PAFS: population-adjusted frequency of sensitization. (I) Influence of sex and age.

Sensitization rates are influenced by sex and age. Crude rates from different departments cannot be compared without taking into account these variables. However, the influence of sex and age has never been considered quantitatively. In 2 hypothetical populations with identical age-specific sensitizations rates, but differing age distributions, the influence of age on the overall sensitization rate (crude rate) is demonstrated. Furthermore, by an abstract reflection on rates, the influence of the proportions of a population category (e.g., age) on crude rates is shown (crude rate = sigma (category-specific rate x proportion of population in category)). To account for differing distributions of sex and age, we propose 2 ways. Sex-specific rates should be presented separately. Age-specific rates should be standardized. The standard rate is defined as: SR = sigma (category specific rate x proportion of standard population in category). Using a standard population with a rectangular structure (i.e., with equal proportions in each of the category (age) specific groups), the standardized rate is the arithmetic average of the category (age) specific rate. Only for simple routine evaluations can a standard population with 2 equal groups be used, namely over 39 years and under 40 years. The standardized rate can easily be calculated as SR: (positive rate (%under 40 + positive rate (%) over 39)/2. The general rule should be to use a "rectangular" standard population with 9 age groups of a 10-year sequence. By using the standardization procedure; remaining differences found in different departments can no longer be attributed to age and sex. Other factors, such as selection of patients or real epidemiological differences, can then be discussed. The application of population-adjusted frequency of sensitization (PAFS) in any publication on prevalences of sensitization is highly recommended.