This is a really good question and not a short answer Im afraid!
The gold standard in blood pressure measurement is a skilled practitioner using the old fashioned mercury sphygmomanometers however, because of the safety risks of mercury these are no longer used in practice.
The guidelines say that a skilled practitioner using a aneroid gauges are the next best thing but only when their calibration accuracy should be regularly checked based on the manufacturer’srecommendation or annually. There is evidence to show that these machines in surgeries are not calibrated as frequently as they should be and can sometimes through being dropped for example, lose their accuracy.
There is also very clear evidence for 'digit preference' among practitioners. For example, how many times have you been told your BP is 115 / 75?
There are no '5's on a monitor so the user has preferred to suggest the 5 rather than to the nearest 2mm mercury which is how the monitor is marked. This gets 'woolier' when a person comes close to a threshold for referral. It is often easier to say the diastolic is 88 because at 90 she needs to be referred to a DAU and of course vice versa.
The practitioner must also be able to clearly demonstrate the Korotkoff V sound. This is when the pulsation sounds disappear. If this cannot be heard then Korotkoff IV should be used.
Assuming the practitioner uses all the correct techniques this is the 'Silver standard'.
Of course digital monitors take away much of the 'human factor' and will give specific readings too - again the monitor has to be appropriate for use.
In pregnancy it should be the 3BTO as this is the only clinically validated monitor and the correct techniques used.
It is not good enough to record an unusual reading and simply get anther machine to check. It must always be followed up by a manual reading.
It is worth bearing in mind that some GPs will have their machines & monitors calibrated by their own Trusts medical physics dept.
(If you feel in devils advocate mood - it is fun to ask when they were last calibrated & watch the reaction of the practitioner!!!!)
In either method the correct size cuff is essential as failure to do this will give an inaccurate result.
Interestingly, research show that home monitoring is the most accurate when the person has an appropriate machine. A woman at home is much more likely to adhere to the correct technique as it is in her interest to do so.
If this is something you feel might be appropriate for you then the home monitoring guidelines recommend you should take your blood pressure ideally, twice a day initially but at the same time each day, perhaps twice in the morning & twice in the evening after you have rested for 1/2 hour or so, 30 min without smoking or caffeine sat down, back supported, arm resting on the table, correct cuff bladder placement on the arm, immobile, legs uncrossed, not talking and repeated readings at 2 min intervals.
Try to keep everything 'like for like' so use the same arm (ideally the left arm) and use the tips above.
Do this for 7-10 days. Discard the 1st days’ readings. The next set of readings should show a 'trend' that is more likely to be accurate.
Doing this Nicki, might give you a truer reflection of what your BP is really doing and whether you can reduce or come off the labetalol.
I hope this helps.....and if you've made it this far ...well done!!!!
Ps currently working on a medicines post...coming soon I promise!
Every Woman is entitled to understand what happened in her pregnancy when pre eclampsia strikes. I hope to be able to support that process.