Beyond Bristol: The BTISS Scale for Evaluating Infant Stool

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"Is this diarrhea, or is it normal?" "The stool seems hard — does that count as constipation?" Parents of infants routinely make these calls without any reliable reference point. Most are relying on instinct and a vague sense of what stool "should" look like.

Clinicians use standardized scales to assess stool consistency. The most widely known is the Bristol Stool Form Scale (BSFS), familiar to many adults from gastroenterology consultations. But this scale has a fundamental limitation: it was not designed for infants. Applying it to a diaper produces systematically misleading results.

This article introduces the Brussels Infant and Toddler Stool Scale (BTISS), a scale developed specifically to assess stool consistency in diapered children — and explains why having the right measuring instrument changes what you can actually conclude about your infant's gut.

The Limits of the Bristol Stool Scale

Lewis and Heaton published the Bristol Stool Form Scale in 1997, classifying adult stool into seven types ranging from Type 1 (hard, separate lumps) to Type 7 (entirely liquid), with photographs of each form [1]. The same Bristol group had earlier reported, in 1990, using stool form as a simple bedside estimate of intestinal transit rate [6]. The scale demonstrated that stool consistency correlates strongly with colonic transit time, and it remains widely used in clinical research and practice.

The problem, when applied to infants, is structural.

• A breastfed newborn's stool — characteristically yellow, watery to pasty in consistency — is entirely normal, but applying Bristol criteria places it squarely at Type 6 or 7, which in adults would indicate diarrhea.
• Formula-fed infants tend toward firmer stool than breastfed infants, but their normal range is also not captured by the Bristol scale.
• After the introduction of complementary foods, the diversity of texture and consistency during the transition period falls outside what Bristol was designed to describe.
• Critically, stool spread across a diaper simply looks different from stool deposited into a toilet. Comparing the two as if they were equivalent introduces a visual assessment error that no amount of careful observation can correct.

The Bristol scale has been described as inadequate for non-toilet-trained children [3]. An earlier infant-specific tool came from Amsterdam: in 2009, Bekkali and colleagues developed the Amsterdam Infant Stool Scale, which describes infant stool by consistency (4 points), amount (4 points), and color (6 categories) [2].

The BTISS: Design and Validation

The Brussels Infant and Toddler Stool Scale was developed by Huysentruyt, Vandenplas, and colleagues at UZ Brussel (Vrije Universiteit Brussel) in Belgium to address this gap directly [3].

The validation study published by Huysentruyt and colleagues in the Journal of Pediatric Gastroenterology and Nutrition in 2019 tested interobserver reliability across 18 centers in Europe, Asia, and the Americas, with 2,462 participants — 1,181 parents, 624 nurses, and 657 physicians [3]. Participants rated seven photographs of diapers containing infant and toddler stool against the Bristol types. The weighted kappa was 0.72 (95% CI: 0.59–0.85), indicating good reliability across rater groups. When the photographs were then grouped into hard, formed, loose, and watery stools, correct allocation for each photograph ranged from 83 to 96% (average 90%) [3].

The scale is deliberately photograph-based: participants assess visual representations of stool in diapers, which matches the actual observation context of parents and caregivers.

What BTISS Measures

BTISS is simple: it consists of seven photographs of stool in diapers. After the validation study, the seven photographs were organized into four consistency groups [3].

1. Hard: three photographs, corresponding to Bristol types 1–3
2. Formed: one photograph, corresponding to Bristol type 4
3. Loose: two photographs, corresponding to Bristol types 5–6
4. Watery: one photograph, corresponding to Bristol type 7

BTISS assesses consistency only. When amount and color also need to be recorded, the earlier Amsterdam Infant Stool Scale (consistency, amount, color) has been used [2]; what the colors mean is covered in the preceding article in this series ("Why Your Baby's Stool Changes Color"). Consistency is assessed visually from photographs showing stool as it appears in a diaper, not as it would look in a toilet.

This design reflects a simple observation: the assessment context for infant stool is the diaper, not the toilet bowl. Building the scale around that reality is what allows it to produce reliable results.

Normal Ranges Vary by Feeding Type

One of the most important findings from research on infant bowel habits is that the normal range for infant stool differs substantially by feeding type. In a study of bowel diaries from 600 healthy Dutch infants, breastfed infants in the first three months had softer, more frequent, and more often yellow stools than formula-fed infants [7].

