The peak expiratory flow (PEF), also called peak expiratory flow rate (PEFR), is a person's maximum speed of expiration, as measured with a peak flow meter, a small, hand-held device used to monitor a person's ability to breathe out air. It measures the airflow through the bronchi and thus the degree of obstruction in the airways. Peak expiratory flow is typically measured in units of liters per minute (L/min). Function Peak flow readings are higher when patients are well, and lower when the airways are constricted. From changes in recorded values, patients and doctors may determine lung functionality, the severity of asthma symptoms, and treatment. Measurement of PEFR requires training to correctly use a meter and the normal expected value depends on the patient's sex, age, and height. It is classically reduced in obstructive lung disorders such as asthma. Due to the wide range of 'normal' values and the high degree of variability, peak flow is not the recommended test to identify asthma. However, it can be useful in some circumstances. A small portion of people with asthma may benefit from regular peak flow monitoring. When monitoring is recommended, it is usually done in addition to reviewing asthma symptoms and frequency of reliever medication use. When peak flow is being monitored regularly, the results may be recorded on a peak flow chart. It is important to use the same peak flow meter every time. Scales or reference values To interpret the significance of peak expiratory flow measurements, a comparison is made to reference (normal, predicted) values based on measurements taken from the general population. Various reference values have been published in the literature and vary by population, ethnic group, age, sex, height and weight of the patient. For this reason, tables or charts are used to determine the normal value for a particular individual. More recently, medical calculators have been developed to calculate predicted values for peak expiratory flow. There are a number of non-equivalent scales used in the interpretation of peak expiratory flow. Some examples of Reference Values are given below. There is a wide natural variation in results from healthy test subjects. Wright scale EN 13826 or EU scale NHANESIII reference values provided by the US Centers for Disease Control (CDC) In 2004 the UK switched from the original Wright scale to the newer, more accurate European scale. Wright values may be converted to the EU scale using