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How to conduct a spirometry measurement

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Instruments in spirometry

Usually, spirometers are devices that use a variety of physical technologies (turbine, pneumotachograph, ultrasonic) to measure flow and then calculate volume with respect to time.

Hand-held digital spirometers have emerged relatively recently. An example is the MESI mTABLET SPIRO. It uses pneumotachograph technology – it calculates the airflow by measuring the difference between the pressure inside the mouthpiece and ambiental pressure. This also offers other advantages explained below.

Device preparation


For successful spirometry measurement, it must be checked if the device is reading correctly. Before performing spirometry, the equipment must be calibrated, or at least the calibration checked. Calibration is advised in the American Thoracic Society (ATS)/European Respiratory Society (ERS) Task Force recommendations. [16]

It is crucial because environmental factors (humidity, air pressure, temperature) can significantly affect the measurement.

Depending on the type of equipment, calibration is performed either with a 3-L syringe that is pumped through to see if the readings are correct (within a tolerance of 3%) or a 1-L syringe pumped a litre at a time to a maximum of 7 L.

Some portable devices do not require calibration, e.g. ultrasonic or pneumotachograph ones. In this case, calibration is just a checking function.

The MESI mTABLET SPIRO uses pneumotachograph technology. This enables self-calibration: it checks the environmental conditions once per second and compensates for the changes of environmental factors (humidity, air pressure, temperature). It also uses precalibrated mouthpieces. Each mouthpiece has a calibration code that calibrates the spirometer; the spirometer unit has inner sensors for measuring the ambient conditions.

Selection of reference values (prediction models)

In initial spirometry testing, the results obtained from lung function tests have little clinical relevance unless compared against reference values or predicted values. In later spirometry tests, comparisons with the patient’s previous results provide more information (e.g. about the success of the treatment). [11] [12]

Reference values or predicted values represent the typical or expected lung function for a healthy individual with similar characteristics as the patient. The values are derived from very large population surveys.

On the basis of reference values, prediction models or reference equations are created. They are algorithms or equations for estimating the expected lung function values for a patient based on their demographic information.

The main determinants of the reference values in adults:

Height: The taller the person, the bigger the lungs.

Weight: Certain reference equations use weight to calculate reference values. Increasing weight increases lung function; however, when obesity is reached, it has the opposite effect.

Age: Lung function generally increases up to approximately 25 years of age and declines afterwards.

Sex: Pre-pubescent males and females generally have the same lung function. Post-puberty, the growth of the thorax is greater in males, contributing to marked differences in lung volumes.

Ethnicity: It affects the patient’s physical constitution (lung capacity).

The selection of the prediction model depends on the local guidelines. In 2012, the Global Lung Initiative (GLI) published multi-ethnic reference values for spirometry for the 3-95-year age range [13]. These are becoming standard in spirometry. Before the spirometry measurement, the patient’s age, sex, height and weight need to be noted and an appropriate prediction model selected:

Select an existing patient or create a new patient profile with correct birthdate and sex information. They are important for correct measurement interpretation.

Enter correct height and weight, and select an appropriate prediction model. They are important for correct measurement interpretation.

This information is optional, but will appear in the final report if added. This provides more data for the clinician.

Please note that no prediction model features references for every single spirometry parameter. Here is an example of the spirometry results in the MESI mTABLET SPIRO:

Empty because no reference value is available. This is because different reference values/prediction models study different parameters.

Patient preparation

Instructing the patient is vital in order to perform the measurement correctly, both before and during the measurement. Children may also benefit from practicing. [14]

  • The patient should loosen any tight-fitting clothing. Loose dentures should be removed.
  • The test is performed with the patient sitting upright in a chair, both feet on the floor, with the equipment at the appropriate height, distance and angle.
  • The patient should rest in a sitting position for 5-10 minutes prior to the testing. Before and during the test, they should be as relaxed as possible.
  • The mouthpiece assembly should be adjusted so that the chin is at 90° horizontal to the chest, thereby ensuring a straight upper airway. Based on local guidelines, the patient may be required to wear a nose clip to prevent unnecessary air leaks.
  • Some predicted values are measured for standing; the patient should therefore be standing when they are used.

NOTE: Besides the correct patient data, the correct prediction model and suitable patient preparation, encouraging the patient during the measurement is key for achieving quality results.

Measurement execution and interpretation

Open-loop and closed-loop spirometry

Open-loop or open-circuit spirometry is the method of spirometric measurement where the patient takes a maximal inspiration, inserts the mouthpiece into the mouth, and then blows out either slowly (SVC) or fast (FVC) until the end-of-test criteria are met. [15]

In closed-loop or closed-circuit spirometry, the patient can breathe in and out with the mouthpiece in their mouth. This provides data on both the expiratory and inspiratory parts of the lung function (aka full-loop spirometry).

