraceptives by factor V L e i d e n mutation
carriers is associated with an 8- to 10-fold increase in
the risk of having a thrombotic episode. 14,15
AJCP • October 1997
WASSERMAN ET AL
Testing for ActivaU
Protein C Resistance
The presence of antiphospholipid antibodies is
likewise associated with acquired APCR.16"18 Potzsch
et al 1 6 defined APCR as the amount of factor VIII
inactivation found when adding activated protein C
to plasma of patients with lupus anticoagulant. They
found apparent APCR in patients with histories of
thrombotic episodes and presence of a lupus anticoagulant. Matsuda et al 17 found that addition of (^"Sty"
coprotein I antigen-antibody complexes to pooled
normal plasma led to a dose-dependent decrease in
the APCR ratio. (^-glycoprotein I is believed to represent the cofactor involved in the binding of antibodies
to cardiolipin. Table 4 summarizes the various correlates of apparent or acquired APCR.
We have demonstrated that a cooperative effort
b e t w e e n the C o a g u l a t i o n L a b o r a t o r y a n d the
Molecular Diagnostic Laboratory can provide costeffective examination of the patient with thrombophilia for the presence of APCR due to the factor
^Leiden m u t a t i o n . The use of a testing algorithm
combining a less-expensive coagulation assay with
m o r e - e x p e n s i v e m o l e c u l a r genetic t e s t i n g w a s
developed to optimize diagnostic sensitivity while
keeping overall testing costs down. When the normalized ratio as a measure of APCR was greater
than the reference value, genetic testing was not
necessary. Only patients in whom normalized ratio
values approaching or below the reference value
were obtained were referred for genetic evaluation.
I m p l e m e n t a t i o n of this a l g o r i t h m identified all
patients w h o had the factor V L e i d e n mutation. We
anticipate that future developments to simplify the
molecular testing method will provide further cost
savings and diagnostic efficiency while maintaining
the current level of sensitivity in identifying factor
V L e i d e n mutation carriers.
Whether further reduction in costs of molecular
testing for the factor V L e i d e n mutation will eliminate
t h e u t i l i t y of d e t e r m i n i n g APCR is d e b a t a b l e .
Whereas the most common cause of APCR is the factor V L e i d e n mutation, other as yet undefined factor V
or factor VIII mutations could lead to APCR. APCR
remains a relatively inexpensive laboratory test to
measure thrombotic risk.
433
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