Venous thromboembolism: maintaining the risk

One of the banes of modern NHS hospital life is filling in VTE assessments. These have become ever more intrusive since June 2010 as a result of the Department of Health decreeing that hospitals will be fined unless 90% of all patients have a VTE assessment done. Is this evidence based stick wielding, or simply political point scoring to claim that the DoH is being proactive about a potentially preventable cause of death?

This is the letter that first got me thinking. There is no doubt about the benefits of thromboprophylaxis in surgical patients, but the evidence for medical admissions is less clear. It seems that for every 400 medical patients treated with LMWH, one symptomatic PE is prevented at the cost of two serious haemorrhages.

We also only give prophylactic LMWH whilst medical patients are in hospital, yet DVT/PEs can form up to 6 weeks after an acute inflammatory condition.

This looks bad. Let’s see if there’s anything else to fly the flag for VTE prophlyaxis.

It turns out there is a greater success rate against DVTs – from the same review, for every 16 medical patients given thromboprophylaxis, one DVT is prevented.

We know that there is a 6% mortality rate within one month of diagnosis of a DVT, and a 12% mortality rate within one month of a PE, assuming adequate anticoagulation. Interperating this is complicated by the fact that having a DVT/PE is a pretty strong marker of generally being unwell when you think of the risk factors: cancers (especially metastatic), advanced age, worsening mobility etc. This makes it hard to be certain how much the DVT/PE actually contributed to the death compared to how much the DVT/PE was a marker of impending death from co-morbidities. The paper generally tried to use autopsy proved PE as a cause of death, which should help reduce other causes of death being ascribed to PE. It does not seem to include the mortality from hemorrhage from the anticoagulation, but I could not be certain from reading it.

For every 400 medical patients given LMWH, at one month we have saved 6% of the 25 who did not get a DVT thanks to us, and about 12% of 1 patient who was spared a PE. Total number of patients saved = 0.06 x 25 + 0.12 x 1 = 1.62 patients

The 26 people who just got a diagnosis of DVT/PE would have been on full dose anticoagulation for that month, at a monthly bleeding risk of 0.3% for a major bleed and just over 0.06% for a fatal bleed. Over three months, this is nearly 1% of the patients having a major bleed and 0.2% having a fatal bleed.

(3 months’ anticoagulation is what most proximal DVTs get. I know I only could find the mortality figures for 1 month and I’m using 3 months’ anticoagulation. I can’t find the perfect papers for this. Sincere apologies.)

Total of number of patients saved from having a major bleed by preventing the DVT/PE = 0.01 x 26 = 0.26, and 0.002 x 26 = 0.05 patients saved from a fatal bleed.

The risk of LMWH is bleeding, of which two per 400 were major. It’s not very clear what is defined as major in the summary.

In summary of this completely ad hoc and massively flawed analysis, out of 400 patients given LMWH, we probably saved 1.6 patients or so from a clot based death. We probably caused about 2 to have a major bleed, but saved 0.26 major bleeds in the process by sparing them anticoagulation to treat a clot.

NICE themselves did a better job of it, and even they concluded that in medical patients, there was no overall benefit at all from VTE prophylaxis. I can’t tell if the studies they used considered the effects of sparing patients from full dose anticoagulation for months on end if you prevent a clot. This is on page 360 of the full guidance:

NICE: Maintaining the evidence

That said, the response in BMJ letters made me realise I may have been a bit too hasty to completely disregard the other side to the debate.

What do you think?

(To add  an off topic point that does not follow from anything that I have just written, recent immobility that is worse than the patient’s normal state is a significant risk factor for VTE. That said, being generally immobile is not significant, especially if they have been that way for over 4 months. This is something I had not appreciated before.

More mud in the water. I’m glad I’m on surgery now so I can just fill VTE assessments without thinking about how we may be pawns being manipulated by politicians eager to prove that they are doing something about something important somewhere without any evidence base.)


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