A spirometry printout can look deceptively simple: a few curves, several percentages and a traffic-light report. Yet knowing how to interpret lung function is not the same as reading whether a value is labelled “normal”. In Occupational Health, the clinical meaning depends on test quality, symptoms, exposure history, job demands and whether the worker is being assessed for diagnosis, surveillance or fitness for safety-critical work.
The most useful approach is structured. First establish whether the test is technically acceptable. Then identify the physiological pattern. Finally, translate the finding into a proportionate workplace decision without overstating what the data can prove.
Before reviewing any result, be clear why lung function has been performed. Pre-placement assessment, statutory or risk-based health surveillance, respiratory symptom assessment and review of a worker with known asthma are different clinical tasks. A mildly reduced measurement may matter very differently for a worker using respiratory protective equipment, a firefighter, a laboratory technician with sensitiser exposure or an office-based employee.
Lung function testing does not diagnose occupational asthma, chronic obstructive pulmonary disease or interstitial lung disease on its own. It provides evidence that must sit alongside a focused history, examination where indicated, exposure assessment and, when needed, primary or specialist respiratory care. For occupational asthma in particular, work-related variation in symptoms and serial peak flow monitoring may be more informative than a single normal spirometry test.
Poor-quality spirometry can imitate disease. The trace and testing record deserve attention before any clinical conclusion is made. Was the worker able to understand and follow instructions? Did they achieve a rapid, forceful start to expiration and continue long enough to reach a satisfactory end of test? Are the best efforts reproducible?
Common problems include a hesitant start, cough in the first second, early termination, variable effort and leaks around the mouthpiece. These can falsely lower FEV1, FVC or both. The volume-time and flow-volume curves are not decorative extras: they help identify whether a reported abnormality is likely to be physiological or technical.
Check that the individual’s age, height, sex and ethnicity data have been entered appropriately, and confirm which reference equations and lower limits of normal the service uses. Equipment calibration, infection-control procedures, contraindication screening and staff competence also matter. In a surveillance programme, consistency of protocol is central because an apparent decline is only meaningful if results are comparable over time.
If quality is unacceptable or repeatable manoeuvres have not been achieved, the defensible action is usually to repeat the test on another occasion or refer to a competent service. Do not convert uncertain data into a restrictive employment recommendation.
Three spirometry measures form the starting point:
Interpret these against the lower limit of normal, ideally expressed using a z-score, rather than relying solely on a fixed percentage predicted threshold. Percentage predicted remains familiar and useful for communication, but it can misclassify people at the extremes of age or body size. A result below the lower limit of normal suggests that it is less likely to be explained by normal population variation.
A fixed FEV1/FVC ratio below 0.70 is commonly recognised in COPD pathways, particularly in older literature and some clinical settings. It should not, however, replace clinical judgement or age-adjusted reference values in routine interpretation. A fixed ratio may overcall obstruction in older workers and miss it in younger adults.
Obstruction is suggested when the FEV1/FVC ratio is below the lower limit of normal. FEV1 may also be reduced, reflecting the severity of airflow limitation, but the ratio is the defining feature. On the flow-volume loop, the expiratory limb may appear scooped or concave.
Asthma, COPD, bronchiectasis and other airway disorders can produce obstruction. In the workplace, consider whether symptoms began after a new role or exposure, improve away from work, worsen on particular tasks, or occur alongside rhinitis or eye symptoms. These clues may point towards a work-related respiratory condition, but they do not establish causation.
Bronchodilator reversibility testing may support variable airflow limitation, but a positive or negative result is not a stand-alone test for asthma. Its usefulness depends on the clinical question and local competency arrangements.
A reduced FVC with a normal or raised FEV1/FVC ratio can suggest restriction, but spirometry cannot confirm restrictive lung disease. It may also reflect submaximal effort, obesity, air trapping or an incomplete exhalation. Confirmation requires measurement of total lung capacity, usually through a respiratory physiology laboratory.
This distinction matters at work. Labelling a worker as having “restrictive lung disease” from screening spirometry alone may cause anxiety, inappropriate restriction and an avoidable delay in proper assessment. Record the finding accurately: for example, “reduced FVC with preserved ratio; restrictive defect cannot be confirmed by spirometry alone”.
A low FEV1/FVC ratio with low FVC may represent mixed obstruction and restriction, but severe obstruction with air trapping can also lower FVC. Full lung volumes are needed to separate these possibilities.
Some workers have reduced FEV1 and FVC with a preserved ratio and normal total lung capacity when tested further. This non-specific pattern again illustrates why a printout should lead to a clinical question, not a premature diagnosis.
More complete pulmonary function testing may include total lung capacity, residual volume and transfer factor for carbon monoxide, often called DLCO or TLCO. These tests are generally arranged through a specialist service rather than used as routine workplace screening.
A reduced total lung capacity confirms restriction. A raised residual volume can support air trapping or hyperinflation. Transfer factor assesses gas transfer across the alveolar-capillary membrane and can be reduced in conditions such as emphysema, interstitial lung disease or pulmonary vascular disease. Anaemia, smoking status and technical factors can affect its interpretation, so context remains essential.
For Occupational Health practice, the practical question is often whether there is evidence of impairment requiring further assessment, adjustment or temporary control measures, rather than identifying the precise respiratory diagnosis yourself.
Serial results can be valuable in respiratory surveillance, especially where workers are exposed to sensitisers, dusts, fumes or other respiratory hazards. However, a numerical fall is not automatically evidence of work-caused decline. Day-to-day biological variation, different testers, equipment, acute respiratory infections, smoking changes and variable effort can all alter results.
Look for a consistent trend across technically valid tests. Ask whether the worker had a chest infection, had taken bronchodilators, or was experiencing active symptoms at the time. Compare like with like, and ensure any escalation pathway is clearly documented. A surveillance programme should detect possible harm early, not create false reassurance from tick-box testing.
Fitness for work is rarely determined by spirometry alone. Consider functional capacity, symptom control, the likelihood and consequences of sudden breathlessness, the specific exposure, emergency arrangements and the effectiveness of controls. For respirator use, the practical demands of the equipment, face-fit requirements and the worker’s tolerance may matter as much as a borderline numerical result.
A worker with well-controlled asthma and normal spirometry may be fit for most roles with an exposure control plan and access to medication. A worker with significant unexplained breathlessness may need temporary restriction from strenuous, confined-space or emergency-response duties pending assessment. The wording should be functional and proportionate: describe what is needed to work safely, rather than disclosing unnecessary diagnostic detail to management.
Where a possible work-related condition is identified, act promptly. Review exposure controls, involve the relevant occupational and respiratory specialists, and consider whether temporary removal from the suspected exposure is needed while investigations proceed. Delayed action can worsen prognosis in sensitiser-induced occupational asthma.
A clear report separates observation from interpretation and recommendation. State whether test quality was acceptable, summarise the pattern, identify limitations and explain the next step. For example: “Spirometry is technically acceptable and shows airflow obstruction. In the context of wheeze that improves on days away from work, this warrants timely clinical assessment for possible work-related asthma. Pending review, reduce exposure to respiratory sensitisers and confirm adequate workplace controls.”
Avoid declaring someone permanently unfit on the basis of one screening test unless there is an immediate, clearly defined safety concern. Equally, do not allow a normal test to close down a convincing history of work-related respiratory symptoms.
A confident Occupational Health opinion comes from putting physiology, test quality and real job demands together. Treat each trace as the start of a structured clinical conversation, and your recommendations will be safer, clearer and more useful to both worker and employer.
