C5.10 Haematology and biochemistry

Polycythaemia should be confirmed as being secondary to COPD by blood gas measurement that demonstrates hypoxaemia. The possibility of sleep apnoea or hypoventi­lation should be considered if polycythaemia is present but oxygen desaturation or hypoxaemia on arterial blood gas tests are absent when the patient is awake. Hyperthyroidism and acidosis are associated with breath­lessness. Hyperventilation states are associated with respira­tory alkalosis. Hypothyroidism aggravates obstructive sleep apnoea.

The prevalence of severe homozygous (ZZ) alpha₁ antitrypsin deficiency has been estimated at between 1/4,348 and 1/5,139 in European populations (Blanco et al., 2006). Available data from 15 cohorts in Australia and New Zealand suggest that the prevalence of affected individuals is around 1/4000.(de Serres, 2002) Although 75 to 85% of such individuals will develop emphysema, tobacco smoking is still the most important risk factor for COPD even in this group. Targeted screening suggests between 1.0 – 4.5% of patients with COPD have underlying severe a_1 AT deficiency (American Thoracic Society/European Respiratory Society, 2003). The index of suspicion should be high in younger Caucasian patients with predominantly basal disease and a family history. The diagnosis can be made by measuring serum levels of alpha₁ antitrypsin and if reduced, genotyping should be performed.