this is the probability of the disease above which we feel comfortable that the benefits of treatment outweight the harms of treatment.
ie. probability of disease at the treatment threshold = R/(B + R), where:
to understand this, we need to understand:
the probability the patient has got the disease
the harm the disease is likely to cause if left untreated
the benefits of the treatment
the hazards of the treatment
the possibility of being able to intervene later once the disease better declares itself
the patient's risk aversity and other characteristics
time and resources
for classical untreated non-massive pulmonary embolism, it is said that 30% will die in the next 12 months, although 20% will die from recurrent PE (the other 10% die from cancer, etc. which may have been a risk factor for the PE), while anticoagulation will reduce this mortality to 5%.
however:
the 12-month mortality in the PIOPED study was 24% and these deaths were caused by cardiac disease, recurrent pulmonary embolism, infection, and cancer, not just PE!
let's assume that warfarin Rx does indeed reduce death from recurrent PE from 20% to 5%
if we don't anticoagulate our patient
overall, she has a 8% chance of death from recurrent PE in the next 12 months given the 40% probability of PE, and assuming the above maths is reflective of the current CT scanners, and she fails to return to ED if she develops further symptoms
if she is in the 60% group that did not have a PE then she will do well and not be exposed to harm from PE or warfarin
if she is in the 40% group and does have a PE then she has a 20% 12 month mortality from recurrent PE assuming, she ignores future signs of recurrent PE and does not get it managed at a second presentation
if we treat her, she will have the major bleed risk of around 5% plus perhaps 2% mortality from PE's giving an overall nasty outcome of 7%, not much different from not treating her!
thus R = 0.05 (risk of major bleed), B = 0.15 (difference in mortality now 5% not 20%), thus the treatment threshold disease probability should be 5/15 or 33% although this ignores risk of CT-PA and quality of life factors of being on warfarin
one can see that pre-test probability can make an important component of our clinical decision making and yet this is often neglected
95% of cases of recurrence of PE will be non-massive giving the opportunity for re-presentation, diagnosis and commencement of anticoagulation, and secondary prevention
a question today with the earlier diagnosis of PE, and the higher recognition of smaller PE's in the absence of clinically obvious DVT, is how much benefit does anticoagulation really give in this population of patients with almost sub-clinical PE, and does the risks outweigh the benfits?