CoMiSS was determined repetitively in 122 no-CMA infants during the first year of life and showed an inverse relation to age: median 7 for both the original and updated CoMiSS at 1.5 months, 2 original at 12 months, and 0 updated CoMiSS, respectively. Original and updated CoMiSS do not differ in infants younger than 6 months (p = 0.586 at 3 months and 0.999 at 1.5 and 4 months). In comparison to data obtained from previously presumed healthy infants aged up to 6 months [7, 8] (Table 5), our observed median values are higher due to loose and watery stools, which were predominant until the introduction of solids, leading to increased stool consistency and decreased CoMiSS. This finding is similar to Bigorajska's observation[8]. This is the first study using a longitudinal repeated assessment of CoMiSS in no-CMA infants. Previous studies in presumed healthy infants applied a single assessment mode [7,8,9]. Each age group included different numbers of infants (Table 5). Only one study (focused initially on interrater and day-to-day variability) reported CM challenge in presumed healthy infants with CoMiSS values ≥ 10 [10].

The decrease in CoMiSS was more pronounced with the updated CoMiSS (− 0.64 per month) than with the original CoMiSS (dropped by 0.5 per month). Age and type of feeding, such as the introduction of solids, impact the evolution of CoMiSS in this cohort.

The effect of age on CoMiSS values in presumed healthy infants was analysed in three studies [7,8,9]. The European study (data from four countries, infants up to 6 months) detected a trend towards differences across ages, with higher scores in the 1–2 months and 3–4 months age groups (p < 0.01). Also, in the study carried out in Poland with identical enrolment parameters [8], age had an impact on the total CoMiSS (p < 0.001). However, the number of infants included in some age groups was small (15 subjects), complicating the comparison of outcomes. A study in 609 European infants aged 6 to 12 months showed higher total CoMiSS in infants aged 6 months compared to 10 months (p = 0.001). Furthermore, the highest median CoMiSS was observed in 6-month-old infants, while the lowest median value was observed at 12 months [9]. These studies used a single assessment model — the number of included subjects and assessments was identical. The comparison of age-appropriate CoMiSS values, including our data, is summarised in Table 5.

The type of feeding did not impact the CoMiSS in the European cohort study (p = 0.43) [7]. However, the type of feeding (exclusive breastfeeding, formula, mixed feeding) influenced CoMiSS (p < 0.001) in the Polish study [8]: the median (Q1–Q3) CoMiSS showed a trend to be higher, with more prevalent loose stools in breastfed than in formula-fed infants (median (IQR) 4.0 (2–7) vs 3.0 (1–4), respectively). No difference was found regarding the type of feeding in Polish infants aged 6–12 months [9].

Original and updated CoMiSS observed in this study does not differ significantly in infants under 6 months of age, although there was a decreasing trend with a median (IQR) of 7.0 (5–8) at 1.5 months dropping to 5 (4–7) at 6 months. This finding aligns with the study comparing the CoMiSS, including the Bristol Stool Form Scale (BSFS) and the CoMiSS values obtained by substituting the stool scale with the Brussels Infants and Toddlers Stool Scale (BITSS) [11], reporting a difference in CoMiSS-BSFS and CoMiSS-BITSS for values ≤ 5 (p < 0.001), but values ≥ 6 remained unchanged (p = 0.81) [11].

This is a single-centre study in Czech infants. Our outcomes may differ from those of other countries, as there might be a difference between CoMiSS in different countries [7]. We cannot rule out that the natural evolution of CoMiSS values may also differ in relation to the region.

The limitation of this study is that it is a single-centre observation and has an uneven distribution of feeding types, as there is a low ratio of non-breastfed infants (4.1%). In 2021, 3452 out of 103.068 with birthweight ≥ 2500 g were not breastfed (3.4%) [12]. The eight infants suspected to have developed CMA were treated with a CM-free diagnostic elimination. Unfortunately, four parents declined the CM oral food challenge because “that would make the child sick again”. The infants who were put on dietary restrictions by the parents were excluded because we could not confirm that these infants were no-CMA.