The correlation of FEV1, max to AUC for all allergens combined during the EAR(0-2 h) and LAR(3-7 h) were both strong and did not differ statistically (r = 0.807 and 0.798 respectively; difference p = 0.408). Thus, a large FEV1, max correlates to a large AUC for both the EAR(0-2 h) and LAR(3-7 h). This result is useful for future ABP tests, since it establishes a strong relationship between FEV1, max and AUC for both the EAR(0-2 h) and LAR(3-7 h) in the context of all the allergens examined. Thus, a greater degree of bronchoconstriction, as measured by the FEV1, max, will likely result in a longer recovery period, as seen by the AUC. Future pharmacological studies aimed at measuring the recovery after an allergen-induced asthmatic response might pick either measure to demonstrate recovery, or choose a specific measurement based on the correlations outlined for the chosen allergen.
The LAR(3-7 h) occurs over a longer time period than the EAR(0-2 h), which we believed would allow for greater variability in AUC, thereby reducing the correlation between FEV1, max and AUC; since these two correlation values did not differ, the AUC method can provide insight on the magnitude of response even during longer response periods. A previous study also found that the correlation between FEV1 and the area under the expiratory flow-volume curve in a methacholine challenge was strong (r = 0.939) . Some key differences from that study are that the expiratory flow-volume curve and AUC are not direct substitutes for each other, and methacholine and ABP challenges do not cause bronchoconstriction through the same pathway with substantially different time courses; methacholine-induced bronchoconstriction is more rapid in both onset and recovery. Methacholine is a direct bronchoconstrictor, it binds to receptors on airway smooth muscle, while an ABP challenge is an indirect test that leads to bronchoconstriction through inflammatory mediators via the IgE pathway . Nonetheless, our findings are in accordance with showing FEV1 and area under a curve describing air flow should correlate.
Pairwise comparisons of individual allergens’ correlation of FEV1, max vs AUC showed grass (r = 0.935) had a statistically significant higher value compared to cat, HDM, horse, and ragweed during the EAR(0-2 h) (p values range from < 0.0001 to 0.042). The between-participant variability of FEV1, max and AUC for grass allergen tended to be less than that for other allergens. The allergen with the second highest correlation during the EAR(0-2 h) was ragweed (r = 0.839), although this value only differed statistically to cat and grass (p = 0.035 and 0.042, respectively). Both grass and ragweed are seasonal allergens. ABP testing in these individuals was performed outside allergy season to avoid the potential for increased allergen responsiveness resulting from recent exposure. This is in contrast to perennial allergens like HDM, wherein exposure is chronic, potentially leading to enhanced responsiveness to ABP testing . HDM had a lower correlation coefficient (r = 0.778) but only differed statistically to grass (p = 0.001). Perhaps the difference in correlation is due to the type of exposure: chronic exposure to an allergen leading to more responsive airways maybe associated with more between-participant variability, leading to a lower AUC vs FEV1, max correlation. Nonetheless, cat allergen, had the lowest correlation (r = 0.650), and would only be a perennial allergen if the participant lived with a cat, but this value only differed statistically to grass. The choice between using FEV1, max and/or AUC to demonstrate recovery is also dependent on the individual allergen being studied. The lower correlation for the cat allergen can be a guide for future ABP tests. It may be more useful to calculate both the FEV1, max and AUC for the EAR(0-2 h) of cat allergen compared to other allergens, since a large FEV1, max does not have as strong of a correlation to a large AUC, and vice versa. Overall, only grass, a seasonal allergen, had statistically significant higher correlation than every other allergen tested.
Correlation and slope values of FEV1, max vs AUC during the LAR(3-7 h) did not show a significant difference between most allergens, only the HDM vs ragweed slope comparison was statistically significant (slope = 140.2 and 216.2 respectively; p = 0.034). The slope of the regression lines for AUC vs FEV1, max for all allergens during the LAR(3-7 h) was steeper than that of the EAR(0-2 h) (slope = 161.1 and 81.9 respectively; p < 0.0001). During the EAR(0-2 h), the only significant pairwise comparisons for the slope of FEV1, max vs AUC was cat vs grass (slope = 70.6 and 102.9 respectively; p = 0.013) and grass vs HDM (slope = 102.9 and 71.7 respectively; p = 0.002). The EAR(0-2 h) slope is related to the recovery period following allergen inhalation (once FEV1, max is reached, FEV1 would approach baseline and AUC would decrease), whereas the LAR(3-7 h) slope is a function of the magnitude of the LAR(3-7 h) (i.e., the development of the response as a sustained drop in FEV1 which would result in a large AUC). No one allergen resulted in a difference in recovery after allergen inhalation (i.e., EAR(0-2 h)), or in the magnitude of the LAR(3-7 h), than all other allergens. Specifically, since HDM did not have significantly different slopes, we cannot conclude that HDM caused a longer recovery period, or a larger LAR(3-7 h) magnitude. The slope values may also be influenced by outliers, especially at larger FEV1, max values (≥ 45) where the points were much more dispersed.
