Table 2

Key recommendations to avoid frequent pitfalls in the interpretation of TLCO, KCO and VA in clinical practice

  • Always rule out anaemia and recent smoking (i.e. CO inhalation) as a cause for a low TLCO.

  • Decreases the number of haemoglobin sites available for CO binding and higher CO back-pressure, respectively.

  • The TLCO/VA ratio does not represent “TLCO corrected by lung volume”. To avoid confusion and misinterpretation, KCO should always be used instead of TLCO/VA.

  • A one unit change in VA does not necessarily lead to one unit change in TLCO.

  • A “preserved” KCO should never be interpreted as indication of no major pulmonary pathology.

  • In both obstructive and restrictive diseases VA may decrease out-of-proportion to TLCO leading to a “pseudo-normal” KCO.

  • The first step in the interpretation of KCO in the presence of a low VA is to check the VA/TLC ratio.

  • If low (<0.8) there is maldistribution of ventilation, frequently leading to a “pseudo-normal” KCO.

  • Rule out submaximal inspiration (inspired volume should be at least 85% of the largest VC) as the cause of a low VA before interpreting a high KCO.

  • Due to the marked increase in KCO as VA decreases, relatively small decrements in the latter has a major impact on KCO.

  • Always grade the functional impairment based on decrease in TLCO not in KCO.