CALCULATING % PREDICTED PEAK FLOW RATE
- Take the best of 3 of the patient’s observed peak flow rate:
- e.g. 200, 180, 190 performed – so take 200.
- Find the patient’s sex, age and height predicted value from nomogram or table:
- e.g. 440 for a woman of age 25 years and height 167 cm
- Divide patient’s observed peak flow rate over their predicted peak flow rate:
- e.g. 200/440 = 0.45
- Multiply by 100:
- e.g. 0.45X100 = 45%
So, in this example, the patient’s observed peak flow rate is 45% of predicted.
CALCULATING PEAK FLOW VARIABILITY
There are a number of methods for calculating PEF variability.
One method is described below:
- Subtract the lowest from the highest reading.
- Divide by the highest reading.
- Multiply by 100.
So, in this example, where a patient has readings of 300 to 400, the variability is 25%. If these readings were taken before and after a test dose of salbutamol, asthma is diagnosed. See Chronic asthma .