Spirometry is used to help establish a clinical diagnosis, quantify the severity of pulmonary function impairment, assess the response to treatments, and monitor disease course and patient prognosis.

It is considered a low-risk and well-tolerated procedure. However, it may cause discomfort and potential harm. Harm is usually related to increased intrathoracic, intra-abdominal, and intracranial pressure during the forced maneuver. This maneuver requires maximal, forced inspiratory and expiratory effort, hence the possibility of complications in the situations described below. It may affect abdominal and thoracic organs, increase myocardial oxygen demand, cause changes in venous return and systemic blood pressure, and modify chest wall and lung expansion. Traditionally, thoracic and abdominal aneurysm were also considered relative contraindications for spirometry. Limited data did not show adverse events in aortic aneurysms measuring 5–13cm and in thoracic aortic aneurysms measuring 5–8cm [7]. These contraindications are considered relative, and the decision to perform spirometry is made by the prescribing physician based on an individual risk–benefit assessment for the specific patient. Potential contraindications should be included in the spirometry request form.

The setting in which spirometry is performed should influence the decision: performing spirometry in the presence of a relative contraindication is not the same in a facility with access to emergency care as in one without it. In the latter case, referral to a specialized center may be necessary.

Since spirometry requires cooperation between the patient and the operator, it is sometimes not possible to perform it in confused, demented, or severely ill patients. Spirometry should be discontinued if the patient experiences pain during the maneuver, has a syncopal episode, or if FEV1 falls by ≥20% from baseline [8].

Nevertheless, complications related to spirometry are very rare, and it is considered a very safe test when the aforementioned situations are taken into account. The incidence of complications is 0.6 per 1000 spirometry tests. The most frequent complications are cardiopulmonary events, particularly syncope.

Suspension of inhaled medication (if indicated) and absence of acute respiratory symptoms or any other condition listed in the test contraindications must be confirmed. Smoking, occupational, and pharmacological treatment history will be collected. Identifying data (name, medical record, date of birth, biological sex, ethnicity) and anthropometric data (height in centimeters, barefoot, straight back; weight with light clothing, precision 0.5kg) will be recorded. In cases of significant chest or vertebral deformity and/or inability to stand, height may be estimated by arm span [12,28].

A quiet and comfortable environment should be provided, offering the patient 5min of rest before the test. The procedure and instructions should be clearly and concisely explained, adapted to the functional diversity of each person. A mouthpiece with a filter should be placed on the spirometer and its height adjusted. The patient should sit with a straight back, feet supported, and jaw at 90°. If necessary, the test may be performed standing, always recording the position in which it was performed. Lip seal, use of nose clip, and presence of poorly fixed dental prostheses should be checked. In cases of limitations (hemiparesis, neuromuscular diseases, tracheostomy), anatomical rubber mouthpieces, facial masks, or adapters should be considered.

Forced spirometry: a rapid maximum inspiration to total lung capacity (TLC) will be performed, with a post-inspiratory pause of less than 2s, followed by a forceful and explosive expiration, prolonged until a plateau is reached on the volume–time curve, a flow of less than 25ml/s, or a maximum time of 15s. Then, a new maximum inspiration to TLC should be performed [5]. The maneuver must be continuously supervised by the technician to assess its acceptability. A minimum of 3 acceptable and reproducible maneuvers should be performed according to the criteria described below, and a maximum of 8 maneuvers before selecting the best for interpretation.

Tables 6 and 7 summarize the acceptability and reproducibility criteria for the maneuvers.

Slow spirometry: slow spirometry, although less used than forced spirometry, can be used in patients where vital capacity (VC) needs to be known and a forced maneuver is not possible (for example, for diffusion testing), or in certain patients with decreased FVC where obstructive origin is suspected [29–31]. The maneuver is performed with the patient breathing at tidal volume, then instructed to perform a deep inspiration to TLC, followed by a deep expiration to reach residual volume.

Supine spirometry: forced spirometry in the supine position is mainly performed in neuromuscular patients, comparing lung function in sitting and supine positions, as its decrease in this position has been related in some studies to diaphragmatic involvement, although its relationship with hypoventilation is not clear [32–35]. The maneuver is performed the same as in the sitting position, but with the patient lying down, and the head may be slightly tilted for greater comfort.

