Randomized Prospective Trial Comparing the Native Prothrombin Antigen with the
Prothrombin Time for Monitoring Oral Anticoagulant Therapy
By Bruce Furie, Caryn F. Diuguid, Margaret Jacobs, David L. Diuguid, and Barbara C. Furie
The dosage of t h e anticoagulant warfarin sodium is based
upon t h e prolongation of t h e prothrombin time into an
optimal therapeutic range. We have developed a new assay
for t h e native prothrombin antigen t h a t measures t h e fully
y-carboxylated prothrombin using a radioimmunoassay.
Based on preliminary data that indicated that t h e native
prothrombin antigen predicted both bleeding and thrombotic complications more accurately than t h e prothrombin
time in patients anticoagulated with warfarin sodium, w e
have performed a randomized prospective trial comparing
the complication rate in warfarin-treated patients monitored with t h e native prothrombin antigen or t h e prothrombin time. Patients with indications for anticoagulation were
randomized to be monitored by t h e native prothrombin
antigen (therapeutic range, 12 to 24 pg/mL) or t h e prothrombin time index (therapeutic range, 1.5 to 2.0). Of t h e
prothrombin time group (N = 80). seven (8.8%) had
bleeding or thrombotic complications, with a complication
rate of 9.5%/patient-year. In t h e native prothrombin antigen group (N = 76). one subject (1.3%) had a bleeding
complication. The complication rate per patient-year w a s
1.5%. These results indicate an 85% reduction in the
complication rate of t h e native prothrombin antigen group
compared with t h e complication rate of t h e prothrombin
time group. This difference is statistically significant by t h e
Fisher exact test (P= .037) and by Kaplan Meier survival
analysis (P = .040).
This study suggests that t h e use of t h e
native prothrombin antigen assay has the potential to
decrease the complications associated with anticoagulation therapy with warfarin sodium.
W
tions but provides the necessary antithrombotic effect, successful efforts to reduce the incidence of bleeding complications
have been based on the reduction in the intensity of warfarin
therapy.3.5.i0
We have previously evaluated a new strategy for the
monitoring of warfarin therapy that is based upon the
measurement of the fully carboxylated, biologically active
prothrombin.” Prothrombin is synthesized in the liver in a
precursor form.I2 In a reaction directed by the y-carboxylation recognition site on the propeptide of the vitamin
K-dependent protein~,”.’~
a hepatic vitamin K-dependent
carboxylase converts glutamic acid residues near the aminoterminus of these proteins to y-carboxyglutamic acid. In the
presence of calcium, these proteins assume a metal-stabilized
conformation that facilitates the protein-membrane interaction required for biologic activity. Anti-prothrombin-Ca(I1)
antibodies are conformation-specific for the metal-stabilized
conformer of prothrombin.''^'^ These antibodies, specific for
antigenic determinants on the fully carboxylated native
prothrombin, do not bind to poorly carboxylated or uncarboxylated species that are functionally inert.I6
We have previously demonstrated that the native prothrombin antigen correlates closely with prothrombin coagulant
activity.” We have also measured the native prothrombin
antigen (NPT) and the prothrombin time in samples obtained from patients treated with warfarin to determine (1)
the levels of the N P T and the prothrombin time that are
associated with bleeding and thrombotic complications; and
(2) the levels of the N P T and the prothrombin time not
associated with complications.” In 20 patients with complications, the serum N P T was below 12 pg/mL in all samples
associated with bleeding complications (13 of 13) and above
24 pg/mL in all but one sample associated with thrombotic
complications (6 of 7). On this basis, a therapeutic range of
12 to 24 pg/mL was suggested, in contrast to the native
prothrombin level of 108 19 pg/mL in normal individuals.
By comparison, a prothrombin time in excess of 2.0 was
associated with an increased risk of bleeding, but no therapeutic range could be identified that separated bleeding and
thrombotic complications. These results implied that the
ARFARIN SODIUM is a widely used vitamin K
antagonist that is used as an oral anticoagulant.’
