(Circulation. 1997;96:2578-2585.)
© 1997 American Heart Association, Inc.
Articles |
From the Department of Medicine and Cardiology (M.S.L., K.T., J.R.) and Department of Clinical Biochemistry (L.H.), Aarhus University Hospital, Denmark.
Correspondence to Michael Stausbøll Lüscher, Department of Medicine and Cardiology, Aarhus University Hospital, Tage Hansens Gade 2, DK-8000 Aarhus C, Denmark.
| Abstract |
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Methods and Results Patients (n=516) suspected of
having unstable coronary artery disease were investigated.
Follow-up was done after 30 days, and the occurrences of cardiac death,
acute myocardial infarction, refractory angina pectoris, and recurrent
angina pectoris were registered. Elevated levels of troponin T (
0.10
µg/L) were associated with an increased risk of cardiac death at 30
days compared with patients with normal levels, 3.2% versus 0.4%
(P=.014). Troponin I values above the chosen cutoff (2.0
µg/L) were similarly found to be an indicator of increased risk of
cardiac death, 3.2% versus 0.7% (P=.026). With regard to
the composite end point of cardiac death/acute myocardial infarction,
the troponins were strong independent indicators of adverse
outcome.
Conclusions In patients suspected of having unstable coronary artery disease, both troponin T and troponin I provide independent prognostic information with regard to cardiac death and acute myocardial infarction.
Key Words: prognosis myocardial infarction angina troponin
| Introduction |
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| Methods |
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Inclusion Diagnosis
The study population of 516 patients, as a part of the main TRIM
protocol, was retrospectively categorized as either unstable AP
(n=309), nonQ-wave AMI (n=190), or other diseases (n=17). The
diagnoses were based on the clinical course, ECG changes, and plasma
levels of cardiac markers available at each participating center (total
CK, CK-MB mass, or CK-MB catalytic activity, obtained within 6 hours
after inclusion).
Definition of End Points
All patients were subjected to a follow-up examination 30 days
after enrollment. AMI, cardiac death, refractory AP, and recurrent AP
were considered cardiac events. All cases of cardiac events were
evaluated by an independent End Point Committee. The End Point
Committee had no knowledge of the cTnT and cTnI values. AMI was
considered present when at least two of the following criteria were
complied with: (1) typical prolonged chest pain; (2) recently occurring
pathological Q waves or loss of R waves, or ST-segment elevation
followed by T-wave inversion in at least two leads in two consecutive
ECG recordings; and (3) an increase in cardiac enzymes
exceeding the upper reference level. Special attention was paid by the
End Point Committee to separate the inclusion event (unstable
AP/nonQ-wave AMI) from AMI occurring in the follow-up period. Cardiac
death was defined as fatal AMI and death caused by arrhythmias
or heart failure occurring in the period from inclusion up to 30 days
after inclusion. Refractory AP was defined as recurrence of
chest pain lasting
5 minutes despite optimal medication, including
intravenous nitroglycerin and ß-blockers
or calcium antagonists, associated with transient ECG
changes indicative of myocardial ischemia and leading to
coronary angiography. Recurrent AP was defined as recurrent
chest pain of
5 minutes duration, typical of myocardial
ischemia and responding to sublingual
nitroglycerin but not fulfilling the criteria for
refractory AP. After hospital discharge, recurrent AP was defined as
readmission because of anginal chest pain. Refractory AP and recurrent
AP occurring during the period from inclusion up to 30 days were used
in the statistical analyses.
If more than one event of the same type occurred, only the first was considered.
Treatment
The patients of the TRIM study were randomized to a regimen of
either low-dose, medium-dose, or high-dose Inogatran or heparin (active
control group). All received low-dose aspirin daily, provided that no
contraindications were present. Nitrates, ß-blockers, calcium
channel blockers, antiarrhythmic drugs, and analgesics were given
according to the routine of each participating center. The medication
the patients received just before inclusion is listed in Table 1
. The need for coronary
angiography, CABG, and PTCA was determined by the treating physician.
All clinical decisions were made without knowledge of cTnT and cTnI
values.
