You are at work in a busy little ED. It’s just you and a bunch of junior MOs on the floor. You are seeing all the patients that you can and handing out advice as you go. Just after lunch one of your JMOs catches your eye… “Can I run this patient past you boss?”
Mrs Clot is a 49 y.o. tourist in Broome. She flew in from Sydney via Perth (4 hr flight, 1 hour lay over, then a 2.5 hr flight) about 3 days ago.
She has been doing all the usual things tourists do – laying on the beach, riding camels at sunset, drinking cocktails and eating too much.
She has also met the local mosquitoes [as is the norm] and has nasty, itchy, red looking ankles – the left maybe a bit more oedematous than the right.
This morning she experienced an episode of chest pain, it was moderately-severe, retrosternal, sharp and eased when she took some paracetamol and relaxed.
Her PMHx: hypertension [untreated], hysterectomy 3 years ago for menorrhagia, no current medications
So – sounds pretty standard. You review the ECG which is plum normal. A CXR is also “unhelpful” – that is, shows no obvious cause for the pain.
The plan is to check a highly sensitive Troponin (6 hours post-pain) and then discuss the disposition if it is normal.
“I am a bit worried about the plane flight…” he says “Doesn’t that make a PE more likely?”
Well there is surprisingly not a lot of really high quality evidence. Lots of trials comparing TEDS to no TEDS, various anticoagulants etc. But the baseline risk is extremely low – so really need a massive trial to detect any real difference in PE or PE-mortality rates.. A few studies have shown modest relative risks around the 1.12 – 1.6 risk ratio level – but this is still a rare event – so from a handful per million flights to 2 handfuls in a million travellers? It does appear that risk factors are cumulative – e.g.. being obese, on hormones, having cancer, previous VTE etc will all add up to make the risk significant. Most of the trials look at “long-haul” – ie > 6 hours in a plane. And some would suggest a dose-risk curve – longer flight more risk. So hard to say what this means to domestic flights even in big countries like Australia
You notice that your JMO is looking a little worried as he checks the lab results….
You ask – “was the troponin up?”
“No. The trop was negative. But….. I added a D-dimer, just to be sure.. and… it was 0.49” [our normal range is < 0.36]
“I was just going to ignore it if it was positive – ‘cos it isn’t really a useful test then is it…?”
You decide to take a closer look at the patient. She is as advertised – normal aside from a few midgy bites on her legs and maybe some early cellulitis as a result.
You check her PERC score – she has 0/8 criteria
Her Well’s score is firmly in the “low-risk” group.
The original Well’s paper would put it at ~ 1.3 % The PERC rule should get her down to 1.8 – 2 % The Geneva score was less discriminatory – it just states low as < 8 % risk of PE
Well it depends on the manufacturer and the “cut off value you use. BUt often used numbers are: Sensitivity = 98%, Specificity = 38 %, + LR = 1.58 ; – LR = 0.05
So we need to work out our TEST THRESHOLD ( the post-test probability below which we would be doing more harm than good if we were to pursue the diagnosis with further testing.
Test thresholds vary from disease to disease and can be tough to work out. They are based upon the prevalence of the disease, the pretest probability, the predicted rate of false positives and the potential harm from treatment. Pines et al (Acad Emerg Med. 2012 Sep;) calculated the “mortality benefit threshold” for various risk-levels. That is the pretest probability you would need to have in order to make embarking upon a PE work-up worthwhile […i.e. when is PE more likely to kill the patient than all the potential downsides of diagnostic interventions and therapy.]
So what does this mean for our traveller?
Pines and colleagues would say that our “mortality threshold” would need to be greater than 3.7 %.
So a bit of maths here – if her Well’s and PERC put her at (lets be generous) 2 % risk – worst case scenario. We can choose to ignore the D-dimer… or we can use the known +LR of a D-dimer and plug it into the formula.
Pretest probability X LR+ = Post-test prob
So for our patient – roughly… 2 % X 1.58 = 3.16 %
This is a beautiful thing. We have not ignored the D-dimer. We have accepted a positive and applied it to our pre-test risk and guess what – we are still below the “test threshold”. She still has a risk which would make it more harmful to continue to a CTPA.
In fact – doing a CTPA on this patient would more likely result in a false positive result than a true positive if the radiologist reported it as showing a PE!
Turkey is unstuffed.
As Dr Anand Senthi stated in his wonderful ACEM WS lecture – always prognosticate before you investigate!
Now I am reasonably ceertian that I hve made at least 1 or 2 errors in the logic here – and this is the beauty of #FOAMed. I would love to have your input and corrections to allow me and the other readers to learn.
Let me know