This drug inhibits the action of vitamin K, thereby lowering
the biological activities of the plasma vitamin K-dependent
blood coagulation proteins and causing an antithrombotic
effect. Warfarin is commonly used to treat patients who have
developed thromboembolic disease and are prone to recurrences or to treat patients for disorders known to be associated with thromboembolic complications.’32 Although
warfarin as an anticoagulant is highly efficacious, its toxictherapeutic ratio is narrow. In current practice, the dosage of
warfarin given is titrated by monitoring the prothrombin
time and maintaining the prothrombin time in a therapeutic
range. Despite careful attention to the prothrombin time
measurements, 10%to 20% of patients treated with warfarin
develop a bleeding complication attributed to warfarin therapy or a thromboembolic complication due to inadequate
warfarin the rap^.^.^ Although there remains considerable
discussion concerning the optimal prolongation of the prothrombin time, ie, that which minimizes bleeding complica~~
From the Center for Hemostasis and Thrombosis Research,
Division of HematologylOncology. New England Medical Center;
and the Department of Medicine, Tufts University School of
Medicine, Boston, MA.
Submitted June 16,1989: accepted September 21.1989.
Supported by Grants No. HL21543 and HL18834 from the
National Institutes of Health. The clinical study, including data
storage and analysis with CLINFO, was supported by Grant No.
RROO54 from the General Clinical Research Centers Branch of the
Division of Research Resources of the National Institutes of
Health, Bethesda, MD.
Address reprint requests to Bruce Furie, MD. Center for H e m e
stasis and Thrombosis Research, New England Medical Center,
750 Washington St, Boston, MA 021 1 1 .
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be hereby marked
“advertisement” in accordance with 18 U.S.C.section 1734 solely to
indicate this fact.
0 I990 by The American Society of Hematology.
0006-4971/90/7502-O010$3.00/0
344
0 1990 by The American Society of Hematology.
Blood, Vol 75,No 2 (January 15). 1990: pp 344-349
345
CLINICAL TRIAL OF NATIVE PROTHROMBIN ANTIGEN
NPT correlated more closely with complications than the
prothrombin time, and that patients monitored with the NPT
assay might be predicted to have a 7.5- to 8-fold lower
number of complications than patients monitored with the
prothrombin time. However, the practical problems of applying the NFT assay to the control of anticoagulant therapy
were not evaluated. To determine the role of the NPT assay
in warfarin monitoring during anticoagulant therapy, we
have now performed a controlled prospective randomized
trial to compare the complication rates in patients monitored
with the NPT assay or the prothrombin time. In the study
design used, the warfarin-associated complication rates of
two random subgroups were measured. One group was
monitored exclusively with the native prothrombin antigen
assay; the other group was monitored exclusively with the
prothrombin time. Clinically relevant endpoints, bleeding,
and thromboemboliccomplications, were documented. Other
issues, including the time for assay of the NPT, the relationship in vivo of changes in warfarin dosage to changes in the
NPT, the stability of the NPT-monitored dosage, and the
comparative accuracy of a chemical assay (NPT) and a
biological assay (prothrombin time), which might influence
the performance of the NPT assay in comparison with the
prothrombin time, were addressed in a direct comparison in
the randomized prospective clinical trial. These results indicate a significant 85% reduction in the complication rate in
the group monitored with the NPT assay compared with the
group monitored with the prothrombin time.
EXPERIMENT
Study Design
Adults with indications for warfarin treatment that were considered eligible for the study were referred to members of this study
team. Participants were inpatients or outpatients at New England
Medical Center, Boston, MA. Each patient was randomized to
Group A, to be monitored by the NPT assay, or to Group B, to be
monitored by the prothrombin time index (PTI), using a system of
sealed envelopes. Those patients initially treated with heparin were
all monitored with the prothrombin time targeted to 1.5 to 2.0 times
the control value during the brief transition from heparin to
warfarin. Subsequently they were monitored by the assay lo which
they had been randomized. The warfarin dosage was regulated by
the nurse member of this study group. The therapeutic range of the
NPT was defined as 12 to 24 pg/mL, with a target of 18 pg/mL."
