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Availability of troponin
testing for cardiac patients in New Zealand 2002 to 2011: implications for
patient care
Mohammad Latif, Chris Ellis, Alexei Chataline, Greg Gamble,
Cam Kyle, Harvey White
The American College of Cardiology (ACC) and the European
Society of Cardiology (ESC) have published a joint consensus statement
redefining acute myocardial infarction (MI).1
The cornerstone of the new definition of MI is the elevation of cardiac troponin
(T or I) concentration in the appropriate clinical context and, due to the range
of other causes of troponin elevation, evidence of a rise and/or
fall.2,3 Troponins are not just useful for the
diagnosis of MI but also have been shown to be prognostically
important.4–6
Over the past 2 decades, immunoassays have been developed
and refined for cardiac troponins T and I which
are increasingly sensitive and specific for cardiac muscle injury. A large
number (10–20) of manufacturers have produced assays for troponin I. Only
Roche Diagnostics has produced and marketed a troponin T assay, although it has
licensed the assay for one point of care analyser marketed by another company
(Radiometer AQT90).
Different manufacturers use their own antibodies raised
against different antigen epitopes on the
cardiac troponin I protein, and also use different calibrator and control
materials.7 While there has been progress in
standardising troponin I
assays using an international reference preparation, this
has not resolved the inherent difficulties in comparing troponin results between
different methods for an individual patient. It is very difficult, and may be
dangerous, to try to diagnose a rise and/or fall in troponin when different
troponin assays and platforms are used.
Use of different methods and different assay platforms could
lead to diagnostic confusion and inconsistencies between centres and/or regions,
affect treatment, and could confound health statistics. Hence we conducted an
audit looking at the various troponin assays (including point of care),
providers, analyser types and the local cutpoints used across New Zealand, in
hospitals and private providers and reviewed how this has changed over time
(2002, 2007 and 2011).
MethodsWe reviewed the troponin tests, analysers and cutpoints
available at hospital laboratories across New Zealand which admitted patients
with a probable acute coronary syndrome (ACS) in 2002, 2007 and 2011. These
hospitals also participated in the comprehensive Cardiac Society of New Zealand
Audit in 2002 and 2007 which represented all hospitals admitting ACS patients in
New Zealand.
The development of the New Zealand Acute Coronary
Syndrome (NZACS) Audit Group and the methodology for the national audits has
been published previously.8–11 A few
smaller hospitals, not routinely admitting ACS patients, had troponin testing
facilities and these were also included in this survey.
Data were collected from the hospital laboratory staff
and/or clinicians by e-mail or telephone in order to establish which assays and
analysers were being used, as well as the local cutpoints for a positive test.
Community laboratory staff were contacted to provide similar data.
We consulted representatives from a number of major New
Zealand troponin testing providers to aid in data collection, to enquire about
manufacturer cutpoints, and identify other sites where point of care analysers
measuring troponin were being used for patient management.
ResultsDistribution of assays used in hospital
laboratories—In 2011, the 43 hospitals across New Zealand which
admit possible ACS patients used 10 different troponin analysers provided by
five companies. In comparison there were 11 analysers in use in 2007 and 8
analysers in 2002 (Table 1). Additionally, the Siemens Centaur assay is in use
by a private laboratory (Labtests) in 2011, making a total of 11 different
analysers in current use.
Troponin analysers from 2002, 2007 and 2011 are shown in
Table 2 by geographical distribution. In 2011, 58% of the hospitals (25/43) use
troponin T assays and 42% (18/43) use troponin I assays as their first-line
method. Troponin T assays also slightly predominated in 2002 (68%) and 2007
(70%).
In 2011 Ortho and Bayer are no longer present in New
Zealand, and there are two new commercial providers: Siemens and Beckman
Coulter. Siemens now provide a reformulated troponin test on the Centaur
(previously Bayer) platform, in addition to the Dimension and Vista analysers.
Radiometer provides a bench top analyser, the AQT90, which is standardised
against the Roche Troponin T assays and uses an identical antibody
configuration.