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Blood Sampling
Venous whole blood was drawn into tubes containing heparin at
the following times: at inclusion and 6, 12, 24, 36, 48, 72, and 96
hours later. The blood samples were immediately centrifuged at
2000g for a period of 20 minutes. The plasma was kept at
-80°C and sent for analysis to the core laboratory at Aarhus
University Hospital, Denmark. The main study was designed to
investigate the occurrence of AMI taking place from 6 hours up to 30
days after enrollment and cardiac death, refractory AP, and recurrent
AP from inclusion up to 30 days. Therefore, the patients were
considered positive for cTnT and cTnI if the highest value on inclusion
or 6 hours after enrollment exceeded the chosen discriminator
values.
Analytical Methods
Measurements of cTnT [ELISA Troponin(e) T] were carried out
with an ES 300 analyzer (Boehringer Mannheim GmbH). The
method is based on a single-step sandwich principle with
streptavidin-coated tubes as the solid phase and two monoclonal
anti-human cTnT antibodies. The method is described in detail
elsewhere.18 In our laboratory, the range and median of
cTnT values in 95 healthy blood donors were 0.00 to 0.02 µg/L
and 0.00 µg/L, respectively. We used 0.10 µg/L as the
cutoff value for cTnT, which is the discriminator recommended by the
manufacturer. In addition, we evaluated the prognostic value of cTnT at
alternative cutoff values of 0.05, 0.15, and 0.20 µg/L. cTnI
was measured with an Opus Magnum device (Behring
Diagnostics Inc). The method for the detection of cTnI is
based on the principle of two-site immunoassay using polyclonal
antibodies to recognize epitopes unique to cTnI. A discriminator value
of 2.0 µg/L was used for cTnI, as recommended by the
manufacturer. In accordance with this recommendation, we found a 97.5
percentile of 2.0 µg/L in 95 healthy blood donors (median, 0.0
µg/L; range, 0.0 to 8.4 µg/L). We also evaluated the
applicability of 1.0, 1.5, and 2.5 µg/L as cutoff values for
cTnI.
The discriminator values were used to separate patients with significant myocardial damage and increased risk of cardiac events from patients without myocardial damage and at low cardiac risk.
Statistics
Statistical assessment of the prognostic value of cTnT and cTnI
was carried out by the log-rank test. In patients in whom more than one
cardiac event occurred, the most serious event was used in the
calculations. These were weighted as follows: cardiac death >AMI
>refractory AP >recurrent AP. The prognostic importance of the
troponins and other parameters available at inclusion with
regard to the composite end point of cardiac death/AMI was evaluated
with a univariate regression analysis
(parameters listed in Table 2
). To determine the independent
prognostic value of the troponins at different cutoff values in
comparison with the other parameters, forward
"stepwise" multivariate logistic regression
analyses were used. A multivariate
analysis was performed for each cutoff value of cTnT or cTnI.
Accordingly, eight separate models were built to evaluate the
independent prognostic significance of each cutoff value with regard to
clinical outcome in comparison with the variables used in the
univariate analysis. In the
multivariate models, parameters with a
value of P<.20 were used as criteria for selection. ORs and
95% CIs were calculated. A value of P<.05 was considered
significant.
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| Results |
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0.10
µg/L. A similar pattern was seen in patients with cTnI <2.0
µg/L, of whom 76% had a history of AP and 55% had suffered
an AMI previously compared with 52% and 40%, respectively, in the
group of patients with cTnI
2.0 µg/L. In the 30-day follow-up period, 8 patients died (1.7%), 34 patients developed AMI (6.6%), refractory AP was reported in 21 patients (4.0%), and 242 patients had recurrent AP (46.8%). CABG or PTCA was performed on 102 patients (19.5%).
Appearance of cTnT and cTnI in Plasma
The median of cTnT values obtained within the initial 6 hours
after inclusion was 0.07 µg/L; it ranged from 0.00 to 13.63
µg/L. Two hundred forty-nine patients (48%) had a highest
value exceeding the cutoff value for cTnT (0.10 µg/L). For
cTnI, the median of values obtained within 6 hours was 0.4
µg/L; range, 0.0 to 155.1 µg/L. Two hundred fourteen
patients (41%) had a highest value of cTnI
2.0 µg/L. In 213
patients, both markers were above the chosen cutoff values; in 36
patients, only cTnT was elevated; and in 1 patient, only cTnI was
elevated. There was a strong positive relationship between values of
cTnT and cTnI within 6 hours after inclusion (Spearman's coefficient,
=.93; P<.0001). The time courses of the cumulative
proportions of patients with elevated values of cTnT and cTnI,
respectively, are shown in Fig 1
.