The therapeutic range of the PTI was defined as 1.5 to 2.0, with a
target of 1.75. This PTI therapeutic range represents standard
intensity warfarin therapy?
Patients were monitored two times per week for the first month,
one time per week for the second month, one time every two weeks
for the third and fourth month, and one time per month thereafter. If
either the NPT or PTI were outside of the defined therapeutic range,
the warfarin dosage was reconsidered and adjusted; and the test was
repeated one week later. Patients were continued on study until the
onset of a complication (failure), the subject was unable to comply
with the study protocol. therapy was completed, the patient died of a
disease process unrelated to anticoagulant therapy, or the study was
terminated. The study was terminated when, on semiannual review,
the complication rate of two study arms (NPT group and PTI group)
showed statistically significant divergence, as tested using the Fisher
exact test.
Hemorrhagic complications were identified by clinical observation
by a member of this study team. Internal bleeding was confirmed by
computerized tomography scans. Thrombotic and embolic complications were diagnosed by venography or arteriography. Informed
consent was obtained from all participants. The study protocol was
reviewed and approved by the Human Investigational Review
Committee of New England Medical Center.
Laboratory Evaluation
All coagulation assays were performed in the Special Coagulation
Laboratory at New England Medical Center. The prothrombin time
was performed using Simplastin reagents purchased from General
Diagnostics Div. (Organon Teknika, Durham, NC). The NPT assay,
a radioimmunoassay specific for the carboxylated form of prothrombin, was performed as previously des~ribed.'~.''In this assay,
conformation-specific anti-prothrombin antibodies that do not bind
to abnormal prothrombin are used to measure native prothrombin in
serum using a competitiorr immunoassay.
Statistical Analysis
Characteristics of the two study groups were compared by the
Fisher exact test. Treatment failures were evaluated by the Kaplan
Meier survival analysis. Differences between the treatment groups
were evaluated by the log rank test.
RESULTS
Patients
Over the course of the study, 156 patients at New England
Medical Center requiring warfarin sodium therapy met
study eligibility criteria and were willing to give informed
consent. Seventy-six patients were randomly assigned to
Group A to be monitored with the NPT assay, and 80
patients were randomly assigned to Group B to be monitored
with the prothrombin time. The clinical characteristics of
these two patient subsets are compared in Table 1 with
regard to age, sex, and indications for anticoagulant therapy.
No statistically significant differences were observed.
Table 1. Clinical Characteristics of 166 Warfarin-Treated
Patients Monitored With t h e Native Prothrombin
Antigen or the Prothrombin Time
Characteristic
G~OUD
A NPT
Grow B: PTI
N
Sex (M/F)
Age (Y)
(mean)
(minimum)
(maximum)
Indications
Atrial fibrillation
Prosthetic valve
CVA
Venous thrombosis
Pulmonary embolism
Arterial thrombosis
Cardiomyopathy
Ventricular aneurysm
Myocardial infarction
Arterial embolism
Graft
76
3514 1
80
36/44
54.9
21.5
80.1
51.8
18.5
85.5
17 (22.4)
9 (11.8)
2 (2.6)
25 (32.9)
12 (15.8)
l(1.3)
4 (5.2)
l(1.3)
4 (5.3)
0 (0.0)
l(1.3)
17 (21.8)
8 (10.2)
2 (2.6)
29 (36.3)
15 (19.2)
0 (0.0)
5 (6.4)
0 (0.0)
2 (2.6)
l(1.3)
2 (2.6)
~~
Values in parentheses indicate percentage of group.
346
FURlE ET AL
Table 2. Comparisonof Results in the Two Subsets
Maintenance of Anticoagulant Therapy Based
on the NPT or PTI
Group A ( N = 76) was maintained on the appropriate
dosage of warfarin based on titration of the NPT. Of this
group, one patient of 76 (1.3%) developed a complication. In
the absence of complications related to warfarin, 32 of 76
patients (42%) completed their course of warfarin therapy,
14 of 76 patients (18.4%) developed contraindications to
anticoagulants or died from their primary disease process,
and 20 of 76 patients (26%) were active participants when
the study was terminated. Nine of 76 (11.8%) were terminated from continued participation in the study after they
were no longer able to comply with the monitoring protocol.