Of hospital laboratories measuring troponin I, the Abbott
Architect assay is the most widely used in 2011. Abbott provided 78% (14/18) of
the assays measuring troponin I as the first-line test, mostly using the
Architect assay. Other troponin I assays are also provided in regional North
Island locations; Siemens Vista assay at North Shore and Waitakere, and Beckman
Coulter DXI and Access platforms in Bay of Plenty and Whakatane.
Table 1. Major providers, number and different
types of analysis methods in use at hospitals admitting acute coronary syndrome
patients across New Zealand in 2002, 2007 and 2011
* Brackets () denote number of times instrument is also
used as backup instrument
Note:
** One other method
(Siemens Centaur assay) is being used in the Auckland community by Labtests.
Table 2. Analysers and cutpoints across New
Zealand in 2002, 2007 and 2011×
Figure 1A shows the geographical distribution of troponin
use across the 20 New Zealand district health boards (DHBs) in 2011. Even within
the same DHB, different troponin assays and analysers are sometimes used.
Distribution of assays used in private
laboratories—There is an even distribution of troponin T and I
users among private laboratory providers across New Zealand. Almost half of
them, (8/15) use the Roche troponin T assays in their main laboratories, whereas
the rest use Abbott (4/15), Beckman Coulter Access (2/15) and Siemens (1/15)
troponin I assays.
Table 3 and Figure 1B show the analysers and cutpoints of
the private laboratory providers across New Zealand. Note that some peripheral
hospitals are also run by private providers, and may have assays that differ
from the main laboratory. For example, Southern Community Laboratories (SCL)
runs the service for Queenstown, Oamaru and Clyde-Dunstan, all of which have
Abbott i-stat instruments, whereas their main Dunedin and Christchurch labs use
Roche Troponin T.
Introduction of Roche high sensitivity
(5th generation) assay—Since the
introduction of the 5th generation high
sensitivity assay (hs Troponin T), from May 2010 onwards there has been a rapid
uptake among existing Roche users, initially in the lower North Island, then in
Auckland/Northland and later in the South Island. By February 2011 all Roche
laboratory analyser platforms had switched to the hs Troponin T test.
Despite the readiness to accept this improved assay, the
changeover was delayed in some places through uncertainty among clinical staff
on the diagnostic and workflow implications of the new test, and there were
concerns over a possible significant increase in “abnormal” results
and therefore emergency department presentations, before diagnostic and
management algorithms were established.
Both 4th and
5th generation tests were typically available
for a short time to enable clinicians to become more familiar with the more
sensitive 5th generation test during the
changeover. In some regions, such as the upper North Island, the introduction
was coordinated between laboratories to occur at the same time.
Figure 1A. Distribution of troponin T, troponin
I and mixed assays by District Health Board Hospital laboratories as at 2011
(Chatham Islands [troponin T] not shown)
 Figure 1B. Distribution of troponin T and
troponin I assays used by community laboratories as at 2011
DML (Labplus)
Labtests
     Table 3. Main testing platforms of private
community laboratories: 2007 and 2011
Laboratory reporting practices—With
the introduction of the Roche hs Troponin T method, the reporting units have
changed from micrograms per litre (ug/L) to nanograms per litre (ng/L), to
reflect greater sensitivity and minimise confusion. However, troponin I tests
are still reported in ug/L by most laboratories in New Zealand.
The exception is Auckland where all four laboratories
measuring troponin I (Middlemore, North Shore, Waitakere, Labtests) report
results in ng/L, effectively by multiplying the analyser result by a thousand
using their laboratory information system.
The number of different ‘cutpoints’ used by
different NZ hospitals in 2002, 2007 and 2011 was reviewed. Table 4 shows the
recommended manufacturer and laboratory cutpoints at these times. During these
periods there were differences in quoted cutpoints for major assays in use. For
example in 2007 three cutpoints (0.03, 0.04 and 0.15ug/L) were reported for the
Architect platform. There were also three cutpoints for the Roche Troponin T
assay (0.03, 0.04 and 0.01ug/L).
