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General equipment and test information for lab and clinic staff and curious readers. It is not medical advice; your own test instructions come from the lab or clinician who ordered it.
Short answer
A body plethysmograph is the sealed booth, mouthpiece and shutter used for body plethysmography, a lung function test that measures how much air the lungs hold. The person sits inside the booth and breathes against a mouthpiece that is briefly blocked. Spirometry measures airflow; the booth is added when a lab needs lung volume, including air that stays in the lungs after a full breath out.
What a body plethysmograph is
UCSF Health describes lung plethysmography as a test used to measure how much air you can hold in your lungs. You sit in a large airtight cabin known as a body box and breathe or pant against a mouthpiece, which may be open at first and then closed (UCSF Health). The American Thoracic Society (ATS) patient page describes the same test as finding out how much air is in your lungs after you take in a deep breath, in a box with large windows, like a telephone booth (ATS).
Cambridge University Hospitals NHS adds that the test measures airway resistance as well as the total amount of air inside the lungs. A research paper on reference values for static lung volumes lists body plethysmography and gas dilution as the two ways lung volumes are determined (Respiratory Research 2024, PMID 38570835).
How the box, shutter and mouthpiece work
The patient pages describe a similar sequence. The person wears a nose clip and breathes normally through the mouthpiece to settle, then pants softly or takes short, shallow breaths while the mouthpiece is blocked. Cambridge says a shutter blocks the mouthpiece for two seconds. In a published protocol, functional residual capacity (FRC) was measured by closing the shutter at the end of a normal breath out, and the person then breathed out fully so that expiratory reserve volume could be recorded (Respiratory Research 2024).
What is a plethysmogram?
The recording the equipment produces during these breaths is the test tracing. In one published protocol, the operator registered pressure-flow diagrams and obtained at least two FRC loops. The 2023 ERS/ATS statement (below) adds example tracings to its acceptability and grading system.
Plethysmography vs spirometry
The ATS patient page describes spirometry as measuring how much air you can breathe in and quickly blow out, and plethysmography as measuring lung volume in the enclosed box. The ATS page also notes that it is normal not to get all the air out of your lungs, no matter how hard you try. That is the reason a lab adds the box: the air that remains after a full breath out (residual volume) cannot be blown into a spirometer. In the 2024 protocol, total lung capacity (TLC) was calculated by adding residual volume (RV) to the best vital capacity, which could come from either plethysmography or spirometry.
For spirometry-side equipment, see spirometer mouthpieces, filters and calibration, and for hand-held devices see peak flow meters and incentive spirometers. These are different devices from a lab-grade plethysmograph.
What are the 4 lung volumes and 4 lung capacities?
The volume of air in the lungs at different phases of breathing is divided into four volumes and four capacities (StatPearls, “Vital Capacity”, David and Sharma, 2023, PMID 31082143). The four volumes are inspiratory reserve volume (IRV), expiratory reserve volume (ERV), tidal volume and residual volume (RV). The four capacities are total lung capacity (TLC), vital capacity (VC), inspiratory capacity (IC) and functional residual capacity (FRC). A plethysmograph is used mainly for the quantities that include the air that stays in the lungs: FRC, RV and TLC.
How three providers describe the test
Labs describe the same manoeuvre differently. The comparison below is limited to what each provider’s patient page states about the equipment.
| Source | The enclosure | Mouthpiece | Stated duration | Notes |
|---|---|---|---|---|
| UCSF Health | Large airtight cabin, “body box” | May be open at first, then closed; nose clips | Not stated | Lists anxiety in a closed box as a possible discomfort |
| ATS patient page | Box with large windows, like a telephone booth | Short shallow breaths against a briefly blocked mouthpiece; nose clip | About 15 minutes | Page also notes it is normal not to get all the air out of the lungs |
| Cambridge University Hospitals NHS | Cubicle, door closed but not locked, can be opened at any time | Shutter blocks the mouthpiece for two seconds | About 10 to 30 minutes | Door closed throughout but not locked |
Where it matters for labs and clinics
For staff who run or equip a pulmonary function lab, the sources below name these parts of the setup: the cabin, the mouthpiece, the nose clip and the shutter. The 2023 ERS/ATS technical statement on standardisation of the measurement of lung volumes (Eur Respir J 2023, DOI 10.1183/13993003.01519-2022, PMID 37500112) updates the 2005 ATS/ERS standard. Its abstract says the update guides laboratory directors, physiologists, operators, pulmonologists and manufacturers, and its key updates include standardisation of linked spirometry, new equipment quality control and validation recommendations, and a new acceptability and grading system with example tracings. Labs that need acceptance criteria or equipment validation requirements should work from the statement itself rather than from a summary page. For the wider clinical field, see what pulmonology covers.
Common misunderstandings
- It is not a bigger spirometer. It measures a different thing: volume in a sealed box, not airflow.
- Descriptions differ between providers. The table shows three different wordings and durations for the same test.
- The door is not locked. Cambridge states that the cubicle door is closed, not locked, and can be opened at any time.
FAQ
Can I drink coffee before a PFT test?
The patient pages from UCSF Health, ATS and Cambridge University Hospitals do not address caffeine. Instructions for any individual test come from the lab that scheduled it.
What is the difference between spirometry and plethysmography?
Spirometry measures airflow from a deep breath in and a fast blow out. Plethysmography measures lung volume in an enclosed box, including air that remains after a full breath out (ATS patient page).
What is a plethysmogram?
It is the recorded tracing from plethysmography. In the 2024 protocol described above, pressure-flow diagrams and FRC loops were registered while the person breathed against the shutter.
What are the 4 lung volumes and 4 lung capacities?
Volumes: inspiratory reserve volume, expiratory reserve volume, tidal volume and residual volume. Capacities: total lung capacity, vital capacity, inspiratory capacity and functional residual capacity (StatPearls, 2023).
How long does a body plethysmography test take?
ATS gives about 15 minutes and Cambridge gives about 10 to 30 minutes.
Sources
- UCSF Health, Lung plethysmography: https://www.ucsfhealth.org/care/medical-tests/lung-plethysmography (checked 2026-11-01)
- American Thoracic Society, Pulmonary function tests (patient resources): https://site.thoracic.org/patient-resources/pulmonary-function-tests (checked 2026-11-01)
- Cambridge University Hospitals NHS, Body plethysmography: https://cuh.nhs.uk/patient-information/body-plethysmography (checked 2026-11-01)
- ERS/ATS technical statement on standardisation of the measurement of lung volumes, 2023 update. Eur Respir J 2023;62(4). DOI 10.1183/13993003.01519-2022: https://pubmed.ncbi.nlm.nih.gov/37500112/ (checked 2026-10-31)
- David S, Sharma S. Vital Capacity. StatPearls, 2023: https://pubmed.ncbi.nlm.nih.gov/31082143/ (checked 2026-10-31)
- Updated reference values for static lung volumes from a healthy population in Austria. Respir Res 2024: https://pubmed.ncbi.nlm.nih.gov/38570835/ (checked 2026-10-31)