Based on Fig. 1, it is possible that grass is the only allergen undergoing recovery during the 6-to-7-h period during the LAR(3-7 h). All other allergens appear to still be increasing or reaching their maximum FEV1 at 7 h, but we would need to have data past this time point, until the maximum response is reached, to be able to comment on LAR(3-7 h) recovery. During the EAR(0-3 h), ragweed had both the largest FEV1, max and AUC, followed by HDM. However, these values are not significantly different compared to the other allergens. During the LAR(3-7 h) the allergen with the largest FEV1, max and AUC were not the same: HDM had the largest FEV1, max while grass had the largest AUC. Importantly though, neither the LAR(3-7 h) FEV1, max nor the AUC values differed statistically between allergens.
The absolute difference in the highest and lowest percent fall in FEV1 during the EAR(0-3 h) did not differ between allergens (p = 0.180), while the EAR(0-3 h) AUC did (p < 0.0001). However, HDM did not result in a larger AUC than all the other allergens; the only significantly different pairwise comparisons were cat vs HDM, cat vs ragweed, and horse vs ragweed. No single allergen had a statistically larger EAR(0-3 h) AUC than the rest. Thus, we cannot conclude from these findings that the recovery after the EAR(0-3 h) for HDM allergen is longer than other allergens, the same conclusion we reached when comparing EAR(0-2 h) FEV1, max vs AUC slopes. We suspected that HDM would result in a delayed or slower recovery during the EAR(0-3 h) because of previous research showing more severe ABP results [6, 7], as well as the activation of additional proteolytic pathways that other allergens may not induce [9,10,11]. Our findings may not be in accordance with previous data due to ragweed having the largest EAR(0-3 h) AUC, followed by HDM (3120.8 and 2510.2, respectively) with ragweed having a much smaller sample size compared to HDM (n = 26 and 72, respectively). Perhaps the smaller group of participants challenged with ragweed had more severe responses and thus larger EAR(0-3 h) AUCs than average. Despite the smaller sample size of the ragweed group, HDM still did not cause a larger AUC than all other allergens. The additional mechanisms previously described to account for HDM-induced bronchoconstriction may still be occurring, we simply did not find evidence to suggest this bronchoconstriction or recovery thereafter was more severe or prolonged.
A limitation of our research is the small sample size of some allergen groups, preventing us from analysing them as individual allergens. For example, Alternaria and tree had 5 (all had LARs) and 2 (only one dual responder) participants. These participants were included in the combined allergen correlation analyses for both the EAR and LAR, but Alternaria and tree could not be compared as individual allergens to the other groups (cat, grass, HDM, horse, and ragweed). Some differences in the data may be due to the small and variable group sizes rather than true differences. Nonetheless, the Scheffe test is considered a more conservative post-hoc test . Additionally, the missing time points for three participants meant we had to interpolate these percent fall in FEV1 values using a weighted mean. However, this is unlikely to influence the overall trend of the data when we analyzed all 221 participants.
Overall, the correlation between AUC vs FEV1, max is strong and did not differ during the EAR(0-2 h) and LAR(3-7 h) (r = 0.807 and 0.798 respectively; difference p = 0.408). This result allows us to better understand the endpoints used to measure ABP tests. Participants with severe bronchoconstriction, as seen by a large percent fall in FEV1, will also likely have a large magnitude of response, as seen by a large AUC during both the EAR(0-2 h) and LAR(3-7 h). Although we predicted HDM to cause a slower recovery and thus have a larger EAR(0-3 h) AUC due to possible additional proteolytic pathways and perennial exposure, we did not find evidence to support this claim [6, 9,10,11]. The EAR(0-3 h) and LAR(3-7 h) AUC as well as the EAR(0-3 h) and LAR(3-7 h) FEV1, max for HDM was not larger than all the other allergens tested.
Future research could be done to understand if the correlation between FEV1, max and AUC also exists in challenges outside of ABP testing, such as direct acting stimuli like methacholine. In addition, the recovery after an HDM challenge could be measured in terms of the absolute change in in FEV1, max to FEV1, min (L) rather than percent fall in FEV1. Using liters rather than percent fall from baseline would control for allergens that cause a larger fall in percent FEV1 and thus their recovery can be smaller to meet the same absolute difference as other allergens.