Personnel working with children must have adequate training, motivation, and experience. It is essential to measure children's height before spirometry. It is not appropriate to use height from previous visits or height reported by parents or caregivers. In children who cannot stand, height should be estimated from ulna length [36]. The child should be placed in the correct posture. As in adult patients, the recommended position is sitting with a straight back. It can also be performed standing, always noting the position adopted [37]. In neuromuscular patients undergoing supine spirometry, this should also be specified. The use of a nose clip is not essential in the forced expiratory maneuver; it is recommended to note whether it was used or not.

Depending on the child's abilities and the available equipment or program, one of the two described techniques can be performed:

• 1

  Rapid but not forced inspiration, taking all the air until the child reaches total lung capacity; then, insert the mouthpiece held with teeth and close the lips. Without pausing for more than 2s, perform a forceful (rapid and strong) expiration continuously until reaching residual volume. The test should be completed with a strong inspiration to total lung capacity.

• 2

  Holding the mouthpiece between the teeth, sealing it with lips, breathe at tidal volume for 2–3 cycles, then rapidly but not forcefully inspire to total lung capacity, and without pausing for more than 2s, perform a forced expiration with maximum effort and speed of all air in the lungs until reaching residual volume. The test should be completed with a strong inspiration to total lung capacity.

The child should be stimulated with words, gestures, and especially body language to encourage maximum inspiration, abrupt initiation of expiration, and sustained expiration until the expiratory plateau is reached. It is highly recommended to use pediatric incentive software that will reinforce specific parts of the maneuver (start, course, or end of forced expiration) [37].

Next, the acceptability and reproducibility criteria for the test in children are specified (Tables 8 and 9).

A grading system has been proposed to assess the quality of spirometry based on the number of acceptable and reproducible FEV1 and FVC variables (Table 10). Variables of good quality are those of grades A and B, sufficient quality those of grade C, and variables of grades D and lower are not useful for interpretation [1]. The latest ATS standardization includes nomenclature for FEV1 or FVC measurements that do not fully meet the described criteria, but where quality limitations are considered to pose little risk of underestimation or overestimation of the measured value. For example, if the FVC from a single maneuver has a normal value but does not meet the indicators of adequate completion, it could be classified as usable. In this case, it can be assumed that it is unlikely that this measurement would be below the lower limit of normality if it had met the proper completion criteria. However, since the FVC could have a higher value if completion criteria were met, the FEV1/FVC ratio may be overestimated, so it will only be useful if it is decreased, as it could only be lower if the FVC reached a higher value by meeting completion criteria.

This classification system has proven useful in both epidemiological studies and clinical practice, having been implemented automatically in some spirometers and referenced in numerous publications [38–41]. However, it should be noted that in approximately 10–20% of cases, it is not possible to obtain maneuvers of good quality despite the technician's effort and the patient's good cooperation [40,41].

The interpretation of spirometry is based on comparing the patient's measured values with the expected theoretical values for a healthy individual with similar demographic characteristics, allowing identification of deviations from normality that may indicate pulmonary pathology.

Therefore, it is essential to accurately record the following demographic data of the patient:

• •

  Age: Should be calculated from the date of birth and the date of the test and expressed in years with one decimal.

• •

  Height: Expressed in centimeters with one decimal. Measurement is performed without shoes, with feet together and back straight against a stadiometer. In cases of inability to stand or significant chest deformity, the patient's height can be measured by arm span (distance between the third finger of each hand with arms outstretched), or by measuring the heel-knee distance (applying a reference equation), recognizing differences by sex, age, and ethnicity in these estimations [42].

• •

  Weight: Expressed in kilograms without decimals, rounded to the nearest 0.5kg. Measured with the patient without shoes. Using weight and height, the Body Mass Index (BMI) is calculated, expressed as kg/m2.

• •

  Biological sex: Should be included in the spirometry request form, and if not, it should be requested from the patient at the time of the test. Biological sex is the determinant of predicted lung size, crucial for reference values.

• •

  Race: The races currently used for GLI reference values are Caucasian (European ancestry), African American, Northeast Asian, Southeast Asian, and other/mixed.

Currently, it is recommended to use the GLI reference equations for all ages (3–95 years) [12]. These equations are based on a large international cohort (97,759 spirometries from 72 centers in 33 countries) and are recognized for minimizing age and ethnic biases. The adoption of GLI 2012, instead of the SEPAR reference equations proposed by García-Río [1], represents a significant improvement in standardization and international and multi-ethnic applicability. The use of previous SEPAR reference equations is only indicated if there is a very specific clinical or research justification for certain subpopulations, and must always be documented in the test report (Table 11).

