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Letter to the Editor
Editor, – I have read Dr Pat Phillips' article (Aust Prescr 2009;32:43-6) with interest. He points out that an individual's laboratory result may be abnormal for them, but still lie within the reference interval. This can occur when the individual's biological or 'intra-individual' variance is small compared with the 'inter-individual' or group variance. The 'index of individuality' which is the ratio of the intra-individual coefficient of variation (CVi) to the group CV (CVg) is used to estimate this variance. If the index is less than 0.6, the population-derived reference interval will not be of great use and the variable is said to show high individuality. If it is greater than 1.4 it should be useful.
The example used in the article on alkaline phosphatase is unfortunate, as this variable shows high individuality and the population reference interval is of limited value. For a variable such as ionised calcium, where the intra-individual variation is close to the inter-individual variation and therefore has a high index of individuality, it will be useful.
Another detail worth mentioning is that the appropriate CV for calculating the least significant difference is the combined intra-individual and analytical CV. This is obtained by squaring the respective CVs to obtain the variances, adding them and taking the square root to obtain the combined CV.
There is much published information on these sources of variation.12 The possibility that individuals vary significantly in their intra-individual variances is recognised. Nevertheless taking these combined values into consideration can be helpful, as Dr Phillips shows, in interpreting successive laboratory results in patients on treatment.
Mount Pleasant, WA
- Fraser CG. Biological variation: from principles to practice. Washington (DC): AACC Press; 2001.
- Desirable specifications for total error, imprecision, and bias, derived from intra-and inter-individual biologic variation. 2009.http://www.westgard.com/biodatabase1.htm [cited 2009 Sep 4]