Primary endpoint analysis showed that the new diagnostic method (Spirometry Holter) proposed in our study could detect obstruction in 42% of patients reporting respiratory symptoms without prior confirmation in spirometry and allow the objective diagnosis of asthma. Moreover, the reported symptoms were not significantly related to the evidence of obstruction.
The study showed that the probability of obstruction detection increased in time, with 21 days to detect all obstructions in this cohort and an optimal time of about 10 days to catch most obstructions. Moreover, the study showed that the newly proposed method is feasible to organise in an outpatient clinic environment, and asthma patients can perform spirometry at home, with over 88% of patients achieving over 50% of technically correct examinations. Therefore, the Spirometry Holter emerges as a promising tool for further investigation in cases of indeterminate asthma cases.
GINA Guidelines provide comprehensive criteria and protocols for asthma diagnosis [2]. That includes the medical history of typical, variable respiratory symptoms, documented obstruction and confirmed variable expiratory airflow limitation with various methods differing in sensitivity, specificity, access and invasiveness [23]. Although confirming variable respiratory symptoms is clear, documenting obstruction and confirming variable expiratory airflow limitation is hard to achieve in an outpatient clinic, though mandatory for preventing misdiagnosis.
Therefore, GINA provides a wide range of measures such as MCT, reversibility tests, excessive PEF variability over two weeks, increase in lung functions after four weeks of treatment, excessive variation in lung function between visits, or exercise challenge tests [2].
This study investigated a few measures using a newly proposed structured diagnostic approach. The most valuable diagnostic tool for asthma remains the direct challenge test with methacholine (MCT), followed by the mannitol challenge test [24]. Yurkadul et al. showed 89% diagnostic accuracy for MCT, with the highest negative predictive value (93,5%) [25]. Nevertheless, the method's utilisation seems poor due to its invasiveness and need for an in-hospital setting during the test.
The second diagnostic accuracy was achieved by PEF variability (71%). Diurnal PEF variability over two weeks was measured in our study using the Holter method. The finding of this study showed that using a 10% cut-off value for average diurnal PEF variability, proposed by current guidelines, resulted in many false positive results (9/15, 60%) that may lead to overdiagnosis. Similar results have been shown by L Tilemann, with low specificity, sensitivity and predictive value of diurnal PEF variability compared to the MCT [26].
With all study limitations, our analysis showed that the optimal threshold indicating at least one obstruction in this group of patients was > 16%. Moreover, some studies have shown that PEF poorly correlates with FEV1, especially in children [27]. Therefore, we suspected FEV1, a non-effort-dependent metric, may be a more adequate parameter for airway flow variability. In the additional analysis using an equation originally proposed by GINA for diurnal PEF, we presented a new digital biomarker available in the spirometry Holter method - average diurnal FEV1 variability over 14 days - establishing an optimal threshold for obstruction detection - >8%. Nevertheless, compared to both diurnal PEF variability cut-off values, 10 and 16%, it was less effective in obstruction detection and requires further validation in population studies to be used in clinical practice.
The bronchodilation test in the Yurdakul study achieved only 57% accuracy but was characterised by the highest specificity of all tests (95%). Unfortunately, a direct comparison was impossible because the bronchodilation test was not a part of our study.
Another airflow variability measure described in the literature is excessive variation in lung function between two visits. Although this study's results align with the GINA statement and literature regarding the limited usefulness of excessive variation in lung function between two visits, in this study, 100% of patients who performed Spirometry Holter met this criterion, with a very high false positive rate. Unlike Dean et al., who reported good specificity with only 17% sensitivity [28]. Differences in calculation methods may explain possible bias. We compared maximal and minimal FEV1 from 30 days using our method, and there was no actual difference between the two visits; nevertheless, our approach covers a wider range of spirometry results. Therefore, the current cut-off value for this criterion may need to be further investigated and used cautiously in clinical practice.
This study has several limitations. First, it was a non-controlled, observational study on a relatively small population of patients designed as a feasibility study of a new innovative method. Regarding the lack of healthy control subjects, it was impossible to reliably analyse a method's diagnostic accuracy and directly compare it to other methods proposed by GINA. Secondly, the average age in the investigated population was 37, and the exclusion criteria included the patient's inability to use a digital spirometer with a mobile app. Although digital exclusion may be a substantial limitation of methods utilisation in elderly patients, it is worth noticing that asthma is diagnosed mainly in middle age, with an average of 50 in men and 38 in women [29].
Despite the described limitations, we see our results as highly valuable and, if confirmed, may contribute to developing a new diagnostic approach for this particular group of patients.
Further research should investigate the clinical value of the method, including healthy subjects, to be compared to other measures mentioned by GINA. Additionally, comparisons should include time to asthma diagnosis and the cost-effectiveness of different approaches in a randomized and multicenter manner. A non-inferiority study with the MCT may be desirable regarding the new method's potential place in the asthma diagnostic pathway. Spirometry Holter is a novel tool for monitoring airway limitation variability that is feasible to implement in outpatient clinic settings.
It may be useful in objectively confirming asthma diagnosis in naive to-treatment patients without prior documented obstruction. Further studies are needed to compare its diagnostic accuracy and cost-effectiveness to other methods mentioned in the guidelines.