By February 2011, all laboratories using Roche hs Troponin T
had the same assay cutpoint, with all but one reporting <14ng/L with their
report (Kaitaia reported 0-13 ng/L as their reference interval). Importantly,
however, a different cutpoint is still currently used for troponin T point of
care instruments (0.03ug/L).
Table 4. Quoted cutpoint levels for New Zealand
laboratories and recommended manufacturer cutpoints 2002, 2007 and
2011×
Troponin I assays also have different cutpoints in different
laboratories. For example, for the Abbott i-stat machine four different
cutpoints are quoted (0.03, <0.04, 0.08 and <0.09ug/L).
Point of care testing—Point of care
testing (POCT) is widely used in a number of rural hospitals both for initial
patient management and also as backup when laboratories cannot be viably staffed
for a full 24 hours. Table 1 includes laboratories where POCT analysers are
available after hours. The Abbott i-stat machine is a common choice with 7 rural
labs using this instrument for routine first-line testing and 5 using it as a
backup.
POCT is also used in some general practice settings where
obtaining a laboratory result urgently is otherwise impractical, especially
after hours, e.g. Waiheke Island (Cardiac reader), and in rural practice
settings, e.g. Hawera, Waipukurau (i-stat) (data not shown).
Results reported by POCT are less precise, and therefore
less sensitive, than laboratory analysers. Therefore laboratory reports using
POCT analysers have a comment emphasising that the test is helpful to diagnose
myocardial damage if positive, but a negative result does not exclude it, i.e. a
positive test is useful to ‘rule in’ myocardial injury in the right
clinical setting, but a negative result also does not rule it out.
POCT methods using both Troponin T (Cardiac reader,
Radiometer AQT) and troponin I (Abbott i-stat) currently continue to be
expressed in ug/L, although the Roche POCT analysers will be calibrated in ng/L
in the near future.
The quoted sensitivity cutpoints for POCT analysers
measuring troponin T (Roche cardiac reader/H232, Radiometer AQT) are similar at
0.03ug/L, although the Cardiac reader provides a semi-quantitative
(‘low’) result in the lower abnormal range (0.03-0.1ug/L), while the
AQT instrument provides a numerical result. For the i-stat machine, the
manufacturer (Abbott Diagnostics) quotes two different cutpoints, either
0.08ug/L corresponding to the 99th population
percentile, or 0.03ug/L, representing the
97.5th population percentile.
The individual choice of POCT instrument depends on a range
of factors, including clinical preference, cost (of analyser, reagents and staff
time/training) and alignment with other laboratory needs such as the ability of
the POCT analyser to measure other analytes besides troponin. For example, the
Abbott i-stat, Roche H232 analyser and Radiometer
AQT instruments can measure other tests, making them useful
options in an emergency department or rural/community setting. Thus the i-stat
is used in Ashburton and Buller, while in the Waikato the Radiometer AQT is used
as the first-line method in Tokoroa and Te Kuiti, and as a backup in Thames and
Taumarunui.
In some centres (e.g. Rawene), the wide menu of other tests
offered by the i-stat means that this instrument is used to measure troponin I,
even though the referral hospitals (Whangarei, Auckland) measure hs Troponin
T.
DiscussionTroponin measurement is pivotal in the diagnosis, risk
stratification and management of ACS. The definition of
MI1 requires a rise and/or fall in cardiac
troponin concentration. Strictly, this means measurement by the same method, in
the absence of close correlation between different assays. The magnitude of
troponin elevation correlates with risk of death or non-fatal MI following
ACS.12–14 Patients with elevated troponin
levels also benefit more from antithrombotic therapy, glycoprotein 2b/3a
receptor antagonists and revascularisation than patients with normal troponin
levels.15–17
Troponin cutpoints—In 2000 the
ESC/ACC Consensus group recommended that the appropriate cutpoint for troponin
levels for diagnosis of MI should be the 99th percentile of a healthy
population.18 They also stated that the assay
reproducibility (the imprecision measured as coefficient of variation) should be
10% or less at that concentration. The International Federation of Clinical
Chemistry (IFCC) Committee on Standardisation of Markers of Cardiac Damage
(C-SMCD) also supports these guidelines.19
Since 2000 there have been major advances in assays. With
improved assay precision and sensitivity, recommended cutpoints have changed
progressively from CKMB-derived thresholds (using the now outdated WHO
definition of MI), to those based on 10% assay precision, to the 99th population
percentile. Until recently, no assay in clinical use had sufficient sensitivity
to satisfy this 99th percentile cutpoint. The
Roche hs Troponin T and Siemens ‘Ultra’ now have this
precision.