The advantages of closed-loop spirometry are quite a few:

  • it enables the analysis of rest breathing patterns as a comparison point for evaluating the results of forced maneuvers (tests that involve either forced inspiration or forced expiration, i.e. inspiring or expiring as forcefully and completely as possible);
  • maneuvers with forced inspiration are useful in diagnosing and monitoring upper airway obstruction;
  • it is easier for the patient, especially if they struggle with the process of maximum inspiration, which is quite difficult with open-loop technology (consisting of breathing in, then correctly inserting the mouthpiece into one’s mouth and then making a maximum expiration).

American Thoracic Society (ATS)/European Respiratory Society (ERS) Task Force recommendations [16], the most commonly adopted guidelines, state the following process for the maneuver of full-loop spirometry:

Acceptability criteria


There are a variety of criteria for successful spirometric measurements. American Thoracic Society (ATS)/ European Respiratory Society (ERS) Task Force recommendations [16] state that 3 acceptable maneuvers should be achieved. An acceptable maneuver is defined as follows [16] [11]:

  • An explosive start (without hesitation) with a <150 mL back-extrapolation volume (i.e. the volume of air expired before the actual start of the forced expiration). Maximal expiration (and inspiration if included in the measurement) is required.
  • No closure or cessation of airflow occurred during the maneuver (e.g. due to hesitation or blocking the mouthpiece).
  • The patient did not start coughing (particularly during the first second), performed no additional inspirations during the measurement, and there is no evidence of leaks.
  • The maneuver should meet the end-of-test criteria (expiring for ≥6 s and continuing to expire until the flow or speed of expired air falls below 25 mL/s). [16]

On the MESI mTABLET SPIRO, the acceptability criteria are implemented through maneuver quality warnings: after each expiration, notes appear to help the healthcare professional direct the patient to improve the next conducted maneuver.

N121: Hesitation
The hesitation symbol is shown when the start of the expiration is not fast and strong enough at the beginning of the effort.

N122: Lazy blow
The lazy blow symbol is shown when the start of the expiration is not fast and strong enough. The expiratory peak is not high and sharp on the curve.

N123: Cough
The cough symbol is shown when coughing occurs during the first second of the expiration.

N124: Early termination
This warning appears if the patient did not expire all the air from their lungs (i.e. their full lung capacity) during the maneuver.


Out of all the maneuvers performed, two need to be repeatable for the measurement to be regarded as conclusive. For FEV1 and FVC, the best two values should be within 5% or 150 mL of each other, whichever is higher. If FVC is <1.0 L, then the values should be within 100 mL. [11]



At least 3 maneuvers should be conducted, but normally not more than 8. Sometimes, patients may show a progressive reduction in FEV1 or FVC with each subsequent maneuver. If the FEV1 from an acceptable test drops below 80% of the start value, the spirometry test should be terminated for the patient’s safety. [12] [16]

When examining children, more than 8 attempts may be required because they may not perform a full maneuver at every attempt. Children may benefit from practicing the different phases of the maneuver before attempting a full one. The test operator should pay attention to the patient’s enthusiasm and effort in order not to exhaust them or discourage them from future testing.


The Z-score is a mathematical combination of the percent predicted value and the variability observed between individuals.

With this, we get a single value that accommodates the expected age– and height-related lung function variations among healthy individuals of similar demographics. [17]

SD = standard deviation (how much an individual value in a dataset deviates from the average value in a dataset)

The lower limit of normal (LLN) for a Z-score is set at -1.64. Unlike percent predicted, where different cutoffs apply to each specific outcome, the consistent -1.64 Z-score cutoff is applicable across all age groups, genders, ethnic backgrounds, and various spirometric pulmonary function parameters.

In certain lung function measures (e.g. plethysmographic lung volumes), illness can be indicated by an elevated value; in such cases, an upper limit of normal (ULN) or the 95th percentile (Z-score 1.64) would be employed.

Regardless of whether Z-scores or percent predicted values are employed, the age-specific normal range should be included in the spirometry report. At result interpretation, it is crucial to keep in mind that there will always be a degree of variability within individuals: a result could fall just outside the ‘normal range’ on one occasion and be just within it on the next. For this reason, caution applies in the case of single tests. [17] [18]

On the MESI mTABLET SPIRO, the Z-score and predicted value for the best result are shown in the form of a scale.

When a Z-score is available, the graphical presentation indicates the Z-value compared to the normal range (+/- 1.645). The green area indicates that the Z-value is within or above the normal range (Z ≥ -1.645). The yellow area indicates mild decrease (-2.0 ≤ Z < -1.645). The red area indicates more severe decrease (Z < -2.0).