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Prognostic Value of cTnT and cTnI in Plasma
The occurrences of cardiac events in patients with normal and
elevated levels of cTnT and cTnI are listed in Table 3
. Patients with cTnT
0.10
µg/L had a significantly higher incidence of cardiac death
than patients with a highest value of cTnT <0.10 µg/L
(P=.014). Furthermore, AMI occurred significantly more
frequently in patients with cTnT
0.10 µg/L
(P=.045). When cardiac death and AMI were combined, the
difference was highly significant (P=.006). Kaplan-Meier
curves of the occurrence of cardiac death and AMI in patients with and
without elevated levels of the cardiac troponins are shown in Fig 2
. However, cTnT contained no prognostic
information with regard to the combined end points of cardiac
death/AMI/refractory AP/recurrent AP (P=.41).
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As for cTnI, we found that patients with elevated levels within 6 hours
after inclusion had a significantly higher risk of cardiac death than
patients with normal levels (P=.026). In contrast to the
result obtained for cTnT, patients with cTnI
2.0 µg/L did
not have an increased risk of AMI (P=.15). However, when the
composite end points of cardiac death and AMI were considered, patients
with cTnI
2.0 µg/L had a significantly poorer prognosis than
patients with cTnI <2.0 µg/L (P=.02). cTnI did not
possess prognostic information when the combined end points cardiac
death/AMI/refractory AP/recurrent AP were taken in consideration
(P=.32).
Logistic Regression Analyses of Biochemical and Clinical
Parameters Predictive of Outcome
To investigate the applicability of cutoff values other than those
recommended by the manufacturers, we also included 0.05, 0.15, and 0.20
µg/L as cutoff values for cTnT and 1.0, 1.5, and 2.5
µg/L for cTnI in the logistic regression analyses.
ORs, 95% CIs, and probability values for the evaluated
parameters are shown in Table 2
. Age, cTnT above cutoff
within 6 hours (0.05, 0.10, 0.15, and 0.20 µg/L), cTnI above
cutoff within 6 hours (1.5, 2.0, and 2.5 µg/L), and ST-segment
depression on the enrollment ECG were found to be significant with
regard to a poor prognosis in the univariate regression
analysis. We performed a multivariate logistic
regression analysis for each of the cutoff values for cTnT and
cTnI in comparison with the clinical parameters tested in
the univariate analysis (see Table 4
). Only age, cTnT, and cTnI level and
ACE inhibitor treatment persisted in the final models. Age
was found to be an independent predictor of poor outcome in all eight
multivariate models (OR, 1.07; 95% CI, 1.02 to 1.11;
P<.002). cTnT was also found to contain independent
prognostic value at all the cutoff values evaluated. As for cTnI, only
cutoff values of 1.5, 2.0, and 2.5 µg/L contained independent
prognostic information. None of the other parameters
evaluated in the multivariate analyses provided
any extra prognostic information. The ORs, 95% CIs, and probability
values are listed in Table 4
.
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Prognostic Value of cTnT and cTnI in Patients With Unstable
AP
In the subset of patients who were retrospectively categorized as
unstable AP (n=309), both cTnT and cTnI contained prognostic
information regarding the occurrence of cardiac death/AMI (Table 5
).
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Clinical Outcome in Patients Receiving Study Drug Versus
Heparin
In the group of patients with cTnT
0.10 µg/L, 5 of 67
patients (8.0%) in the heparin group suffered a cardiac event (cardiac
death/AMI), compared with 22 of 182 patients (12.1%) receiving
Inogatran (P=.3). No statistically significant difference
was found in the cTnI-positive group; the event rate was 7.2% in the
patients who where treated with heparin and 11.9% in the Inogatran
group (P=.3). In patients with normal levels of the
troponins receiving heparin or Inogatran, no statistically significant
differences in the event rates were found.
| Discussion |
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=.93,
P<.0001), which further supports the theory that elevation
of these markers reflects the same pathological process in the
myocardium. Because of the relatively high incidence of refractory AP and recurrent AP in the group of patients with cardiac troponins below the cutoff values, neither cTnT nor cTnI levels were predictive of the combined end points of cardiac death/AMI/refractory AP/recurrent AP. The high occurrence of refractory AP and recurrent AP in the group of patients with normal levels of the cardiac troponins is probably caused by the higher prevalence of AP and AMI at baseline compared with patients with elevated levels of the markers. Thus, patients with normal values of the troponins would also be prone to episodes of AP during the follow-up period. The frequent reports of recurrent AP and refractory AP in the patients with normal levels of the troponins probably explains why there was no association between cTnT levels and the performance of PTCA and CABG, inasmuch as invasive procedures are directed primarily by the presence of recurrent chest pain.