Rather than provide a single ‘cutpoint’, it is
widespread practice for manufacturers to provide a summary of their own
‘in-house’ data (usually validated by independent published
studies), showing the 10% CV and 99th population percentiles for their assay.
There is routinely a recommendation that each laboratory validate a reference
range for its own population, which is a difficult task, particularly for small
rural laboratories with staffing and cost-constraints.
These reporting difficulties are also compounded as
laboratories generally report ‘rounded’ results generated by their
analysers to two decimal places (e.g. 0.032 becomes 0.03ug/L, while 0.037
becomes 0.04ug/L). Some laboratories also reported a reference range with an
upper normal limit, while others report a ‘less than’ number (e.g.
<0.04ug/L).
Some manufacturers still quote a threshold that is
“unequivocal” or “diagnostic” of MI, referring back to
comparisons with previous CKMB results. For example Abbott uses an
‘unequivocal’ threshold of 0.3ug/L and Beckman a threshold of
0.5ug/L. Both are about 10 times higher than ranges routinely quoted by NZ
laboratories, although some still refer to this “unequivocal”
threshold with a comment in their reports. While hard to quantify, anecdotal
evidence suggests local clinician preferences played a role in some cases in
influencing laboratory cutpoints.
Interpretation and standardisation of troponin
results—Despite significant assay improvements, there are
persisting problems with interpretation and standardisation of serum troponin
measurements. This is more so for cardiac troponin I as different manufacturers
use different antibodies raised against different epitopes on the cardiac
troponin I protein.21
These different antibodies vary in their ability to bind
troponin I. It is also important to note that troponin I can exist in multiple
forms in the serum:
There are potentially several additional forms
that also exist for each of these three forms, representing N- and C-terminal
degradation products, oxidised and reduced forms, and phosphorylated
forms.22
Different assays do not react on an equimolar basis with
these different molecular species. Therefore, different assays
do not produce equivalent concentration results and comparisons of absolute
troponin I concentrations cannot be reliably
made.23
There have been considerable efforts towards international
standardisation of troponin I assays.7 However,
troponin I results can still differ by at least several-fold, sufficient to make
direct comparisons of results between assays unhelpful and potentially dangerous
when serial samples are taken in an ACS setting.24,25
By contrast assay standardisation issues are minimal with
troponin T because the same antibodies are used—even when the assay has
been released for use on another manufacturer’s platform (e.g. Radiometer
AQT).
Given the heterogeneity of troponin assays,
Apple26 has proposed a cardiac troponin assay
scorecard. This lists the various assays according to the total imprecision at
the 99th percentile and whether the assays are
“clinically usable” or “guideline acceptable”.
This scorecard may be useful in providing some guidance to
clinicians and laboratories regarding the strengths and weakness of various
assays. He suggests only two assays are guideline compliant (Siemens 'Ultra'
troponin I and Roche hs Troponin T) and, of the two assays available in New
Zealand only one, hs Troponin T, is classified as high sensitivity. However, a
recent comparison of several 'sensitive' troponin assays suggested comparable
performance in the diagnosis of MI in patients presenting acutely with ACS but
superiority to the 4th generation troponin T assay.27
This study included three of the assays currently in use in
New Zealand, the Abbott Architect troponin I, Roche hs Troponin T, and Siemens
troponin I Ultra assays. Receiver operator curve (ROC) analysis showed high
diagnostic accuracy with identical areas under the curve of 0.96 for these three
assays. A separate recent comparison study also indicated that the Beckman
Coulter assay had excellent diagnostic
sensitivity.28
Use of these high sensitivity troponin tests allows for
earlier diagnosis, or exclusion of MI. The recent update of the Cardiac Society
of Australia and New Zealand ACS Management Guidelines suggest that clinicians
now take specimens 3 hours apart, with 1 at least 6 hours after symptom
onset.29
Troponins and clinical
management—Although laboratories have provided education to users
as well as interpretive comments with their results, clinicians have taken time
to become familiar with the newer assays and the implications of elevated
levels, especially in the low range.30 Assay
99th population percentiles quoted by
manufacturers are based on a ‘healthy’ (typically relatively young)
population. However, many elderly patients have small degrees of troponin
elevation above this ‘healthy’ cutpoint for multiple reasons. This
can cause diagnostic difficulty if symptoms are non-specific and accompanied by
“mild elevation” on a first test.