When a Z-score is not available, the measured values are shown on a scale as the percentage of the predicted value. In this, 100% of the predicted value is represented as 0 in the middle of the scale. The green area indicates that the value is within or above the normal range.

Bronchodilation test

A bronchodilation test is a diagnostic procedure used to measure the changes in lung capacity after the patient inhales a bronchodilator medication, used to relax and widen the airways. It is employed in evaluating and diagnosing obstructive pulmonary disorders (asthma, COPD). [19]

Here is how a bronchodilation test is typically conducted:

The response to bronchodilation is regarded as positive when there is an increase of ≥10% of the individual’s predicted value in either FEV1 or FVC. [66]

Measurements/parameters and their definitions

FEV6 (forced expired volume in 6 seconds)The amount of forced vital capacity (FVC, i.e. the maximum amount of air expired forcefully from a fully inflated lung) expired by the 6th second of expiration. Measured in litres BTPS (body temperature, ambient barometric pressure, saturated with water vapor). In a healthy person, the FVC and FEV6 values are about the same. [16]
PEF (peak expiratory flow)The point of maximum expiration speed (in L/min) during a forced maneuver.
FEV1 (forced expired volume
in 1 second)
The maximal volume of FVC expired in the first second of forced expiration. Measured in litres BTPS.
FEV1/FEV6This ratio is a simplified alternative to the TiffeneauPinelli index (the FEV1/FVC ratio). Usually expressed as a percentage. [16]
FEVC (forced expiratory vital
capacity) or FVC (forced vital
The maximum amount of air expired forcefully from a fully inflated lung. (In contrast to the 6 seconds in FEV6, the full forced expiration is required). The patient is encouraged to expire as long as possible. On the MESI mTABLET SPIRO device, FVC measurement comprises both the inspiration and the expiration. Here, both FIVC and FEVC are measured due to closed-loop spirometry.
VC (vital capacity)In contrast to FEVC or FVC, VC is the measurement of the maximum amount of air slowly expired from a fully inflated lung (the deepest inspiration).
FEV0.5The forced expired volume at 0.5 s of forced expiration (normally used for children).
FEF25Forced expiratory flow or mid-expiratory flow at 25% FVC.
FEF50Forced expiratory flow or mid-expiratory flow at 50% FVC.
FEF75Forced expiratory flow or mid-expiratory flow at 75% FVC.
FEF(25-75)Forced expiratory flow or mid-expiratory flow from 25% to 75% FVC.
FET (forced expiratory time)The total time it takes the patient to expire FVC.
VEXT (extrapolated volume)The extrapolated volume of the expired air that is outside the tangent on the Flow-Volume curve. According to the ATS/ERS guidelines, it serves as the hesitation criterion (if it exceeds 5%, the expiration was too slow). [16]
FEV1/FVCThe Tiffeneau-Pinelli index, usually expressed as a percentage.
PEFT (time to peak expiratory flow)The time it takes an individual to reach peak expiratory flow – the point of maximum expiration speed (in L/min) during a forced maneuver.
FIVC (forced inspiratory vital
The total inspired volume after performing th expiratory FVC and reinspiring completely.
TV (tidal volume)The amount of air moved into or out of the lungs during inspiration or expiration while the patient rests (breathes normally).
SVC (slow vital capacity)The volume of air expired through an unforced maneuver (slow blow). Similar to VC, but this measurement is more advanced.
PIF (peak inspiratory flow)The point of maximum inspiration speed.
FIV1 (forced inspiratory volume in 1 second)The volume of air inspired in the first second of performing the forced inspiratory vital capacity). [20]
FIV0.5 (forced inspiratory volume in half a second)The volume of air inspired in the first half a second of performing the forced inspiratory vital capacity). [20]
FIV6The volume of air inspired in the first six seconds of performing the forced inspiratory vital capacity. [20]
FIT (forced inspiratory time)The time required by patient for forced inspiration.
IVC (inspiratory vital capacity)The maximum amount of air that the patient can inspire after a full expiration.
IC (inspiratory capacity)The maximum amount of air that can be inspired.
EC (expiratory capacity)The maximum amount of air that can be expired.
IRV (inspiratory reserve volumeThe difference between maximum inspiration and rest breathing.
ERV (expiratory reserve volume)The difference between maximum expiration and rest breathing.
TZEROThe time point when the spirometer output returns to zero at the end of expiration. Used in the analysis of FEV1.
TIZEROThe time point when the spirometer output returns to zero at the end of inspiration.
FEV1% (FVC)The ratio of the forced expiratory volume in one second (FEV1) to the forced vital capacity (FVC).
FEV1% (FEV6)The ratio of the forced expiratory volume in one second (FEV1) to the forced expiratory volume in six seconds (FEV6).

Here is a user-friendly SPIRO e-book full of illustrations!