What should my child's Peak Flow be?
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If you find it difficult to use this page, ask your doctor or your asthma nurse to tell you what your normal predicted Peak Flow is.
Check your child's peak flow against this chart
This page is based on the standard chart which most doctors use to find out what is a normal peak flow for a child. Unfortunately it does not give such a useful prediction as we can get for adults because children of the same height and sex vary a lot in peak flow. Any reasonably experienced specialist in asthma in children will also use other evidence to help to estimate whether your child's peak flow is reduced. Peak flow readings in young children are often inaccurate compared to those of older children or adults. This is another reason why guidance from a specialist is important. Nevertheless, this chart is of some use. Peak flows don't depend on whether your child is a boy or a girl. The "predicted" value just depends on your child's height. On the chart, height is given in centimetres only. If you have measured your child in inches and feet, you must first convert to centimetres.
Draw or imagine a straight line vertically from your child's height at the bottom of the chart. (See example below.)
Where your vertical line crosses middle of the three red lines, imagine a horizontal line.
The horizontal line will cross the left axis at the predicted average figure for your child's Peak Flow. What this means is that in a group of English children in the late 1960s this was the average result.
This is only a rough indication; many healthy people have higher Peak Flows than the chart suggests. There is a lot of variation between normal people, more so in children.
The upper and lower red lines show values within which 95% of the original group of children fell. So 5% of the children (one child in 20), who were apparently well, fell outside these limits. Because human beings vary, we have to use this kind of method to separate values which are likely to be normal from those which may not be. A doctor with the right expertise will be very helpful in deciding whether your child truly has asthma or an abnormal peak flow.
Often it is more useful to check whether the peak flow changes when your child seems to have difficulty in breathing or during a cold, and whether asthma treatment can increase the peak flow.
All these things only help if your child is really able to do peak flow readings well. This depends both on the child and on how he or she is taught.
Here is an example of how to use the chart.
If your child is 135 cm tall:
In this example, the child's average predicted Peak Flow is roughly 290 litres per minute. 95% of normal children of this height will have peak flows from 220 to 370 litres per minute. Notice this enormous range. This shows how imprecise the chart is for prediction. Even then, one child in 20 falls outside the range. A child could be normal and fall outside the range, or have narrowed airways from asthma and fall within the range. In cases of doubt you will need the help of a doctor or specialist nurse.
It was created 25 March 2003
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