Prognostic Value of cTnT in Unstable AP
As in previous reports, we found that cTnT identifies a subgroup
of patients diagnosed as having unstable AP with elevated cTnT, who
have a poorer prognosis than patients with normal
levels.4 27 28 However, the proportion with an event
(11.8%) in the patients with elevated cTnT levels was somewhat lower
than reported in other studies.27 28 These differences
might be due to different designs and study populations. The prognostic
capacity of cTnI seems to be comparable to that of cTnT in the subset
of patients suffering from unstable AP.
Choice of Cutoff Values for the Present Study
In the evaluation of the prognostic value of cTnT and cTnI, we
applied the cutoff values for myocardial damage recommended by the
manufacturers of the assays. As for cTnT, several studies have
confirmed the applicability of 0.10 µg/L as the cutoff value
for risk stratification in patients with acute ischemic heart
disease.3 19 27 A value of 0.20 µg/L was used as
the discriminator limit in the early studies of the prognostic value of
cTnT.16 28 29 However, as the cTnT assay has improved, the
detection limit has been successively lowered. We found that cTnT also
provides independent prognostic information with 0.05, 0.15, and 0.20
µg/L used as cutoff values. The multivariate
analysis indicates that use of 0.20 µg/L as the cutoff
value provides the best separation between high-risk and low-risk
patients. Thus, the optimal cutoff values extracted from different
study populations might vary because different inclusion criteria and
blood sampling protocols are used, rather than being unequivocal and
constant. It seems that a cutoff value as low as 0.05 µg/L can
be used for risk stratification in patients with UCAD.
In the multivariate logistic regression analysis, cTnI provided independent prognostic information at 1.5, 2.0, and 2.5 µg/L. Assessed by the ORs, it seems that 2.5 µg/L is the best cutoff value for cTnI for early risk stratification in patients with UCAD. However, increased levels of cTnI were found in 3 of the 95 blood donors. This may have been due to unspecific binding of the cTnI antibodies to other plasma components of the blood donors.
Comparison With Other cTnI Assays
Several other immunoassays for the detection of cTnI exist that
use miscellaneous methods and antibodies.7 20 21 22 24 The
cTnI assays use different discriminator limits, ranging from 0.10 to
3.1 µg/L. This underlines the substantial differences in
methodology and emphasizes difficulties in comparing studies that use
different cTnI assays. Consequently, the prognostic capacity must be
documented for each assay, and a standardization of cTnI assays is
needed. A conversion factor for each cTnI assay could be calculated
from standard reference samples containing predetermined concentrations
of purified cTnI. cTnI concentrations determined by different assays
would thereby be comparable.
Optimal Timing of Blood Sampling for Risk Stratification
Two large studies have shown that the samples of cTnT and cTnI
taken on enrollment provide important prognostic information regarding
cardiac death.3 7 In our study, we used the highest value
within the first 6 hours after inclusion. When only the admission
sample is used for risk stratification, a considerable number of
patients who are suffering from significant myocardial damage is
overlooked; this is probably due to the delay in the release of the
cardiac troponins from injured myocardial cells. During the 96 hours in
which blood samples were collected, a total number of 270 patients had
cTnT values exceeding the cutoff value of 0.10 µg/L. We found
that at 12 hours after enrollment, the cumulative number of patients
with cTnT
0.10 µg/L was 256 (95%); at 24 hours, it was
found that 259 patients (96%) had cTnT
0.10 µg/L, and only
81% of patients had an elevated value on admission. In 77% of the
patients with an elevated value of cTnI in the 96-hour sampling period,
cTnI exceeded the cutoff value on admission, whereas 97% were elevated
at 12 hours. On the basis of these data, it seems that an in-hospital
observation period of at least 12 hours gives an accurate
identification of the majority of patients suffering from myocardial
damage.