Data from all 76 patients were used in the analysis.
In the absence of prior experience with the use of the N P T
assay to monitor anticoagulant therapy, we observed that
patients could be regulated with this assay. The NPT,
expressed in pg/mL, was obtained from serum samples and
was reproducible to +7%. Over the course of the study, we
accumulated 66.92 patient-years of experience with this
assay, in 76 individual patients. The mean time on study was
45.8 weeks. A typical pattern of the NPT over time for a
single patient is shown in Fig 1. During a 13 month period,
this patient was maintained with a warfarin dosage that
averaged 5.0 to 6.0 mg per day. Of the 19 NPT determinations, 14 were within the defined therapeutic range. The
N P T value of 8.3 pg/mL can be associated with bleeding
complications,” but was not in this instance. Despite minimal variability of the warfarin dosage, there was variation in
the measured NPT. This likely represents biological variations due to minor changes in diet or warfarin metabolism.
Total N
Complications
Bleeding
Thromboembolic
Time on study (cumulative wks)
(Mean wks)
Complications/lOON
Complications/patient-year (%)
Warfarin (mg)/d (average)
Group A NPT
Group 6: PTI
76
1
1
EO
7
5
2
3,822
47.8
8.8
9.5
7.2
0
3,480
45.8
1.3
1.5
7.1
The intensity of warfarin therapy was measured by determining the average daily warfarin dose of all patients in the
Group A subset. In the N P T group, the average daily
warfarin dose was 7.1 mg/d.
Group B ( N = 80) was maintained on standard intensity
warfarin therapy based on the prothrombin time. In this
study, our goal was a therapeutic range of 1.5 to 2.0. In this
group, seven patients of 80 (8.8%) developed a complication.
In the absence of complications related to warfarin, 35 of 80
patients (43.8%) completed their course of warfarin therapy,
10 of 80 patients (12.5%) developed contraindications to
anticoagulants or died from their primary disease process,
and 20 of 80 patients (25%) were active participants when
the study was terminated. Eight of 80 (10.0%) were terminated from continued participation in the study after they
were no longer able to comply with the monitoring protocol.
Data from all 80 patients were used in the analysis. In the
PTI group, the average daily warfarin dose was 7.2 mg/d.
Complications
Group A: NPT
During the course of the clinical trial, none of the patients
(0/76; 0%) monitored with the N P T assay had thromboembolic complications. One of 76 patients (1.3%) in this group
had a bleeding complication. The total complication rate for
this group was 1.3% (Table 2). Corrected for the time a t risk
to anticoagulant therapy, the complication rate was 1.5% per
patient-year.
Complication 1 . E.H. was a 57-year-old woman with
hemolytic anemia and recurrent thrombophlebitis. She was
treated with warfarin over a 5 month period without incident
before the drug was discontinued. One month later, she
developed deep venous thrombosis in the left leg and warfarin
therapy was reinitiated. She was randomized to be monitored
by the N P T assay. After 32 months on continuous warfarin
therapy, she developed a conjunctival hemorrhage.
h
I-
%
40
Group B: PTZ
100
200
300
~~
460
DAYS
Fig 1. Monitoring of warfarin dosagewith the native prothrombin antigen. A patient on chronic warfarin therapy because of
aortic and mitral prosthetic valves was evaluated with the NPT
assay, and the warfarin dose adjusted accordingly. The goal was to
maintain the NPT at 1 8 pg/mL. within the therapeutic range of 1 2
to 24 pglmL. The NPT in normal subjects is 108 ? 1 9 pglmL.
During the course of the clinical trial, two of 80 patients
(2.5%) in the group monitored with the prothrombin time
(Group B) had thromboembolic complications. Five of 80
patients (6.3%) in this group had hemorrhagic complications. The total complication rate for Group B was 8.8%
(Table 2). Corrected for the time a t risk to anticoagulant
therapy, the complication rate was 9.5% per patient-year.
