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Spirometary Interpretation



https://www.youtube.com/watch?v=yNDKD_xI684 if you want a refresher or if audio/visual makes more sense for you.  US based but mostly the same language, using GOLD criteria as we do. Probably watch then look at the 5 steps below.

Interpretation should only be in conjunction with the history and consultation notes.  E.g. smoking history; pollution/ toxic exposure; personal and family history of atopy; childhood respiratory symptoms?  Work now and previously; variability of symptoms; triggers; medications esp aspirin/ nsaids/ b-blockers; symptoms e.g. cough; sputum and the timing of these; night cough; wheeze; shortness of breath; has FBC been done, if so raised eosinophils?  These all give you clues about what you’re likely to be looking for ie COPD or Asthma... or is it something else?

Firstly, don’t (necessarily) believe the spirometer’s interpretation!  I’ll explain why later.

Then LOOK at the graphs, has the patient really done their very best?  FVC of best 2 blows should be within 100ml of each other, if widely variable then ask for repeat.  If slow start may need to repeat, but if cough is affecting every blow, then don’t repeat now but treat then repeat after 2-3 months.  Code as ‘suspected Asthma/ suspected COPD’ in the meantime. 


(Ignore the last of these with an inspiratory loop as we only do expiratory.)


Next look at Volume/ time graph – ours will show predicted also, which on here is the ‘normal’ line.  In obstruction (COPD) they will often blow for over 6 seconds, can be 12-15!  Your first clue that it’s likely to be COPD.


Then look at the flow/ volume graph,





Now look at the numbers

  1. Is the VC higher then FVC? If so, use FEV1/VC ratio next. 

If the FVC is higher than VC, use the FEV1/FVC ratio. 

Circle/ highlight whichever is the lower ratio (ignore percentage of predicted on these!)

  1. Ratio – as per 1. - is either below 70? = Obstruction.  Circle the lower ratio.

Is the ratio normal but both FEV1 and FVC reduced = Restriction. 

Restriction can be caused by abdominal obesity or spinal curvature i.e. not allowing full inspiration; also by conditions such as pulmonary fibrosis and tumours. Restriction needs CXR and possibly referral.

  1. Is the FEV1 reduced, if so, by how much? Circle the percentage of predicted


  1. Is the FEF 25-75 reduced? >50% of predicted is the limit of normal.  This can be reduced in both Asthma and COPD but this may be low even with a normal ratio in Asthma.  Circle the percentage of predicted


  1. Now you can look at their interpretation – if it says Obstruction, then this is based on FEV1/FVC ratio only, it doesn’t take into account a higher VC so it could say normal and still be Obstruction. If it says Restriction then you can believe this, and you will see the reduced FEV1 and FVC but the ratio is normal, or if mixed pattern this could be reduced also.



Some of the above info in table form -


So, if it shows obstruction (ratio <70), but the FEV1 % predicted is over 80%, or even somewhat under, which is it – COPD or Asthma? 

So - look again at history and symptoms.  But either way, if there’s uncertainty, TREAT and REPEAT!  Use (depending on severity of symptoms) either ICS (Clenil or QVAR) or ICS+LABA (Fostair or Symbicort generally), and repeat 2-3 months later, then you get variability over time and the required 12% and 200ml improvement in FEV1. Or 400ml improvement of FEV1 if there was obstructive ratio on first spirometry. Please give a SABA also for any symptomatic relief.


Asthma – Salbutamol reversibility testing is occasionally useful, but only if patient is symptomatic at the time!  Do not delay treatment to do reversibility testing.  It’s fine to start ICS/ ICS +LABA and do 2 weeks of peak flows, whilst waiting to do spirometry also.  If 2 weeks of peak flows are done, then do a further week of peak flows after a month of treatment.  See if you get diurnal variation which improves with treatment, or other measures of improvement, including symptoms of course.  

This shows obstruction, FEV1/FVC ratio lower than 70%, obstructive (scalloped) curve, with good improvement after 400mcg Salbutamol (repeated 15mins later).


(but ignoring the inspiratory flow loop on here as we don’t do them)


COPD diagnosis – so this is a definite diagnosis if you have severe or very severe limitation as per GOLD chart below.  It’s likely to be correct if there is moderate obstruction ie FEV1 in the 50-70% predicted but be careful of those above 70%, occasionally there’s an Asthmatic hiding out in there!  So as above, if it’s no absolutely certain, then treat and repeat.



Also remember about potential for other causes of symptoms.

Remember to refer if spirometry is not showing cause but symptoms are troublesome, or if the patient does not improve with treatment - there are many other potential causes of respiratory symptoms! Think tumour, Bronchiectasis, Interstitial Lung Disease, vocal chord dysfunction, tracheomalacia, foreign body, Cystic Fibrosis, Pulmonary Vascular Disease, Occupational Lung disease, TB and of course others.

Graphs and slides from




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