For the interpretation of spirometry, it is necessary to include the patient's values, as well as the following data:

• •

  Percentage of reference value (%ref.): The obtained values are commonly expressed as a percentage of the reference value, although using a fixed threshold as the lower limit of normality lacks statistical basis and may lead to classification errors.

• •

  Lower limit of normality (LLN): A value is considered below the expected range if it is less than the 5th percentile of the reference population. The LLN should be calculated individually for each spirometry parameter according to the patient's sex, age, and height, providing a more accurate threshold.

• •

  z-Scores: Expressing results as z-scores (z=(observed value−population mean)/standard deviation) is the most accurate and recommended method for interpreting functional abnormalities and their changes. The z-score allows standardized interpretation, where a z-score of −1.645 is equivalent to the 5th percentile, providing a value proportional to the standard deviation.

By adopting GLI and z-scores, misclassification of patients (for example, overdiagnosis of obstruction in the elderly or underdiagnosis in young adults, or incorrect interpretation of bronchodilator response) can be significantly minimized. This leads to more precise diagnoses, more appropriate initiation of therapies, and more reliable monitoring of disease progression over time and across different populations.

The current recommendation is to include all spirometry information, both values and graphs, in a single document. This document should also include the patient's demographic data, as well as the technician's observations and interpretation (Fig. 2) [43].

Fig. 2.

Recommended report of forced spirometry with bronchodilator test and slow spirometry. GLI: global lung function initiative. BMI: body mass index. FVC: forced vital capacity. FEV1: forced expiratory volume in 1second. %FEV1/FVC: ratio between FEV1 and FVC multiplied by 100. FET: forced expiration time. FIVC: forced inspiratory vital capacity. LLN: lower limit of normal. %ref.: % with respect to the mean of GLI reference values. SVC: slow vital capacity. IC: inspiratory capacity. ULN: upper limit of normal.

*Adapted from Culver BH et al. Am J Respir Crit Care Med 2017: 196, 1463–72. DOI: 10.1164/rccm.201710-1981ST [43].

The interpretation of spirometry should be clear, concise, and informative to help determine whether the observed result is within the normal range and, if not, what type of physiological abnormality is likely involved. Additionally, repeated spirometry assessment is important to detect deviations that may be clinically significant. Spirometry evaluates the flow and volume of expired air, with the most important indices being FEV1, FVC, and the FEV1/FVC ratio.

In spirometry, we must qualify and quantify the pattern:

• 1.

  Airflow limitation or obstruction.

Obstructive ventilatory impairment is defined by an FEV1/FVC ratio below the lower limit of normality (LLN), which corresponds to the 5th percentile of a healthy population (Fig. 3) [6].

Fig. 3.

Ventilatory deficiency defined by spirometry: diagnostic algorithm. LLN (lower limit of normal) based on GLI 2012 reference equations [44].

The widely used cut-off points of 80% of the theoretical value for FEV1 (as it is arbitrary) and 0.70 for the FEV1/FVC ratio (as it does not account for age) are discouraged [44,48–50].

To evaluate the severity of pulmonary function impairment, cut-off points based on z-score values for FEV1 are recommended [51]. A z-score value between −1.645 and +1.645 is within the normal range, between −1.645 and −2.5 reflects mild obstructive impairment, between −2.5 and −4 moderate obstructive impairment, and z-score<−4 severe obstructive impairment [6].

Although the use of LLN and z-score is recommended for functional assessment, their application has so far been documented mainly in studies on obstructive pathology.

• 2.

  Suspicion of restrictive pattern.

A restrictive disorder should be suspected when FVC is below the LLN, the FEV1/FVC ratio exceeds its LLN, and the flow-volume curve shows a convex morphology. However, this can only be confirmed if a reduction in total lung capacity (TLC) is objectively demonstrated, i.e., if TLC is below the LLN or the 5th percentile, by measuring static lung volumes [6,52,53].

• 3.

  Unspecified ventilatory pattern and PRISm.

The presence of a pattern with reduced FVC and FEV1, with FEV1/FVC and TLC within the reference range, is known as an “unspecified” ventilatory impairment. Three-year follow-up of subjects with an “unspecified” pattern has shown that two-thirds maintain the alteration, while the remaining third develop either an overt obstructive or restrictive pattern [54]. When TLC is not available, the “unspecified” pattern is termed “preserved ratio impaired spirometry (PRISm)” [55].

