Tlhe variability of repeated house dust mite (HDM) allergen determinations at the same site within 3-24 months was evaluated on previously collected samples. Between two and four repeated measurements of Der p 1, a major allergen of Dermatophagoidespteronyssinus and Der f 1, a major allergen of D. farinae, on 46 carpets and 31 mattresses were analyzed. In 90% of carpets and mattresses, HDM allergen concentrations were cinically relevant (at least one measurement >0.1 pg Der p 1 + Der f l/g dust). The coefficients ofvariation (CVs) for allergen concentrations in repeated samples over time (55.3-82.0% for the two allergens in beds and carpets) were clearly greater than the CVs for multiple samples collected at the same time (4.0-32.6%). Determination ofallergen mass per square meter of surface instead of concentration per gram of dust resulted in an even greater There are some other considerations of practical relevance that have not been addressed so far. First, the Australian study measured variability in 2-week intervals. This design might fail to detect long-term variations of exposure. Second, the allergen concentration in sampled dust might be massively "diluted" with allergen-free dirt from outside. Is the total sampled mass of allergen a measure of less variability? Finally, can the reliability be improved if sampling is always done by the same trained field worker rather than by residents following written instructions?Answers to these questions should help improve methods of future field studies of HDM allergen exposure. For this purpose it is also useful to estimate how many samples are needed to compensate for the variability caused by influences that cannot easily be controlled by study design. Methods Study DesignVariability ofsampling and dust extraction procedures. To describe the real variability of HDM exposure, it is necessary to assess the impact of laboratory procedures on total variability. Besides the variability of the assay itself (interassay and intra-assay variability; see "Laboratory Analysis") the influence of our sampling and dust extraction procedures on the variability of results was examined in a methodical study.Sample pairs from six mattresses (both longitudinal halves) and six carpets (two squares of 1 m2) were collected at the same time. To evaluate the variability of the extraction procedure, each of these two samples and two additional single samples from two mattresses (n = 26) were divided into two to nine dust portions. These portions were analyzed separately, and the mean and the coefficient of variation (CV) were calculated for each set of portions. The mean was interpreted as the value of the whole original sample. The values of the two original samples collected at one site were then used to calculate CVs for sampling at the same site at the same time.Long-term variability ofallergen concentration at the same site. We retrospectively analyzed dust samples from two field studies in which HDM allergen exposure was determined repeatedly (within 3-24 months).

Tlhe variability of repeated house dust mite (HDM) allergen determinations at the same site within 3-24 months was evaluated on previously collected samples. Between two and four repeated measurements of Der p 1, a major allergen of Dermatophagoidespteronyssinus and Der f 1, a major allergen of D. farinae, on 46 carpets and 31 mattresses were analyzed. In 90% of carpets and mattresses, HDM allergen concentrations were cinically relevant (at least one measurement >0.1 pg Der p 1 + Der f l/g dust). The coefficients ofvariation (CVs) for allergen concentrations in repeated samples over time (55.3-82.0% for the two allergens in beds and carpets) were clearly greater than the CVs for multiple samples collected at the same time (4.0-32.6%). Determination ofallergen mass per square meter of surface instead of concentration per gram of dust resulted in an even greater CV (72.3-86.7%). The 95% range of expected values was about 10-fold above and below the result of a single determination. We conclude that single determi-nations of HDM allergen in dust give very limited information about long-term exposure of an individual to the allergen. Repeated measurements are recommended. Studies of factors that affect HDM allergen exposure must be planned with appropriate sample sizes. Key words: asthma , house dust mite allergens, indoor allergen eTosure, reliability, repeated determination, sample size, yariability. Environ Health Perspect 106:659-664 (1998). [Online 11 September 1998] hbfp://ehpnetl.niehs.nih.gov/does/998/106p659-664birshlabstract.hbtmI Some studies have shown that exposure to house dust mite (HDM) allergens at home is associated with prevalence of allergic sensitiza-tion (1-3) and asthma (4,5) in susceptible children, but others did not (6,7). The reliability of the determination of indoor exposure to these allergens has not been investigated extensively. Marks and co-workers (8) calculated that threefold above and below a result was the range within which the true value lies with 95% certainty. The authors referred to 117 duplicate determinations of Der p 1, a major allergen of Dermatophagoides pteronyssi-nus, within 2 weeks in Sydney, Australia. In contrast to most other parts of the world (2,3,6,5), Sydney is characterized by extremely high concentrations of Der p 1 [geometric mean 38.9 pg/g dust on mattresses and 22.4 pg/g dust on bedroom floors (8)] and the absence of other HDM allergens. Two recent studies have shown that the prevalence of allergic sensitization in children correlates with exposure to HDM allergen at concentrations far below these levels [0.1 pg Der p 1 + Der f 1 (a major allergen of D. faninae)lg carpet dust (3) or even lower (2]. Thus, it seems reasonable to compare the Australian results with data from a European region with low to moderate levels of Der p 1 and, in addition, moderate concentrations of the HDM allergen Der f 1 to answer two questions: 1) Is the (relative) variability of allergen concentration higher at lower exposure levels? 2) Is the variability of Der f 1 exposure lo...