Pulmonary function tests

In short

Pulmonary function tests (PFTs) measure how much air the lungs hold, how fast it moves in and out, and how well gas crosses the alveolar membrane. They convert a patient's breathlessness into numbers that can be compared with reference values, repeated over time and used to diagnose asthma, COPD, interstitial lung disease, neuromuscular weakness and upper airway obstruction. For a B.Sc. respiratory therapy student this is the single highest-yield practical topic: every university paper has a spirometry interpretation question, and every hospital interview (AIIMS, PGIMER, CMC Vellore, Apollo, Fortis, Narayana, Medanta) tests whether you can perform a manoeuvre to ATS/ERS standard, grade its quality and read the curve. PFT skills are also required in occupational health surveillance of dust-exposed workers, pre-operative risk assessment, lung-transplant and pulmonary-rehabilitation work, and spirometry over-reading for clinical trials.

Explained

What PFTs measure: volumes, capacities and flows

• Volume is a single compartment; a capacity is two or more volumes added together.
• Tidal volume (TV) - air moved in one quiet breath, about 6-8 mL/kg (roughly 400-500 mL in an Indian adult).
• Inspiratory reserve volume (IRV) - extra air inhaled above a tidal breath; expiratory reserve volume (ERV) - extra air exhaled below a tidal breath.
• Residual volume (RV) - air left after maximal expiration; cannot be measured by spirometry, so TLC and FRC cannot either.
• Vital capacity (VC) = IRV + TV + ERV; inspiratory capacity (IC) = TV + IRV; functional residual capacity (FRC) = ERV + RV; total lung capacity (TLC) = VC + RV.
• Typical Western adult male values: TV 500, IRV 3,000, ERV 1,100, RV 1,200, VC 4,600, FRC 2,300, TLC 5,800 mL. Indian adults are smaller - predicted values run roughly 10-20% lower, which is why ethnicity-appropriate reference equations matter.
• Flows: FVC (forced vital capacity), FEV1 (volume in the first second), FEV1/FVC ratio, PEF (peak expiratory flow), FEF25-75 (mean flow over the middle half of the FVC), FEF50, FIF50.
• Anatomical dead space ≈ 2 mL/kg (about 150 mL); only alveolar ventilation takes part in gas exchange.

Spirometry: technique, acceptability and quality grading

• Equipment and technique follow the ATS/ERS 2019 spirometry standardisation; interpretation follows the ATS/ERS 2021/2022 interpretative strategies statement.
• Calibration verification daily with a 3-litre syringe, accepted within ±3%; check leaks, ambient temperature, pressure and humidity; all volumes reported at BTPS (body temperature 37 °C, ambient pressure, saturated).
• Patient preparation: no smoking or vaping for 1 h, no vigorous exercise for 1 h, no large meal for 2 h, withhold short-acting bronchodilators 4-6 h and long-acting 12-24 h (36-48 h for ultra-LABA) when reversibility is being tested; record height (standing, without footwear), weight, age, sex; use arm span ÷ 1.06 for height if there is kyphoscoliosis.
• Manoeuvre: seated upright, nose clip, tight lip seal on a bacterial-viral filter, maximal inhalation to TLC, a brief pause (<2 s), then a blast-out with maximal effort continued to end of forced expiration, followed by a maximal inhalation (to complete the flow-volume loop).
• Acceptability of a single blow: good start - back-extrapolated volume (BEV) < 5% of FVC or < 0.100 L, whichever is greater; no cough in the first second; no glottic closure, leak, early termination or obstructed mouthpiece; end of forced expiration = a plateau with volume change < 0.025 L for ≥ 1 s, or FET ≥ 15 s, or the patient cannot continue.
• Repeatability: the two largest FVC values within 0.150 L of each other, and the two largest FEV1 values within 0.150 L (0.100 L if FVC ≤ 1.0 L). Minimum 3 acceptable blows, maximum usually 8.
• Quality grades A to F are assigned separately for FVC and FEV1 (A = ≥3 acceptable efforts with the top two within 0.150 L; F = no acceptable measurement). Report the largest FVC and largest FEV1, even if from different blows.
• Flow-volume loop shapes tell the site of the lesion: expiratory plateau = variable intrathoracic obstruction (tracheomalacia); inspiratory plateau = variable extrathoracic obstruction (vocal cord dysfunction, goitre); a squared "box" loop with both limbs flattened = fixed upper airway obstruction (tracheal stenosis, post-tracheostomy); scooped-out concave expiratory limb = small airway obstruction.
• Contraindications (ATS/ERS 2019, mostly relative): recent myocardial infarction (within a week), unstable angina, aortic/cerebral aneurysm, recent pneumothorax, recent eye, thoracic, abdominal or neurosurgery, active haemoptysis, uncontrolled hypertension, and any state where an active respiratory infection could be transmitted.