CLINICAL TRIAL OF NATIVE PROTHROMBIN ANTIGEN
Complication 1. L A . was a 25-year-old woman with
systemic lupus erythematosus and a documented lupus
anticoagulant. Because of deep venous thrombosis in her
right leg, she was treated with warfarin over a 5 month
period. She was randomized to be monitored with the
prothrombin time. On anticoagulant therapy monitored by
the prothrombin time, she developed deep venous thrombosis
in her left leg.
Complication 2. C.D. was an 80-year-old woman who
was begun on warfarin for the onset of atrial fibrillation.
After 4 months on warfarin therapy monitored by the
prothrombin time, she had a cerebrovascular secondary to an
embolus.
Complication 3. P.C. was a 69-year-old man who developed mural thrombi after a myocardial infarction. After 7
months on warfarin therapy monitored with the prothrombin
time, he presented with hematuria. He was begun on Motrin
1 week before this complication.
Complication 4. J.L. was a 39-year-old man with deep
venous thrombosis of the right leg documented by venography. After 3 months of warfarin therapy monitored with the
prothrombin time, he developed a hemarthrosis in the right
elbow in the absence of trauma.
Complication 5. S.A. was a 42-year-old female with
systemic lupus erythematosus and a lupus anticoagulant. She
developed a pulmonary embolus and, after initial treatment
with heparin, was treated uneventfully with warfarin for 9
months. After discontinuing the warfarin, she presented 1
year later with a pulmonary embolus and deep venous
thrombosis of the right leg. She was placed on warfarin
therapy and randomized to be monitored with the prothrombin time. Fifteen months later, she developed a large hematoma on her right thigh and lower leg in the absence of
trauma.
Complication 6. R.G. was a 31-year-old man with coronary artery disease and atrial fibrillation. Because of the
potential risks of thromboembolic complications, he was
placed on warfarin therapy and randomized to be monitored
with the prothrombin time. Three months later, he developed
large hematomas on the left shoulder and left thigh in the
absence of trauma.
Complication 7. A.M. was a 36-year-old woman with
radiation-induced cardiomyopathy complicating her treatment for stage IIA Hodgkin’s disease. After a pulmonary
embolus and deep venous thrombosis in her right leg, she was
begun on chronic warfarin therapy after initial heparin
treatment. She was randomized to be monitored with the
prothrombin time. After 23 months of warfarin treatment,
she developed a large hematoma on the right thigh in the
absence of trauma.
Comparison of the Complication Rates
A summary of the complication rates in the two study
groups is shown in Table 2. The total complication rate in the
NPT group was 1.3%, and the total complication rate in the
PTI group was 8.8%. These differences are statistically
significant (P = .037, one-tailed Fisher exact test). When
corrected for the time on therapy, the complication rate was
347
1.5% per patient-year for the NPT group and 9.5% per
patient-year for the PTI group.
Survival analysis of these data also indicate statistically
significant differences in the outcome in the two study
groups. As shown in Fig 2, the proportion of complicationfree patients is compared with the number of weeks of
warfarin therapy using the Kaplan-Meier method. The NPT
group and the PTI group differ significantly (P = .040;
log rank test).
DISCUSSION
Effective antithrombotic therapy has been hampered by
the morbidity and occasional mortality associated with such
therapy. Two separate approaches have been recognized to
potentially decrease the complication rates associated with
anticoagulant therapy. First, more sensitive and accurate
methods are required to identify those patients with the
hypercoagulable state.I8 In the absence of such diagnostic
modalities in clinical practice, historical practice patterns
dictate that large numbers of patients are subjected to the
risk of anticoagulation therapy who are not likely to require
such therapy. Measurement of prothrombin fragment 1 2,
the thrombin-antithrombin I11 complex, protein C activation, and platelet activation-specific antigens offer some of
the approaches that may prove practical in identifying
patients with a prethrombotic state at risk to the development of thromboembolic disease.”-*’Sparing subpopulations
that are not prone to thrombosis would decrease the total
numbers of patients that incur anticoagulant-induced complications. Second, those patients likely to benefit from antithrombotic therapy must be given an optimal dosage so as
to eliminate thrombosis and minimize warfarin-induced
hemorrhagic complications. It is this latter problem that is
the focus of the current study.
