What are pulmonary function tests (PFTs), spirometry, and peak expiratory flow (PEF) measurements?

Pulmonary function tests (PFTs)—as the name implies—
are tests designed to measure and assess lung function.
PFTs were originally research tools, available
only in specialized academic hospital centers. They
are now widely available and are frequently performed
because of their usefulness in the diagnosis and treatment
of asthma. Keep in mind as you read this answer that the
lung function abnormalities seen in active asthma on PFT
testing are, by definition, reversible.

The term PFTs is used to collectively describe several
different specific tests of lung function. Spirometry is the
single most useful of the PFTs when it comes to asthma
diagnosis and treatment. Spirometry, in turn, includes
two important subtests. The first is called the peak expiratory
flow, abbreviated PEF. The second is the FEV1,
the forced expiratory volume in 1 second. Measurements
of PEF and FEV1 are thus parts, or subtests, of the
spirometry portion of the PFTs. The availability of inexpensive,
highly portable, and easy-to-use peak-flow monitors
makes it possible for every person with asthma
(even children) to measure his or her peak flow at home
on a daily basis in order to monitor asthma activity.
FEV1 measurements, on the other hand, require the use
of a spirometer, which is more costly, requires special
maintenance, and is not presently advised for home use.
Self-monitoring of PEF allows a person with asthma
insight into his or her condition and permits an assessment
of asthma control. Both PEF and FEV1 play a
pivotal role in the National Asthma Education and
Prevention Program’s (NAEPP’s) asthma diagnosis,
classification, and treatment guidelines.
To perform spirometry and PEF, the patient is first
asked to take a deep breath of room air. Then, that
biggest single breath is forcefully and rapidly exhaled
into a mouthpiece connected to the spirometer or peak
flow meter. The maneuver is repeated several times during
testing to ensure accurate and reproducible values.
The spirometer measures the exhaled lung volume, as
well as the flow of air through the mouthpiece for the
time that exhalation takes place.

The spirometry measurements
are recorded by the spirometer and are printed
out and graphed for review and future reference.
Each individual patient measurement is compared to
a predicted value. The predicted values for pulmonary
function tests are based on three variables: age, height,
and gender.Predicted values are different for a 21-year-old,
6-foot-tall man than for a 5-foot-tall, 64-year-old
woman. It follows that the PEF value (and the FEV1) that
would be considered within normal limits for a short,
older female with asthma would be abnormally low if
obtained by a tall, adolescent male with asthma, even
though they both had asthma.
Because asthma is characterized as a disease of lung emptying,
exhalation time is abnormally prolonged in symptomatic
asthma. Anyone with active asthma who attempts
to blow out all the candles on a birthday cake with one
single mighty blow of air knows about impaired lung emptying
firsthand! Depending on the degree of asthma and
other factors, such as how much airway narrowing, or
bronchospasm, is present, full exhalation during spirometry
might last as long as 14 seconds rather than the normal
5 to 6 seconds.The FEV1 and PEF values reflect the
efficiency and status of lung emptying, and thus provide
information about how a person with asthma’s lung
function is affected by his or her condition.
The FEV1 measures the amount (volume) of air that is
exhaled in the first second of forceful exhalation during
spirometry as you breathe out as hard and as fast as you
can after you have taken in a deep breath.When asthma
is poorly controlled, it takes longer than predicted for
the lungs to fully empty. Since the total exhalation time
is prolonged in symptomatic or inadequately controlled
asthma, it follows that the amount (volume) of air exhaled
during the first second of that exhalation is lower than
predicted. The FEV1 decreases in symptomatic or poorly
controlled asthma. With treatment, the lungs empty
more efficiently, and the FEV1 value returns to a normal
range. When asthma is suspected, spirometry is performed
before and after inhalation of a short-acting
bronchodilator medication to look for the normalization
of the FEV1—a phenomenon called reversibility. The
most up-to-date guidelines from the third EPR (Expert
Panel Report) of the National Heart, Lung, and Blood
Institute define a 12% or greater increase in a person’s
baseline FEV1 on spirometry after use of a bronchodilator
as a significant response.
When active or exacerbated asthma prolongs exhalation,
flow of air through narrowed air passages becomes
reduced. Spirometry in active asthma also reveals reduced
flow rates. The peak flow is the single greatest value of
flow measurement that occurs as the lungs start to empty.
Peak flows reflect the flow of air through the larger, so called
conducting airways in asthma. Peak flow generally
tracks asthma activity. Monitoring peak flow at home
allows for comparison of a person’s predicted PEF, with
his or her actual personal best measurements obtained
when the asthma is well controlled.
Home-based PEF monitoring can then help
identify even a mild exacerbation and guide medication
adjustment up or down, depending on how the PEF
value fluctuates from the personal best. Self-administered
PEF measurements over time are a component of asthma
action plans .

A peak-flow meter is an easy-to-use device designed to
help you assess the degree of your asthma control. Persons
who have moderate or severe persistent asthma,
persons with a history of severe exacerbations, and persons
who have difficulty perceiving when their asthma
worsens are most likely to benefit from self peak-flow
monitoring. Monitoring long-term, daily peak-flow
measurements detects early changes in asthma control
that require an adjustment in treatment and helps
gauge the responses to those treatment changes.
Asthma self-monitoring should be neither a bother
nor a nuisance. On the contrary, daily home peak-flow
monitoring has been shown to improve asthma control,
reduce exacerbations, and decrease absences from
school and work. Using a peak-flow monitor may also
increase your confidence as it helps you learn how to
optimize asthma control and achieve greater mastery
over your asthma. Most children can accurately measure
their peak flow under adult guidance starting at about
6 years of age. Peak-flow monitoring also allows for
objective decisions about modifying your asthma regimen
based on information contained in the written
asthma action plan your physician has provided.
If your physician gives you a prescription for home peakflow
monitoring, you will be asked to determine your
personal best value based on measurements obtained
when you feel well and are symptom-free. An asthma
action plan provides instructions on what asthma medication
to take as the peak-flow value falls into one of
three zones labeled green, yellow, or red. The green
zone includes peak-flow measurements in the range of
80–100% of your personal best. Yellow corresponds to
peak-flow measurements in the range of 60–80% of the
personal best value. The red zone includes all peak flow
values below 60% of your best. Peak-flow measurements
in the red zone indicate that your asthma is poorly controlled,
and that you will need to either contact your
physician, proceed to the emergency room, or both.