• 4.

  Suspicion of mixed ventilatory impairment.

A mixed ventilatory impairment is suspected when spirometry shows both FVC and FEV1/FVC below the LLN. To confirm the restrictive component, measurement of TLC, which must be below its LLN, is essential [6,56].

• 5.

  Dysanapsis. Other measurements.

Dysanapsis is characterized by an FEV1/FVC ratio below the LLN, FEV1 within the reference range, and elevated FVC. This pattern may result from disproportionate growth between the pulmonary parenchyma and airways. Although it is believed to be a physiological variant of normality, new data suggest it may be associated with, or represent an early stage in the development of airway obstruction [57,58].

For the evaluation of peripheral airways, especially when FEV1 and FEV1/FVC are within the normal range, traditional measurements such as forced expiratory flow between 25% and 75% of FVC (FEF25–75) have been used; however, these measurements are highly variable, poorly reproducible, and lack specificity for small airway disease [59].

• 6.

  Central airway obstruction.

Central and upper airways are located at the intrathoracic and extrathoracic levels. Obstruction of these airways in early stages may not cause a significant reduction in FEV1 or FVC, but often leads to a marked decrease in peak expiratory flow (PEF) relative to FEV1, as well as a reduction in forced inspiratory flow at 50% (FIF50%). The indices presented in Table 12 can help differentiate intrathoracic from extrathoracic obstruction [60,61], as well as the interpretation of flow-volume curves (Fig. 4).

Fig. 4.

Flow–volume curve patterns suggestive of upper airway obstruction.

It consists of measuring changes in pulmonary function after inhaling a short-acting bronchodilator. It helps diagnose and assess asthma, COPD, and other bronchial diseases, but the response to the bronchodilator test (BDT) is not sufficient to discriminate between asthma and COPD, nor to assess the definitive efficacy of a bronchodilator [6,62–67]. BDT should not be routine when baseline spirometry is normal or when the patient has previous BDT. It should be assessed based on available clinical information.

Indications:

• 1.

  Baseline obstructive or mixed spirometry for the first time [6,49,68].

• 2.

  Unspecified spirometric pattern in an individual with apparent good cooperation [6,49,68].

• 3.

  Evaluation of bronchodilator response during an asthma attack, including bronchoconstriction induced in bronchial provocation tests [69].

• 4.

  Disability assessment when FEV1 is below 65% of predicted [70,71].

• 5.

  Preoperative evaluation when there is airflow limitation [72].

Contraindications:

• 1.

  Known or probable adverse reactions to the bronchodilator to be used.

• 2.

  Those of spirometry.

Bronchodilator drugs should be withdrawn beforehand (Table 5). If preparation has not been met, the test should be postponed. The relative merits of different protocols are unclear; the choice of bronchodilator, dose, and mode of administration is up to the laboratory [4,6,73]. Our recommendation is the use of salbutamol via MDI with a spacer. In children under 5 years, a mask is recommended. Unless contraindicated (known arrhythmias, tremor, or previous adverse reactions), high doses are recommended: 4 inhalations (400μg) separated by 30-s intervals, to minimize variability [4,6,74]. After 15min following salbutamol inhalation, the second spirometry should be performed 4.

Although the use of ipratropium bromide (IB) requires a longer waiting time (30min) and is a less effective drug for demonstrating reversibility in asthma, its use may be accepted in patients who may have intolerance or contraindications to beta-agonists.

Both FEV1 and FVC have been shown to be reproducible if the test is performed by well-trained personnel [6,12,49].

It is recommended to express the change in FEV1 or FVC as a percentage of predicted values, as this normalizes the result for sex, initial FEV1, and age, factors known to influence the response. A response is considered positive if the percentage change exceeds 10%. This criterion is supported by two major studies and is recommended by most respiratory societies [6,64,66,67,75–87].

% Change = (value after bronchodilators - value before bronchodilators) /predicted value × 100.

Regarding pediatric criteria, there are insufficient data to establish recommendations. The test could be considered positive if the percentage change is equal to or greater than 12% compared to baseline or 9–10% compared to predicted. To maintain adequate consistency, one criterion should be chosen and always applied [88–94]. Demonstration of reversibility by measuring PEF is useful for asthma diagnosis and outpatient management and for asthma attacks [95], but it is not recommended in pulmonary function labo