100Measurements of arterial blood gases should be performed on two occasions, with the employee seated.

100     Measurements of arterial blood gases should be performed on two occasions, with the employee seated.

2.1.2    Methods of measurement

101     Measurements must be performed in a manner consistent with the methods used by a respiratory function laboratory accredited by one or more of the following bodies:

a)      the Thoracic Society of Australia and New Zealand;

b)      the Australasian Sleep Association; or

c)       the Australian Council on Healthcare Standards.

102     Methods of measurement should conform to internationally recognised standards in relation to the equipment used, the procedure, and analysis of the data. Reference values (‘predicted’ normal values) should be representative of the healthy population and be appropriate for ethnicity where possible. Laboratories providing measurements used to assess impairment should state the method(s) of measurement used, and the source of the reference values used.

2.1.3    Impairment rating

103     Several professional groups have published criteria for rating the severity of impairment based on spirometry, gas transfer and VO2 max. These professional groups include the Thoracic Society of Australia and New Zealand (Abramson, 1996), the American Thoracic Society (American Thoracic Society Ad Hoc Committee on Impairment/Disability Criteria, 1986), and the American Medical Association (2001). In general, measurements are expressed as a percentage of the predicted value (%P) and, where several measurements are performed, the most abnormal result is used to classify the degree of impairment.

104     Severity of impairment is rated as shown in Table 2.1: Conversion of respiratory function values to impairment. This generic table can be used to assign WPI ratings using any valid measurement for which there are predicted normal data.

Table 2.1: Conversion of respiratory function values to impairment

See note to Table 2.1 for further information.

| %WPI | Respiratory function %P | | 0 | >85 | | 10 | 85 to 76 | | 20 | 75 to 66 | | 30 | 65 to 56 | | 40 | 55 to 51 | | 50 | 50 to 44 | | 60 | 45 to 41 | | 70 | 40 to 36 | | 80 | ≤35 |

Note to Table 2.1

1. %P = percentage of mean value for healthy individuals of the same age, height and sex.

2.2        Asthma and other hyper-reactive airways diseases

105     Assessment of impairment due to asthma can be confounded by the natural history of occupational asthma, by variably severe airflow obstruction, and therefore variable FEV1, and by response to treatment.