Static lung volumes, resistance and compliance

• Helium dilution (closed circuit): patient rebreathes a known He-air volume from FRC until equilibration; He is insoluble and not absorbed, so FRC is calculated from the dilution. Underestimates volume in severe obstruction because poorly ventilated bullae do not equilibrate.
• Nitrogen washout (open circuit): patient breathes 100% O₂ from FRC; all exhaled N₂ is collected and FRC derived from the initial alveolar N₂ fraction (about 75-79%).
• Body plethysmography - the reference method: in a sealed cabin the patient pants against a closed shutter; Boyle's law (P₁V₁ = P₂V₂) relates mouth-pressure and box-pressure changes to thoracic gas volume (TGV ≈ FRC). It measures trapped gas, so plethysmographic TLC exceeds dilution TLC in emphysema - the difference estimates trapped volume.
• Airway resistance (Raw) from the plethysmograph = ΔPalv/flow, normally about 0.6-2.4 cm H₂O/L/s; conductance Gaw = 1/Raw, and specific conductance sGaw = Gaw/TGV corrects for lung volume.
• Compliance: static lung compliance ΔV/ΔP ≈ 0.2 L/cm H₂O (200 mL/cm H₂O); reduced in fibrosis, pulmonary oedema and ARDS; increased in emphysema. Measured with an oesophageal balloon for transpulmonary pressure.
• Interpretation of volumes: TLC below the lower limit of normal (LLN) = restriction; raised RV, FRC and RV/TLC = air trapping; raised TLC = hyperinflation.
• Respiratory muscle tests: MIP/PImax and MEP/PEmax at the mouth, and sniff nasal inspiratory pressure (SNIP) - key in neuromuscular disease, where supine FVC falling >20% from sitting indicates diaphragm weakness.

Diffusing capacity (DLCO) and gas transfer

• DLCO (TLCO) measures CO uptake per unit driving pressure - usually by the single-breath breath-hold method: inhale a test gas (about 0.3% CO + 10% He or methane tracer) from RV to TLC, hold for 10 s, exhale; discard dead space, analyse an alveolar sample.
• Units: mL/min/mmHg (traditional) or mmol/min/kPa (SI); 1 mmol/min/kPa = 2.986 mL/min/mmHg.
• KCO (DLCO/VA) is the transfer coefficient - DLCO corrected for the alveolar volume actually reached; it separates a small lung (lobectomy: low DLCO, normal/high KCO) from a diseased membrane (fibrosis: both low).
• DLCO must be corrected for haemoglobin (anaemia lowers it) and for COHb/smoking; it should be interpreted against altitude and inspired O₂.
• Low DLCO: emphysema, interstitial lung disease, pulmonary vascular disease and pulmonary embolism, pulmonary hypertension, anaemia, post-pneumonectomy.
• High DLCO: polycythaemia, alveolar haemorrhage, left-to-right shunt, asthma (increased pulmonary blood volume), obesity, exercise, supine posture.
• The combination carries the diagnosis: obstruction with low DLCO suggests emphysema, obstruction with normal or high DLCO suggests asthma.