• Breastfed infants: stool is generally soft to watery in consistency. Landing toward the soft end of a scale is normal and expected. Bowel movement frequency is also higher [7].
• Formula-fed infants: stool tends to be firmer than in breastfed infants, landing toward the firmer end of a scale. Frequency is typically lower [7].
• After introduction of complementary foods: dietary fiber and solid foods shift consistency, and formed stool becomes more common. Color diversifies according to what has been eaten.

Gustin and colleagues argued that exclusively breastfed infant stool covers such a wide range of consistencies that even the Amsterdam scale does not capture it well, and proposed a new scale [5].

BTISS has also been compared with Bristol in a clinical setting. In a study by Velasco-Benitez and colleagues, published in Neurogastroenterology and Motility in 2021, parents of 666 non-toilet-trained children in Colombia rated stool with both scales. Among children with functional constipation, BTISS identified hard stools in 57.4% compared with 25.3% for the Bristol scale [4]. The authors concluded that BTISS seems more sensitive for detecting hard stools and functional constipation, while noting that more studies are needed before it replaces Bristol in this age group [4].

Parent-Reported Assessment and Clinical Agreement

A key feature of BTISS is that it was designed for use by non-specialists. Because the scale is photograph-based and depicts stool as it actually appears in diapers, agreement between parent ratings and clinician ratings is relatively high [3].

In practice, if you photograph a diaper at each change, you can walk into a pediatric appointment and show the doctor what has been happening over the past week rather than describing it in words. Language-based descriptions of stool — "soft," "sticky," "runny" — vary enormously between observers; photographs are comparably more objective.

This is the connection between BTISS as a clinical instrument and the everyday habit of keeping a feeding and diaper log. A stool-color and consistency entry in a parenting app such as Memori, recorded at the time of each diaper change, produces exactly the kind of longitudinal data that allows a meaningful clinical conversation. The BTISS framework provides the vocabulary; the log provides the record.

Dismantling "Soft Stool = Abnormal"

It is not unusual for parents of breastfed infants to visit a pediatrician with concerns about diarrhea, only to learn that what they observed was within normal limits. But without a tool calibrated to the infant context, there is no principled way to reach that conclusion — the parent's concern is genuine, and "don't worry" is not an explanation.

Feeding-specific observational data show that breastfed infants normally have softer, more frequent stools than formula-fed infants [7]. Applying Bristol's Type 7 ("watery stool = diarrhea") to a two-week-old breastfed infant is structurally incorrect. BTISS was built to prevent exactly that misclassification.

The same logic applies to constipation assessment. Rome IV criteria for functional constipation in children under 4 require at least two of the following for one month: two or fewer bowel movements per week, excessive stool retention, painful or hard bowel movements, large-diameter stools, or a large fecal mass in the rectum [8]. Hard stool is part of the diagnosis, yet Rome IV recommends the Bristol scale for rating consistency, which has been questioned for children in diapers [4]. Consistency assessment requires a scale designed for the population being evaluated.

Summary

The Bristol Stool Form Scale was developed for adults and has a structural mismatch with the assessment needs of non-toilet-trained infants. BTISS — developed at UZ Brussel and validated across 18 centers and 2,462 raters — addresses this gap directly. Its seven diaper photographs, grouped into four consistency categories, produce reliable results across parent, nurse, and physician raters [3].

A parent's observation that their infant's stool is "soft" can only be interpreted as normal or abnormal with reference to a scale that specifies what normal looks like for that infant's feeding type and age. Having that tool available does not replace a pediatric consultation; it provides the language that makes the consultation more productive.

References

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2. Bekkali N, Hamers SL, Reitsma JB, Van Toledo L, Benninga MA. Infant stool form scale: development and results. J Pediatr. 2009;154(4):521–526.e1. doi:10.1016/j.jpeds.2008.10.010. PMID: 19054528.
3. Huysentruyt K, Koppen I, Benninga M, et al; BITSS Study Group. The Brussels Infant and Toddler Stool Scale: A Study on Interobserver Reliability. J Pediatr Gastroenterol Nutr. 2019;68(2):207–213. doi:10.1097/MPG.0000000000002153. PMID: 30672767.
4. Velasco-Benitez CA, Llanos-Chea A, Saps M. Utility of the Brussels Infant and Toddler Stool Scale (BITSS) and Bristol Stool Scale in non-toilet-trained children: A large comparative study. Neurogastroenterol Motil. 2021;33(8):e14015. doi:10.1111/nmo.14015. PMID: 33094889.
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