Tests of lung funtion, when using a spirometer and measuring the diffusion of carbon monoxide, are for some reason usually referred to as "formal" pulmonary function tests, as if there is something casual and perfunctory about all the other methods of assessing this organ system. The specific set of values produced in the course of these tests keeps appearing in the written exam and viva stations.  Question 21.2 from the first  paper of 2014  and the identical Question 9.1 from the first  paper of 2011 asked the candidates to interpret some formal pulmonary function tests, complete with fancy carbon monoxide diffusion values and faintly remembered variables from that annoying respiratory volumes diagram. Later, in Question 26.1 (and 26.2, and 26.3) from the second paper of 2018, the college presented the candidates with a set of PFT scenarios where they were expected to make a diagnosis on the basis of the results and then discuss how that diagnosis influences their ventilator management. A major improvement in SAQ maturity (and a move towards better assessment practice) between 2011 and 2018 has been the introduction of a "key" which lists the definitions for abbreviated PFT result labels; one criticism of the older SAQ was the fact that one might have scored poorly if one could not remember what "KCO (DLCO/VA)" stands for, even though one might be otherwise quite familiar in with lung physiology. That sort of error represents a form of "unnecessary complexity", where an element is added into an assessment item which makes the question more difficult to answer but does not increase the complexity of the tested content.

An excellent overview of this can be found in a 2005 article by Pellegrino et al.

That diagram again:

respiratory volumes

The college presents the candidates with a series of lung function variables, as well as actual and predicted values. The specific table is reproduced below:

Test

Actual

Predicted

FEV1

1.96 litres

2.66 litres

FVC

2.52 litres

3.11 litres

FEV1/FVC

78%

85%

PEF

7.50 L/sec

6.47 L/sec

FRC

2.18 litres

2.77 litres

RV

1.08 litres

1.84 litres

TLC

3.64 litres

5.17 litres

DLco

10.4 ml/min/mmHg

24.7 ml/min/mmHg

KCO (DlCO/VA)

2.85 ml/min/mmHg

4.77 1/min/mmHg

The meaning of these variables will be briefly discussed:

FEV1 is the forced expired volume over 1 second, and is a measure of maximal air flow. A decreased FEV1 may mean either an obstructive pattern of lung disease, or a diminished expiratory effort (eg. in a patient who has some sort of myopathy or neuropathy).

FVC is the forced vital capacity, from maximal inspiration to maximal expiration. A decreased FVC may reflect poor respiratory effort.

FEV1/FVC ratio is a measure of airway resistance. A FEV1/FVC ratio less than the 5th percentile of predicted suggests obstructive airways disease

PEF is the peak expiratory flow rate. A low PEF suggests obstructive disease.

FRC is the functional residual capacity. A high FRC suggests hyperinflation (eg. in asthma) or large volumes of dead space (eg. emphysema)

RV is the residual volume. As with FRC, a high RV suggests expiratory gas trapping or bullous dead space.

TLC is the total lung capacity. A high TLC may coexist with a very poor FEV1 and FVC in emphysema. A very high TLC suggests hyperinflation.  A low TLC (below the 5th percentile of predicted) suggests restrictive lung disease, such as pulmonary fibrosis.

DLCO is the diffusing capacity for carbon monoxide, a measure of the efficiency of the lung as a gas exchange surface.  It is expressed in ml/min/mmHg, and a value below 40% of predicted suggests a severe diffusion defect. DLCO may  be decreased if there is reduced lung expansion (i.e. a reduced TLC). Normal  spirometry and lung volumes associated with decreased DLCO may suggest anaemia, pulmonary vascular disorders, early interstitial pulmonary fibrosis or early emphysema.  In Question 26.3 from the second paper of 2018, the college presented candidates with just such a situation, where all the other variables were completely normal; the examiner comments were "problem is not in the lungs but with the blood flow i.e. pulmonary vascular disease/pulmonary hypertension".

KCO (DlCO/VA) is the transfer coefficient for carbon monoxide. It is calculated as the DLCO per unit of alveolar volume. As such, the KCO will not be confused by changes in lung volume, and is a more faithful representation of the gas diffusion efficiency.

References

Pellegrino, Riccardo, et al. "Interpretative strategies for lung function tests."European Respiratory Journal 26.5 (2005): 948-968.

The American Thoracic Society has a page which features an excellent bibliography of the articles which support their interpretation standards.