Interpreting the report: pattern, severity, reversibility

1. Check quality first - an uninterpretable blow must never be reported as disease.
2. Pattern from the ratio: FEV1/FVC below the LLN (5th percentile, z-score < −1.645) = obstruction. A fixed cut-off of 0.70 (GOLD) is still used for COPD case-finding but over-diagnoses the elderly and under-diagnoses the young.
3. Restriction = normal or high FEV1/FVC with low FVC and a TLC below LLN. Low FVC with a normal ratio on spirometry alone is only a "possible restrictive" or non-specific pattern until TLC is measured.
4. Mixed = both ratio and TLC below LLN.
5. Severity (ATS/ERS 2021) is graded on the FEV1 z-score: mild −1.65 to −2.50, moderate −2.51 to −4.00, severe worse than −4.00. The older % predicted grading (mild ≥ 80, moderate 50-79, severe 30-49, very severe < 30) still appears in GOLD staging and in Indian exam papers - know both and say which you are using.
6. Bronchodilator reversibility: 400 µg salbutamol by metered-dose inhaler with a spacer, repeat spirometry after 10-15 min. The older criterion is an increase in FEV1 or FVC of ≥12% and ≥200 mL over baseline; the 2021/2022 ATS/ERS criterion is a change > 10% of the predicted value, which removes the bias against people with small baselines.
7. Bronchial provocation when spirometry is normal but asthma is suspected: methacholine PC20 < 8 mg/mL (or PD20 < 400 µg) indicates airway hyper-responsiveness; exercise or mannitol challenge with a ≥10% fall in FEV1 supports exercise-induced bronchoconstriction.
8. Reference equations: GLI-2012 (with the South-East Asian / "other" modules) and the race-neutral GLI Global 2022 are the international standard; Indian practice (Indian Chest Society / NCCP guidance) also uses Indian equations such as the ICMR-supported north Indian set, because Caucasian equations overestimate Indian predicted values by roughly 10-15% and so manufacture false "restriction".

Other tests, and where PFTs sit in Indian practice

• Peak expiratory flow (PEF) by mini-Wright or similar portable meter: effort-dependent, cheap, used for asthma monitoring and workplace (occupational asthma) diaries, not for diagnosis.
• MVV - breathe as deep and fast as possible for 12-15 s, extrapolated to a minute; a global test of muscles, airways and effort, used in pre-operative assessment.
• Slow VC and FEV6: SVC exceeds FVC in obstruction; FEV1/FEV6 is an acceptable surrogate ratio in screening.
• Field and exercise tests: 6-minute walk test (distance and desaturation), shuttle walk, and cardiopulmonary exercise testing for peak VO₂, anaerobic threshold and VE/VCO₂ - used before lung resection and in transplant work-up.
• Infection control: single-use mouthpieces with in-line bacterial-viral filters, hand hygiene, adequate room ventilation; routine since the COVID-19 period and now standard in NABH-accredited labs.
• Occupational and legal use: periodic medical examination of workers exposed to silica, asbestos, cotton dust and coal dust is a statutory occupational-health duty - formerly under the Factories Act, 1948 and the Mines Act, 1952, now under the Occupational Safety, Health and Working Conditions Code, 2020, which is in force with the other three Labour Codes from 21 November 2025. ESIC and employer schemes use spirometry for pneumoconiosis assessment and disability evaluation.
• Devices: spirometers and plethysmographs are regulated medical devices under the Medical Devices Rules, 2017 (CDSCO); laboratory accreditation is through NABL/NABH. Whether a specific device class needs a licence depends on its risk classification - check the CDSCO notification for the particular device rather than assuming.

Key points

• FVC - total volume forcibly exhaled from TLC; FEV1 - the first second of it; the ratio decides obstruction.
• Obstruction - low FEV1/FVC; restriction - low TLC with a preserved ratio; spirometry alone cannot confirm restriction.
• RV, FRC and TLC need dilution, washout or plethysmography - a spirometer can never measure them.
• Plethysmography is the reference for TLC and the only method that counts trapped gas.
• LLN / z-score interpretation is preferred over fixed 80%-predicted and 0.70 cut-offs.
• Severity by FEV1 z-score (ATS/ERS 2021); GOLD % predicted staging is still widely quoted in India.
• Reversibility - older rule 12% and 200 mL; current rule > 10% of predicted.
• Quality grades A-F are reported separately for FVC and FEV1; largest FVC and largest FEV1 may come from different blows.
• BEV < 5% of FVC or 0.100 L defines an acceptable start of test.
• Repeatability - top two FVC and top two FEV1 within 0.150 L (0.100 L if FVC ≤ 1.0 L).
• DLCO - alveolar-capillary gas transfer; always correct for haemoglobin.
• KCO = DLCO/VA distinguishes small lungs from diseased membrane.
• Obstruction + low DLCO = emphysema; obstruction + normal/high DLCO = asthma.
• Flow-volume loop localises upper airway lesions: expiratory plateau (intrathoracic), inspiratory plateau (extrathoracic), both (fixed).
• MIP, MEP, SNIP and supine FVC fall > 20% screen respiratory muscle and diaphragm weakness.
• All volumes are reported at BTPS; daily 3-litre syringe verification within ±3%.
• Indian predicted values are lower than Caucasian - using the wrong equation invents restriction.