Clinical Implications
At present, much effort is put into developing new treatment
modalities for patients suffering from UCAD. Studies that investigated
the utility of low-molecular-weight heparin have shown promising
results.1 2 The applicability of the cardiac troponins as
indicators of high risk in patients with UCAD might provide a useful
tool for the choice of optimal treatment. A large study has recently
shown that elevation of cTnT identifies a subgroup of patients in whom
prolonged antithrombotic treatment with a low-molecular-weight heparin
(Dalteparin) can improve the prognosis.19 However,
additional prospective studies are needed to document whether
antithrombotic treatment and/or invasive procedures can improve the
prognosis of patients with elevated levels of the cardiac
troponins.
Comparison With Previous Studies
In a retrospective study of 1404 patients with unstable
AP/nonQ-wave AMI, Antman et al7 found elevated levels of
cTnI to be a strong indicator of cardiac death. Among the 573 patients
with elevated cTnI levels, 3.7% died during the 42-day follow-up
period, which is comparable to the 3.2% mortality rate we found in the
cTnI-positive group. The event rate in patients with normal levels of
cTnI was also concordant with our results: 1.0% in the study by Antman
et al versus 0.7% in our study.
A recent large prospective study by Ohman et al3 reported
that elevated levels of cTnT were strongly associated with a poor
prognosis. The mortality rate at 30 days in patients with cTnT
0.10
µg/L on admission was 11.8% versus 3.9% among patients with
normal levels. The higher mortality rate found by Ohman et al compared
with our results is probably due to differences in study populations:
we investigated patients suspected of unstable AP/nonQ-wave AMI,
whereas Ohman et al included patients with Q-wave AMI, nonQ-wave AMI,
and unstable AP. Only 36% had elevated levels of cTnT on admission,
but a relatively short period from onset of symptoms to blood sampling
(median time, 3 hours) was reported by Ohman et al, which might have
caused false-negative cTnT values on admission in some patients with
myocardial damage.
Study Limitations
The present investigation was performed as a substudy in a
multicenter trial in which several exclusion criteria were applied.
This may have resulted in the exclusion of patients with concomitant
diseases such as severe heart failure, renal failure, liver disease,
and diseases with increased risk of bleeding. The exclusion of these
patients might favor the troponins, because false-positives could occur
when concomitant diseases are present. Furthermore, the
interpretation of ECG findings was done by the treating physician at
each participating center. A central evaluation by uniform criteria
might have increased the prognostic value of the ECG changes.
Conclusions
In patients suspected of having UCAD, both cTnT and cTnI convey
independent prognostic information with regard to cardiac death/AMI.
The addition of ECG changes did not add any extra information with
regard to the clinical outcome. The predictabilities of the markers
vary at different cutoff values and are already significant at 0.05
µg/L for cTnT and 1.5 µg/L for cTnI. Prospective
trials may indicate whether cTnT and cTnI can identify patients who
will benefit from antithrombotic treatment and/or invasive
procedures.
| Selected Abbreviations and Acronyms |
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| Appendix 1 |
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Sweden: Höglandssjukhuset, Eksjö: Steen Ekdahl, Krister Kihlström, Jan-Olof Magnusson. Östra Sjukhuset, Göteborg: Karl Swedberg, Mikael Dellborg, Karl Andersen. Karolinska Sjukhuset, Stockholm: Lars Grip, Rikard Linder. Borås Lasarett, Borås: Erland Hall, Christer Wettervik, Sven-Åke Forsberg, Hans Thygesen. Länssjukhuset Ryhov, Jönköping: Jan-Erik Karlsson, Olof Svensson. Länssjukhuset, Kalmar: Finn Landgren, Bengt Holmgren, Stefan Ryden. Universitetssjukhuset, Lund: Hans Öhlin, Ole Hansen, Erik Tingberg. Universitetssjukhuset, Linköping: Eva Swahn, Magnus Janzon. Akademiska Sjukhuset, Uppsala: Lars Wallentin, Bertil Lindahl. Lasarettet, Falun: Helge Saetre, Gösta, Greger Ahlberg, Björn Linde, Lars Hageström. Söndersjukhuset, Stockholm: Johan Hulting, Jonas Höijer. Bollnäs Sjukhus, Bollnäs: Eskil Hammarström, Lennart Åström. Skellefteå Lasarett, Skellefteå: Kurt Boman, Jan Remmets. Sundsvalls Sjukhus, Sundsvall: Bengt Hj. Möller, Mona Lycksell.
| Acknowledgments |
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| Footnotes |
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Received February 27, 1997; revision received May 20, 1997; accepted May 28, 1997.
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