Despite adequate monitoring of the prothrombin time,
approximately 10%to 20% of patients treated with warfarin
develop a bleeding complication secondary to warfarin or a
thromboembolic complication because of inadequate warfarin therapy.3*4v6-8.22
Although some of these complications
are predicted by prothrombin times outside of the therapeutic range, approximately half occur despite a therapeutic
3
*O
40
80
120
160
200
WEEKS
Fig 2. Survival analysis of failures in the NPT group and PTI
group. The cumulative percentage of patients without hemorrhagic or thromboembolic complications is compared with the
number of weeks of warfarin exposure. The results were analyzed
according to the Kaplan Meier method. The logrank test of .04
indicates a significant difference between the two survival curves.
348
prothrombin time. In our earlier analysis of warfarinassociated complications, six of 20 complications (30%)
occurred with a prothrombin time index between 1.5 and 2.0
(standard intensity therapy), and five of 20 complications
(25%) occurred with a prothrombin index between 1.3 and
1.8 (low intensity therapy).”
The problem of thromboplastin standardization has been
addressed by the development of the British Comparative
Thr~mboplastin.’~
Studies with human brain thromboplastinbased reagents have emphasized that patients in North
America monitored with the Simplastin-based prothrombin
time are subjected to higher intensities of warfarin therapy.24
For these reasons, Hull et a]**have examined the efficacy and
the complication rates of low intensity and standard intensity
warfarin therapy in prospective controlled studies of patients
requiring anticoagulant therapy. Lower intensity therapy
decreased the bleeding complication rate from 22% to 4% in
patients with proximal vein thrombosis3 and from 13.9% to
5.9% in patients after tissue heart valve repla~ement.~
However, standard and lower intensity therapies were associated with similar thromboembolic complication rates: 2.0%
and 2.1%, respectively, in patients with proximal vein
thrombosis3 and 1.9% and 2.0%, respectively, in patients
after tissue heart valve replacement.’ These well-designed
clinical trials have clearly demonstrated that lower intensity
warfarin therapy decreased the hemorrhagic complication
rate associated with this therapy, and that the efficacy of low
intensity antithrombotic therapy in the treatment of these
disorders is equivalent to standard intensity therapy. The
improvements introduced with low intensity therapy have
been able to reduce bleeding complications. Low dose therapies have not contributed to a reduction in the 2% rate of
thromboembolic complications in the conditions studied.
In the current study, we have employed a new strategy,
unrelated to the prothrombin time, to monitor warfarininduced anticoagulation. Assay of the fully carboxylated
prothrombin using a specific immunoassay allows for the
measurement of a plasma component against a chemical
standard with the precision and reproducibility of an immunometric detection system. W e have previously compared the
NPT assay and prothrombin time in warfarin-treated patients with and without complications.” Regardless of whether
the PTI therapeutic range was defined as 1.3 to 1.8, 1.5 to
2.0, 1.5 to 2.2, or 1.5 to 2.5, a complication rateof 4.5 to 4.9
complications per 100 patients was measured for the prothrombin time while a complication rate of 0.6 complications
per 100 patients was measured for the NPT. This eightfold
difference provided the impetus to compare the N P T assay
and the prothrombin time in a prospective, randomized
controlled trial.
We have now shown that the complication rate of the NPT
group was 1.3% in the current study, an 85% reduction
compared with the complication rate of 8.8% of the prothrombin time group. The complication rate measured in our
prothrombin time group is lower than the complication rate
measured in other recent trials using standard intensity
warfarin therapy, where the prothrombin time is targeted
between 1.5 and 2.0 (Table 3).Values of 22.4%,3 17.0%?