Formulas and examples

Example 1 - obstructive pattern with borderline reversibility. A 58-year-old man, smoker: FVC 3.20 L (predicted 4.00 L), FEV1 1.80 L (predicted 3.20 L, RSD 0.45). Post-salbutamol FEV1 2.10 L.

• FEV1/FVC = 1.80 ÷ 3.20 = 0.56 (56%) - below LLN → obstruction.
• FEV1 % predicted = 1.80 ÷ 3.20 × 100 = 56%; FVC % predicted = 3.20 ÷ 4.00 = 80%.
• z-score = (1.80 − 3.20) ÷ 0.45 = −3.11 → moderate by ATS/ERS 2021 (and "moderate" at 56% by GOLD).
• Reversibility: change = 0.30 L → 0.30 ÷ 1.80 = 16.7% and 300 mL → positive by the old 12%+200 mL rule; but 0.30 ÷ 3.20 = 9.4% of predicted → not positive by the 2021 rule. Report both and treat clinically.
• Predicted MVV ≈ 1.80 × 35 = 63 L/min.

Example 2 - restrictive pattern. FVC 2.00 L (predicted 3.60 L), FEV1 1.70 L (predicted 3.00 L, RSD 0.45), TLC by plethysmography 3.20 L (predicted 5.50 L).

• FEV1/FVC = 1.70 ÷ 2.00 = 0.85 → not obstruction.
• FVC = 55.6% predicted, FEV1 = 56.7% predicted, TLC = 3.20 ÷ 5.50 = 58.2% predicted → below LLN → restriction confirmed.
• FEV1 z = (1.70 − 3.00) ÷ 0.45 = −2.89 → moderate. If DLCO is also low, interstitial lung disease is likely; if DLCO is normal, suspect chest wall, obesity or neuromuscular cause.

Example 3 - FRC and RV by helium dilution. Spirometer gas volume 3.00 L with initial He 10.0%; equilibration He 6.0%. Measured ERV 0.90 L, IC 2.60 L.

• FRC = 3.00 × (10.0 − 6.0) ÷ 6.0 = 3.00 × 0.6667 = 2.00 L.
• RV = FRC − ERV = 2.00 − 0.90 = 1.10 L.
• TLC = FRC + IC = 2.00 + 2.60 = 4.60 L; RV/TLC = 1.10 ÷ 4.60 = 23.9% (normal, roughly 20-35%).

Example 4 - ventilation and dead space. TV 450 mL, respiratory rate 16/min, dead space 150 mL.

• V̇E = 0.450 × 16 = 7.20 L/min; V̇A = (0.450 − 0.150) × 16 = 4.80 L/min; dead-space fraction = 150 ÷ 450 = 0.33.
• Shallow rapid breathing at the same V̇E (TV 300 mL, RR 24): V̇E = 7.20 L/min but V̇A = (0.300 − 0.150) × 24 = 3.60 L/min - 25% less effective ventilation.

Example 5 - DLCO corrected for anaemia. Male, measured DLCO 18.0 mL/min/mmHg, Hb 10.0 g/dL, alveolar volume 4.5 L.

• Factor = (10.22 + 10.0) ÷ (1.7 × 10.0) = 20.22 ÷ 17.0 = 1.189.
• DLCO_corr = 18.0 × 1.189 = 21.4 mL/min/mmHg (≈ 21.4 ÷ 2.986 = 7.17 mmol/min/kPa).
• KCO = 21.4 ÷ 4.5 = 4.76 mL/min/mmHg/L. A DLCO reported as "low" before Hb correction was partly anaemia, not lung disease.

Where it is used

• Pulmonology and PFT labs in government and corporate hospitals - AIIMS, PGIMER, JIPMER, CMC Vellore, Apollo, Fortis, Manipal, Narayana - where respiratory therapists perform and grade spirometry, body plethysmography, DLCO and 6MWT.
• Pre-operative and pre-anaesthetic assessment, especially before thoracic and upper abdominal surgery and lung resection, where FEV1, DLCO and exercise capacity predict post-operative complications.
• Occupational health centres of Coal India, Tata Steel, SAIL, Vedanta, cement and stone-crushing units and textile mills - periodic spirometry for silicosis, asbestosis, byssinosis and occupational asthma under the OSH&WC Code, 2020 framework.
• Asthma and COPD clinics, pulmonary rehabilitation and lung-transplant programmes - serial FEV1, DLCO and walk-test distance drive therapy and listing decisions.
• Clinical research and CROs - ICH-GCP trials of inhalers and biologics require ATS/ERS-compliant spirometry with central over-reading; trained technologists are hired specifically for this.
• Medical device and diagnostics industry - application specialists, installation, calibration and training roles with spirometer and plethysmograph suppliers in India; sleep and respiratory diagnostics in home-care companies.