Diagnosis in such patients requires follow-up tests
performed using the same assay platform to confirm or exclude acute myocardial
injury. In New Zealand, based on biological and analytical variability
considerations, a hs Troponin T initial level of 14-49 ng/L and a change of 50%
or more in serial blood tests is currently required for the diagnosis of
myocardial infarction in the appropriate clinical setting. If the initial hs
Troponin T level is ≥50 ng/L (at some sites 53 ng/L), a change of 20% or
more is required for diagnostic purposes.31
In some referral networks there has been a trend towards
similar methodology as instrument platforms are updated. The Northland regional
centres (Kaitaia, Kawakawa, Dargaville and Whangarei) which routinely send their
patients to Auckland City Hospital have since 2007 changed their assays to Roche
troponin T which is also being used at LabPlus in Auckland.
Peripheral and regional hospitals in Waikato and the far
North also use troponin T, similar to their major referral hospital. Conversely,
in Whakatane hospital troponin testing has moved from troponin T to Beckman
Coulter troponin I, also using the same test performed in the local community.
However, despite uniformity in some areas, in a number of
regions different troponin assays and analysers are being used so that patients
frequently have troponin measurements by more than one (even several) methods.
Since results from different methods cannot be directly compared this has
implications for ACS patients transferred from one centre to another.
Extra costs are incurred in repeating troponin tests and
hospital stay may be prolonged while repeat tests are performed. The same
applies to patients who have had troponin tests done in the community or by a
private laboratory and are subsequently referred to the public hospital. Table 5
illustrates the potential transfer pathways of cardiac patients across New
Zealand and the troponin assays in use among different centres.
Changes in troponin testing are related to multiple other
factors besides the choice of the troponin assay itself. In the past 10 years
national and regional district health board initiatives with tendering for
bulk-funded laboratory services have resulted in significant consolidation of
laboratories in New Zealand, but with little coordination across district health
boards as each has pursued individual solutions for laboratory services.
Troponin tests in some provincial hospitals are routinely
performed by their local private laboratories (e.g. Palmerston North,
Whakatane), as part of a contract to run the hospital laboratory. Conversely
some private laboratories in the community send their troponin requests on to
their local public hospital (e.g. Taranaki). Further, there has been little
attention to the impact of different troponin tests on patient management across
New Zealand's 5 major Cardiothoracic Regions which collaborate to manage ACS
patients who require invasive management at the Regional
Centre9, 11.
For cost and efficiency reasons the choice of assay has
often been determined not just by clinician and pathologist preference, but by
what is considered to be the most cost-effective overall analyser configuration,
due to the need to purchase large and expensive analyser platforms that perform
many different tests.
Table 5. Examples of transfer pathways for
cardiac patients across New Zealand and various troponin assays used
2011
Manufacturer Codes:
R=Roche, Ra= Radiometer, A=Abbott, BC=Beckman Coulter,
S = Siemens
I=Troponin I, T=Troponin T
^ Usually a community laboratory or peripheral
hospital. In these sites the same laboratory usually handles both community and
hospital work. In some cases patients are referred directly from community labs
or peripheral hospitals to referral hospitals. In others they may be referred to
regional hospitals, and then sent on to referral hospitals for intervention or
further evaluation.
