I haven’t had my blood pressure checked for a couple of years. I think the last time was for a life insurance medical so I wasn’t given much option. As with all investigations, it’s always worth having in mind what you plan to do with the result, and I’m pretty sure I wouldn’t be in any hurry to take blood pressure medication. We use QRISK in general practice (it is embedded in some of the GP computer systems) and I’ve been playing around with QRISK online. Here’s my QRISK score based on my current risk factors:
So, over the next 10 years that’s a risk of 0.8% (1 in 125) of a heart attack or stroke. Most of my risk is related to my age and my gender. Can’t do much about them.
Risk calculators can be a bit weird. You’ve got to be careful how you use them. Let’s assume that in fact I’ve got a slightly elevated BP – perhaps a systolic of 160mmHg, which according to NICE guidelines should be treated at that level. According to QRISK-2 (2011), my risk of a heart attack or stroke over the next 10 years is now 1.3%. That is, in my opinion, remains a fairly low risk (and just a 0.5% increase in absolute terms from having a normal blood pressure).
But, say I have a change of heart, my family insist, and I start on medication. I may then revisit the QRISK calculator a couple of months later. I pop in the same details but also check the box ‘on blood pressure treatment’. Even with a systolic BP of 120mmHg my risk is now 2% over 10 years.
It has gone up! That’s not very encouraging and is a good reminder of the limitations of these tools.
The QRISK information page does suggest the calculator can be used this way:
Where patients are on antihypertensive treatment, should a pre-treatment blood pressure be used when calculating their risk?
No. QRISK®2 has been designed such that if a patient is taking antihypertensive medication then their current blood pressure on treatment can be used rather than a pre-treatment value.
I don’t have details of the algorithm used in QRISK. Presumably what is happening here is that either there is more harm than benefit from anti-hypertensives, or more likely, the sub-group ‘on blood pressure treatment’ has a lot of confounders which haven’t all been teased out. That means that being on blood pressure treatment is simply a marker of other medical issues that worsen risk when looked at on a population basis.
Risk calculators like this don’t offer some magical glimpse of the future for individuals – they are just playing with statistics and, as QRISK are at pains to point out, are best used with careful real world interpretation.Read More