Interview questions

• Q: Which lung volumes cannot be measured by spirometry and why? A: RV, and therefore FRC and TLC, because the spirometer only records air that actually moves at the mouth and RV never leaves the lungs.
• Q: How do you decide the pattern is obstructive rather than restrictive? A: By the FEV1/FVC ratio - below the lower limit of normal means obstruction; a preserved or high ratio with a low FVC points to restriction, which must then be confirmed by a TLC below LLN.
• Q: What are the acceptability criteria for a single forced expiratory blow? A: Good start with back-extrapolated volume under 5% of FVC or 0.100 L, no cough in the first second, no glottic closure or leak, and a satisfactory end of test - plateau of under 0.025 L for one second or FET of 15 s.
• Q: What is the repeatability requirement? A: The two largest FVC values and the two largest FEV1 values must each be within 0.150 L (0.100 L when FVC is 1.0 L or less).
• Q: How is bronchodilator reversibility defined today? A: The ATS/ERS 2021/2022 criterion is an FEV1 or FVC increase of more than 10% of the predicted value after 400 µg salbutamol; the older criterion of 12% and 200 mL over baseline is still in common use.
• Q: Why does plethysmographic TLC exceed helium-dilution TLC in emphysema? A: Helium cannot reach poorly ventilated bullae, whereas plethysmography measures all compressible thoracic gas, including trapped air; the difference estimates trapped volume.
• Q: A patient has obstruction on spirometry. How does DLCO help? A: A low DLCO suggests emphysema with alveolar destruction, while a normal or high DLCO favours asthma or bronchiectasis.
• Q: What does a flattened inspiratory limb with a normal expiratory limb mean? A: Variable extrathoracic upper airway obstruction - vocal cord dysfunction or paralysis, a large goitre or extrathoracic tracheal lesion.
• Q: Why must DLCO be corrected for haemoglobin? A: CO uptake depends on haemoglobin available to bind it, so anaemia lowers DLCO without any membrane disease; the correction uses (10.22 + Hb)/(1.7 × Hb) in men.
• Q: How do you test for asthma when spirometry is normal? A: Bronchial provocation - methacholine with a PC20 under 8 mg/mL, or exercise or mannitol challenge with a fall in FEV1 of 10% or more - plus PEF variability over two weeks.
• Q: Why should Indian reference equations or the GLI South-East Asian module be used? A: Indian predicted volumes are about 10-15% lower than Caucasian; using Caucasian equations makes normal Indian lungs look restrictive.
• Q: What tells you a patient has diaphragmatic weakness on PFT? A: A fall in FVC of more than 20% from sitting to supine, together with reduced MIP and SNIP.

Common mistakes

• Diagnosing restriction from a low FVC on spirometry alone, without measuring TLC - it may simply be a submaximal inhalation or poor effort.
• Measuring height wrongly, entering wrong age or sex, or leaving the default Caucasian reference equation on the machine - every % predicted and z-score then becomes wrong.
• Accepting a blow with an early cough, glottic closure or early termination, then reporting the resulting FEV1 as obstruction.
• Reporting FEV1/FVC from different sessions, or forgetting that the reported FVC and FEV1 may legitimately come from different acceptable blows.
• Using a fixed FEV1/FVC of 0.70 and a fixed 80%-predicted cut-off in all ages, instead of the LLN or z-score.
• Interpreting DLCO without the haemoglobin and COHb corrections, or confusing DLCO with KCO in a post-lobectomy patient.
• Omitting the bacterial-viral filter, not verifying the 3-litre syringe calibration that day, or failing to withhold bronchodilators before a reversibility test.

Related topics

Arterial blood gas analysis · Oxygen therapy, aerosol therapy and airway clearance · Mechanical ventilation basics · Respiratory anatomy and physiology of gas exchange · Pulmonary rehabilitation and exercise testing · Occupational lung diseases and health surveillance