* In Hawke's Bay some GP practices get troponin T
(Roche) through Southern Community Laboratories (SCL), while others get Abbott
troponin I (Hawke's Bay DHB laboratory). After hours all GPs get troponin I
through the hospital laboratory.
The difficulties of providing a 24 hour service for this
important test have also led to individual solutions to provide backup testing.
The constraints and cost-implications of doing this have not always been well
understood by funders. Point of care testing (POCT) is a common solution,
depending on the staffing needs of the hospital, the range of other platforms
available, and distance from other laboratories able to also measure troponin.
Clinical trials are increasingly using troponin values as
part of their enrolment as well as outcome criteria. Use of different assays
could lead to diagnostic inconsistencies between different centres and/or
regions, potentially confounding trial results. This also can affect the
accuracy of community wide disease rates and compromise the ability to apply
trial results to the general population.
Mechanisms of troponin release—Our
understanding of the underlying mechanisms of troponin release has also
increased. Very small amounts of troponin may be released by means other than
gross myocyte destruction (infarction). Recently
White20 has suggested six possible major
pathobiologic mechanisms for troponin elevations. Troponin elevations could be
due to myocyte necrosis, apoptosis, normal myocyte cell turnover, cellular
release of proteolytic troponin degradation products, increased cellular wall
permeability or passage of membranous blebs.
Study limitations—A potential
limitation of this study is that the data regarding ‘cut points’ was
obtained via personal communication, and repeated phone and email questioning of
laboratory and clinical staff. Much of the data obtained were also retrospective
from up to 10 years ago.
The implications of varied laboratory testing platforms, in
terms of the cost of retesting troponin results on different platforms for
transferred patients, would require further study. In addition, the risk to a
patient where a cut point has been inappropriately applied, and the frequency
with which errors of interpretation are related to different platforms, would
also require further exploration.
ConclusionTroponin assays have improved throughout New Zealand in the
last 10 years reflecting international improvements in assays and clinical
practice. However, there remain differences in the troponin tests and reporting
practices across New Zealand hospitals.
Point of care troponin tests are widely used in smaller
regional hospitals and in some rural and community settings. Care is required in
interpretation, especially when comparing with follow-up by a more sensitive
test.
The use of different assay platforms, units and sometimes
different cutpoints for the same assay can result in difficulties in
interpretation of changes when referring patients from the community or
peripheral hospitals to tertiary referral hospitals. This is a particular
problem when diagnosis requires clinical interpretation based on a change in
troponin results.
Clinicians and District Health Board management need to be
aware of these issues. Physicians and pathologists need to collaborate in
establishing acceptable and uniform criteria for troponin assays and their use.
Some standardisation of available tests, thresholds and testing protocols is
likely to improve patient management and the accuracy of community wide disease
rates.
A more coordinated national approach may result in better
use of medical resources and improve patient care.
Competing interests: None
declared.
Author information: Mohammad Latif,
Cardiology Registrar, Green Lane Cardiovascular (CVS) Service, Auckland City
Hospital, Auckland; Chris Ellis, Cardiologist , Green Lane CVS Service, Auckland
City Hospital, Auckland; Alexei Chataline, Medical Officer, Green Lane CVS
Service, Auckland City Hospital, Auckland; Greg Gamble, Statistician, University
of Auckland, Auckland; Cam Kyle, Chemical Pathologist, LabPlus and Diagnostic
Medlab; Harvey White, Cardiologist and Director of the Coronary Care Unit &
Green Lane CVS Research Unit, Green Lane CVS Service, Auckland City Hospital,
Auckland
Acknowledgements: We acknowledge and thank
all of the local laboratory staff and clinicians at the participating centres
for helping us with collaboration of this data; the many laboratory company
representatives for their assistance in this study; and all of the doctors and
nurses who contributed to the New Zealand Cardiac Society Acute Coronary
Syndrome Audits for 2002 and 20078-11 which
facilitated the collection of the initial data for this study.
Correspondence: Dr Chris Ellis, Cardiology
Department, Green Lane CVS Service, Level 3, Auckland City Hospital, Grafton,
Auckland 1023, New Zealand. Email: chrise@adhb.govt.nz
References:
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