DLCO - Diffusion Capacity of Lungs for Carbon Monoxide

DLCO measures the ease with which Carbon Monoxide can diffuse across the alveolar lining. Normal would be 100 +/- 20. There are conditions in which it can increase as well as decrease as listed below.

Decreased DLCO:

With restrictive pattern on PFTs (poulmonary function tests) then diagnosis could be restrictive lung diseases such as fibrosis, pneumonitis, elveolitis, idiopathic pulmonary fibrosis, firbrosis secondary to bleomycin therapy.

With obstructive pattern on PFTs then diagnosis is usually emphysema.

If there is no PFTs and clinical scenarios are suggestive the following two could be possible:
1) Pulmonary embolism (long plane journey, after pregnancy, on oral contraceptives etc.)
2) Pulmonary hypertension (idiopathic, h/o heart disease etc.)

Increased DLCO:

With increased circulating lung blood volume e.g. exercise, heart failure

Asthma

Polycythemia

Normal DLCO 

Can be seen in extrinsic conditions affecting the ventilation for example scoliosis. PFTs will show restrictive pattern but DLCO will be normal.