LABORATORY UPDATE
“Pathology that Adds Value”
THE PATHCARE NEWS
Troponin – change of measuring units
Troponin I results will be reported in ng/L, in stead of ng/mL
Example: 0.07 ng/mL = 70 ng/L
Cut-offs will be instrument-specific
Since its introduction in the nineties, cardiac troponin (Tn) assays
have played an important role in the diagnosis of acute myocardial
infarction (MI). The first generation assays were relatively
insensitive, with Tn undetectable in the normal population. As the
analytical performance of the assays improved, lower and lower
concentrations of Tn could be detected, with Tn now detectable in
a significant percentage of the normal population.
Many Tn assays now comply with the recommendation of the joint
ESC/ACC committee for the redefinition of MI in 2000, requiring
imprecision of ≤ 10% at the upper limit of normal, which is the 99th
percentile of a reference control group. These are in fact the
criteria for defining an assay as “highly sensitive”.
Unfortunately, the consequence of more sensitive assays is a
decrease in specificity for MI, i.e. small Tn elevations are
demonstrated in an increasing number of patients without MI,
resulting in unnecessary and inappropriate investigations and
treatment.
This has created uncertainty among many
practitioners, regarding the use of cut-off values for clinical
interpretation.
For this reason, the SA Heart Association recently convened a
meeting of cardiologists, pathologists, ER physicians and industry
representatives under the supervision of an international expert,
Prof. HD White, to discuss the current status of Tn assays and to
formulate guidelines on their clinical interpretation .
One of the recommendations of the meeting was that troponin
results should be reported in ng/L and not in ng/mL any more.
Changing from ng/mL to ng/L:
Troponin T has already been reported in ng/L, since the launch of
the new high sensitivity troponin T (hs-TnT) assay by Roche last
year.
Because most of the troponin I (TnI) assays that are currently in
use also meet the criteria for “highly sensitive” assays, it was
decided that the reporting of Tn results should be standardised,
and all Tn results should now be reported in ng/L. Implementation
is planned for 1 October 2012.
The advantage of measuring in ng/L, is that the result is expressed
in whole numbers, thereby avoiding potential misinterpretation
due to the many decimals e.g. 0.015 ng/mL = 15 ng/L.
September 2012
Cut-offs in ng/L:
According to international guidelines, the upper limit of normal for
Tn is defined by the 99th percentile of a normal reference
population. The WHO cut-off reflects the original cut-off for MI
which was derived from comparative studies with CK-MB.
Because TnI assays are not standardised, these cut-off levels are
assay-specific. The corresponding levels in ng/L for the highly
sensitive Tn assays utilised by PathCare are shown in the table:
Assay
99th percentile (ng/L) WHO cut-off
Upper limit normal
for MI (ng/L)
Beckman AccuTnI
40
500
Siemens Stratus CS TnI 70
600
Roche hsTnT
100
14
Please note that there will be a substantial difference in upper
limit of normal between the two TnI assays.
Other recommendations from the meeting, in brief:
• Because of the many non-acute coronary syndrome causes of
raised levels, Tn should not be interpreted in isolation. Careful
clinical evaluation, risk assessment and accurate ECG
interpretation are required.
• Diagnosis of ST-elevation MI is made by ECG. Treatment should
not be delayed until biomarker assay is completed.
• A normal highly sensitive Tn level at 6h after onset of chest pain,
rules out MI.
• Dynamic change in Tn level (see algorithm) is required for
diagnosis of MI - serial sampling, 3h apart, is recommended.
• For TnI, a change of ≥ 50% is regarded as significant.
• For TnT, a change of ≥ 50% for levels below 53 ng/L and a
change of ≥ 20% for levels above 53 ng/L is regarded as
significant.
• Serial testing should be done on the same instrument platform,
due to the lack of standardisation between assays.
Readers are encouraged to read the original article: SAHeart
2012;9:210-215, or online: www.saheart.org/journal
www.pathcare.co.za
LABORATORY UPDATE
“Pathology that Adds Value”
THE PATHCARE NEWS
With permission from the author.
References: SAHeart 2012;9:210-215, EHJ 2007;28:2525-2538
Compiled by Dr Esmé Hitchcock, Chemical Pathologist, PathCare
September 2012
www.pathcare.co.za