FURlE ET AL
Table 3. Complication Rates Associated Wkh Standard Intensity
Warfarin Therapy Based on Monitoring the Prothrombin Time
Hemorrhagic
Thromboembolic
Total
This study: Group 8
5/80 (6.3%)
2/80 (2.5%)
7/80 (8.8%)
Hull et al, 19823
11/49 (22.4%) 1/49 (2.0%) 12/49 (24.5%)
Hull et al, 198Z4
9/53 (17.0%) 1/53 (1.9%) 10/53 (18.9%)
Turpieet al, 198E5 15/108 (13.9%) 2/108 (1.9%) 17/108 (15.7%)
Hullet al, 1979”
7/33 (21.2%) 0/33 (0%)
7/33 (21.2%)
The prothrombin time therapeutic range is defined as 1.5 to 2.0. The
numerator indicates the number of patients with complications associated with warfarin therapy. The denominator indicates the total number
of patients in the group.
21.2%:’
and 13.9%’ have been observed in patients with
different therapeutic indications when treated with standard
intensity warfarin therapy, but we observed a hemorrhagic
complication rate of 6.3% using equivalent intensity therapy
in the current study. The thromboembolic complication rate
of 2.5% in the current study is similar to that previously
r e p ~ r t e d . ~Although
-j
we do not have an explanation for the
lower incidence of complications in our prothrombin time
group compared to earlier studies, the significant difference
between the complication rates in our NPT group and our
prothrombin time group measured in the current trial is
emphasized since this difference is observed due to decreased
complications in the NPT group and not to an increase in the
complication rate of the prothrombin time group relative to
historical controls. The differences observed (6.7-fold) are
comparable to the eightfold difference that we observed in
our earlier comparative study.” If these data are expressed
as complications per patient-year, discrepancies due to small
differences in group size and time on study are corrected. In
this case, the complication rate of the NPT group is 1.5%/
patient-year, and the complication rate of the prothrombin
time group is 9.5%/patient-year. The complication rate in
the NPT group is lower than those observed in the clinical
trials employing low intensity therapy, where complication
rates of 7.8%’ and 6.4%3 have been reported. Since the
therapeutic ranges of the prothrombin time and NPT have
been independently derived, we were interested to ascertain
whether one subset was subjected to higher intensity warfarin therapy than the other. Group A received an average
daily warfarin dose of 7.1 mg/d; Group B received an
average daily warfarin dose of 7.2 mg/d. These values are
not significantly different (P = .68).
For the past 40 years, the monitoring of anticoagulation
therapy with vitamin K antagonists has been based upon the
prolongation of the prothrombin time. Although the prothrombin time has contributed significantly to the safety of
oral anticoagulant therapy with warfarin sodium, hemorrhagic complications represent a significant problem, even in
patients who are closely controlled. The introduction of low
intensity warfarin regimens has led to a reduction in these
complication^.^^'^ However, this assay, in which rabbit brain
and lung extracts are added to recalcified, relipidated plasma,
is a complex biologic test. These thromboplastins vary in
coagulant potency, composition, and stability. Factors including the concentration of citrate anticoagulant in plasma
CLINICAL TRIAL
349
OF NATIVE PROTHROMBIN ANTIGEN
samples, the chemical nature of the vessel in which the blood
sample is collected, the time delay from venipuncture to
plasma assay, and the sample temperature all contribute to
the laboratory variation of the prothrombin time.25 In contrast, the NPT is an immunoassay that can be configured to
offer rapid and precise measurements of the fully carboxylated prothrombin in serum or plasma samples.” Furthermore, we have previously demonstrated that the NPT correlates to the development of bleeding and thromboembolic
complications more closely than does the prothrombin time.”
T h e current clinical trial confirms that the NPT assay
reduces the complication rate associated with prothrombin
time monitoring when judged in a prospective study with a
clinical end point. Our experience in this study included
monitoring of anticoagulant therapy in 76 patients with the
NPT assay exclusively; over 66 patient-years of experience
were accrued, with only a single complication. T h e results of
our previous work and this prospective randomized trial
suggest that the introduction of t h e NPT into clinical
laboratory practice has the potential to significantly reduce
complications in patients treated with warfarin.
ACKNOWLEDGMENT
We are most grateful to our colleagues at New England Medical
Center for allowing us access to their patients. We also thank Dr T.
Heerens (Center for Health Services Research and Study Design,
New England Medical Center) for his expert assistance with the
statistical analyses.
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