Peak expiratory flow rate (PEFR) is the maximum flow rate generated during a forceful exhalation, starting from full lung inflation. Peak flow rate primarily reflects large airway flow and depends on the voluntary effort and muscular strength of the patient.
Maximal airflow occurs during the effort-dependent portion of the expiratory maneuver, so low values may be caused by a less than maximal effort rather than by airway obstruction. Nevertheless, the ease of measuring peak flow rate with an inexpensive small portable device has made it popular as a means of following the degree of airway obstruction in patients with asthma and other pulmonary conditions.
Forced expiratory volume over 1 second (FEV1) is a dynamic measure of flow used in formal spirometry. It represents a truer indication of airway obstruction than does peak flow rate. Although peak flow rate usually correlates well with FEV1, this correlation decreases in patients with asthma as airflow diminishes. 
Peak flow rate monitoring can be accurately performed by most patients older than 5 years (see the video below). It is most commonly measured by a portable flow gauge device (peak flow meter; see the image below) but may also be obtained by a transducer that converts flow to electric output during spirometry (pneumotachometer). 
The most frequent use of peak flow rate measurement is in home monitoring of asthma, where it can be beneficial in patients for both short- and long-term monitoring. When properly performed and interpreted, peak flow rate measurement can provide the patient and the clinician with objective data upon which to base therapeutic decisions.
There are conflicting data regarding the efficacy of peak flow rate monitoring for improving asthma outcome.  Most studies have shown a benefit when peak flow rate monitoring is linked to a comprehensive program, combined with symptom diaries and patient education. [4, 5, 6]
However, a recent meta-analysis found peak flow rate monitoring to be equivalent to symptom-based asthma action plans.  Another study suggested that with symptom-based monitoring, some patients underestimate the severity of their condition and use medication inappropriately.  Gray et al reported that low baseline peak flow rate predicted unsuccessful treatment of adults with severe asthma. 
In 2007, an expert panel of the National Asthma Education and Prevention Program recommended periodic assessment of pulmonary function by spirometry or peak flow rate monitoring.  If peak flow rate monitoring is used, a written asthma action plan should use the patient’s personal best peak flow, rather than published norms, as a reference value.
The panel recommended consideration of long-term daily peak flow rate monitoring or home peak flow rate assessment during exacerbations for patients with the following:
Moderate or severe persistent asthma
History of severe exacerbations
Poor perception of airflow obstruction and worsening asthma
Preference for peak flow rate monitoring rather than the use of a symptom-based asthma action plan
In managing chronic asthma, long-term daily peak flow rate monitoring may assist with the following measures:
Detecting early changes in asthma that may require therapy
Evaluating responsiveness to changes in therapy
Giving a quantitative measurement of improvement
Identifying temporal relationships between environmental and occupational exposures and bronchospasm
The use of peak flow rate during acute asthma exacerbations is controversial. The 2007 Expert Panel report suggested that measuring peak flow rate in acute asthma episodes helps to determine the severity of exacerbations and assists in guiding therapeutic decisions in the home, school, practitioner’s office, and emergency department.  However, Eid et al have reported that peak flow rate measurement is unreliable for the classification of asthma severity. 
Compliance with peak flow rate monitoring is limited by the difficulty that patients and their caregivers often have with keeping records. In one study of inner city children, monitoring decreased 30% over the course of 3 weeks.  Kamps et al noted that children have poor compliance with recording peak flow rate measurements in symptom diaries.  Self et al reported that peak flow rate meters are frequently incorrectly used by both children and adults.  Peak flow rate measurement may be of lower utility in younger children  and elderly patients, but it has been shown to be of greater benefit in children who are poor or are members of minority groups. 
Ayala et al noted that provider communication about peak flow rate measurement was infrequent, but that such communication predicted accurate peak flow meter use in children with asthma.  In a study of adults, adherence was greatly improved by using an electronic device. 
Compliance with peak flow rate monitoring is also limited by the lack of ability, on the part of most clinicians, to interpret the data in a meaningful way. Numerous scales and charts are available, but many of them are difficult to interpret. [18, 19]
Since peak flow rate measurement depends significantly on patient effort and technique, clear instructions, demonstrations, and frequent review of technique are essential. Due to diurnal variation, peak flow rate should be measured at the same time every day. Peak flow rate declines linearly throughout gestation in pregnancy, especially when it is measured in the supine position. 
Personal best peak flow rate measurements reach a plateau of 95% predicted levels 3 weeks after the initiation of inhaled corticosteroid therapy.  The predicted peak flow rate values for African American and Hispanic patients are 10% lower than reflected in most tables.  Accuracy of peak flow meters may decrease over time.
Indications for peak flow rate measurement are as follows:
Monitoring of asthma 
Monitoring effects of ozone and other air pollutants on respiratory function 
Monitoring of chronic obstructive pulmonary disease
Providing feedback on predicted peak flow rates may improve both perception of respiratory compromise and adherence to controller medications in urban ethnic minority children. 
No contraindications exist to peak flow rate measurement.
No anesthesia is required for peak flow rate measurement.
Many varieties of peak flow rate meters are available. Some devices have electronic measurement and recording capabilities with the ability to transmit measurements to clinicians. These small hand-held devices may yield more information and increase adherence to monitoring.  They are becoming more affordable but are not yet reimbursed by insurance companies. Patients using standard meters should be provided with a diary for home recording of results.
The peak flow rate is conventionally measured with the patient standing. The mouthpiece of the peak flow rate meter is placed in the patient’s mouth and sealed by the lips. The tongue should not be placed in the front opening of the meter.
Peak flow rate measurement depends significantly on patient effort and technique. Therefore, clear instructions, demonstrations, and frequent review of technique are essential.
The indicator is moved to the lowest end of the numbered scale as shown below. For some devices, this requires vigorous shaking. If the device has a separate mouthpiece, it must now be attached.
While standing, the patient takes a deep inhalation, as shown in the image below.
The mouthpiece of the device is placed in the patient’s mouth with lips closed around it. The tongue should not be placed in the front hole. The patient blows out forcefully and rapidly in a single exhalation, as depicted below.
These steps are repeated 2 more times. If the patient coughs or does not perform the technique correctly, the turn is ignored and repeated. The highest number from the 3 attempts is recorded by the patient. Information about medications, symptoms, and any unusual activities should also be recorded.
Formulae for predicted peak flow rate vary across the literature. The patient’s personal best peak flow (as described below) may be used as a reference; population-based equations may also be used. 
Personal Best Peak Flow Rate
The patient’s individual personal best peak flow rate must be reevaluated to account for both growth and disease progression. Peak flow rate measurement should be periodically correlated with office spirometry. 
The patient is instructed to identify his or her personal best peak expiratory flow by recording the highest number achieved within 2 weeks when he or she feels relatively well without respiratory symptoms. Details of measurement are as follows:
Peak flow rate is measured at least twice a day for 2-3 weeks
Peak flow rate should be measured upon awakening and in the late afternoon or early evening
Peak flow rate should be measured 15-20 minutes after use of an inhaled short-acting beta 2 -agonist
After the personal best peak flow rate is obtained, the patient’s healthcare provider may include this information in an asthma action plan to direct the patient’s self-management.
In general, a peak flow rate of less than 80% of the patient’s personal best should trigger the administration of an inhaled short-acting beta2 -agonist. A peak flow rate of less than 50% of the patient’s personal best should trigger both administration of an inhaled short-acting beta2 -agonist and immediate medical attention.
No complications are reported as results of measuring peak flow rate.