12 July 2018

Degrees of suspicion: Troponin testing in general practice.

NOTE: These blog posts are written from the perspective of a GP colleague who still seems to accumulate a list of unanswered questions at the end of each day in practice.  Each post describes how I went about trying to answer the specific questions.  These posts are not intended to be used for specific clinical guidance but hopefully provide links to some useful resources which the registrar can explore for themselves.  

In the tea room, not long ago, I mentioned one of those patients who makes your session that bit longer by raising a potentially serious symptom at the end of an already long consult.  This was an elderly patient of mine who mentioned having had some chest discomfort on the previous weekend. When asked what I had done I said “I did an ECG and sent her for a troponin”.    The other doctor, a recently-fellowed, and evidence-based chap, said “I never do troponin. If I suspect an AMI they go straight to ED”.

Now, this had me worried.  I respect the opinions of people such as him and I think it not unlikely that I could be a bit out of date with emergency management these days and I also aspire to “choose wisely”.    On the other hand, it occurred to me that if I sent every patient of mine who complained of chest discomfort up to ED they would be swamped with 75 year olds with reflux, arthritis, fibromyalgia, URTIs with cough and those who have taken up the rowing machine at the gym.  I tried to take the phrase apart and decide whether I really “suspected” an AMI.  I also decided to find out what is the appropriate use of a troponin test in primary care – particularly in my sort of primary care population.

If my patient population was mainly kids, young adults for travel vaccinations, females for pill scripts and a few middle aged people who rarely complain of chest pain (as it was years ago)  then, if a 45 year old male presented with chest pain, I would certainly suspect an AMI until proven otherwise – and do the needful.  However, I now see an older population with multiple morbidities.  So what is “most likely” in that population?  Well, just about anything is “most likely”.  But they are also more likely to have cardiac problems and, given the predominance of females and quite a few diabetics, maybe likely to present in an “atypical” way.  That didn’t really assuage my anxieties about missing something important and, in the past, I have sent patients to ED whose discharge summaries said “probable GORD.  Follow up with GP”.

Some resources

There are two fairly recent and readable articles which summarise the advice nicely.  One was in the Australian Family Physician in November 2017 and one was in the Medical Journal of Australia in July 2016.

These articles are in the context of current high sensitivity troponin tests.  My history as a doctor goes back to the days of trying to interpret the timing and source of various “cardiac enzymes” (CK, SGOT and LDH from memory?) and, later, CK-MB.  I was well out of the hospital when troponins arrived on the scene.

The MJA article notes that the troponin assay was intended to improve diagnosis and risk stratification of patients in the ED with symptoms suggestive of acute coronary syndrome (ACS) and that the newer assays provide reliable detection of very low concentrations of troponin.  Use and interpretation is always in the context of symptoms, history, examination and ECG.  In regards to the GP context it states testing should be limited to patients presenting with symptoms that occurred more than 24 hours previously. If these patients have no high risk clinical features and a normal ECG, they may be assessed with a single troponin assay but should be able to be referred urgently to hospital if the result is elevated.  This means that the test should be labelled as urgent and the result must be able to be conveyed immediately to the GP, as it has implications re management and prognosis.

Both articles clearly and helpfully specify which patients should be urgently transported to hospital without a troponin test.  These include patients presenting with possible ACS in the preceding 24 hours, including symptoms consistent with either unstable angina or high risk clinical features. These patients need evaluation in a monitored environment with relevant facilities available.

There is a further important caveat that a negative serum troponin result in the absence of high risk features does not necessarily exclude a diagnosis of unstable angina, for which urgent cardiac assessment may still be appropriate (worth reading the articles).

At the risk of sounding repetitive, the take home message I extracted from the AFP article was that “It is reasonable for a GP to order a troponin test to exclude ACS in a patient who is low-risk and asymptomatic, and in whom the symptoms have completely resolved 24 hours prior to presentation.” It also describes patients with atypical symptoms and a low likelihood of ACS, where troponin testing may ‘exclude’ ACS to cover clinical uncertainty. There are the same provisos about the criteria for urgent referral and also the concerns about the possibility of unstable angina. They seem to be a little less strict about situations in which the troponin may be ordered in general practice (again, read the article for further details).

Fortunately my patient seemed to fit into all these appropriate categories.  I got the result urgently and it was negative.  Next time I might be able to articulate my reasoning a bit more specifically.

It is also noted that troponin elevation can occur in other pathologies, too numerous to go into here, and this is more likely with the more sensitive assays and also in older patients.  The history of the development of the laboratory markers is actually quite interesting including its current performance in terms of negative predictive value for MI or cardiac death subsequent to initial hospital assessment.

It is impossible for doctors to clinically assess accurately enough to safely exclude cardiac causes.  There have been attempts to create decision rules.  However the NICE guidelines conclude that meta-analyses suggest that, in isolation, clinical history and examination are not sufficient for the diagnosis of acute MI and ACS (and also that most studies are done in emergency departments rather than low prevalence general practice).

The GP context

General practice is a tricky job, full of dilemmas, and for many conditions it is a low prevalence setting, which affects estimation of risk. Cardiac troponin testing in general practice should be limited to patients presenting with ischaemic symptoms that occurred more than 24 hours previously. If these patients have no high risk clinical features and a normal electrocardiogram (ECG), they may be assessed with a single troponin assay but should be referred urgently to hospital if the result is elevated. Less than 5% of chest pain presentations in primary care are acute coronary syndrome, we don’t want to miss any cases (or other serious conditions) and yet we also have an important role as gatekeepers.  Your degree of “suspicion” is influenced by your patient population and informed by good history taking and examination but requires further appropriate investigations (in the right place) and evidence-based management algorithms as found in the recent literature.7. Nilsson S, Ortoft K, Mölstad S. The accuracy of general practitioners’ clinical assessment of chest pain patients. Eur J Gen Pract 2008; 14: 50-55.

When I was reading up on the topic I also came across an older article from a 2009/2010 study which looked at patients referred from general practice for troponin testing a number of whom, in retrospect, actually fitted into a high or intermediate risk category and some of whom went on to a cardiac event despite a negative troponin. Hopefully, the appropriate place of testing (and the use of high sensitivity troponin) is now more clearly established in the general practice setting.

Some concluding thoughts

In any area it’s appropriate to not be over confident and to seek advice from colleagues and the literature – and it’s great to emerge from that exploration feeling better informed for the future. None of us will ever know enough – and just as we think we do, the knowledge base shifts beneath us. Don’t take advice from me except that of checking things out for yourself from quality resources! It’s a good habit to maintain.

About our guest blogger
Dr Cathy Regan has worked in GP vocational training for more than 20 years.

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