Beyond Bristol: The BTISS Scale for Evaluating Infant Stool

Lead

"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 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 significant 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.

When Bekkali and colleagues developed the Amsterdam Infant Stool Scale, their work demonstrated that applying the Bristol scale to non-toilet-trained infants yields poor interobserver agreement — caregivers and clinicians looking at the same diaper reach different conclusions, in part because the tool they are using was not built for that context [2].

The BTISS: Design and Validation

The Brussels Infant and Toddler Stool Scale was developed by Huysentruyt, Vandenplas, and colleagues at Ghent University in Belgium to address this gap directly [3,4].

The validation study published by Huysentruyt and colleagues in the Journal of Pediatric Gastroenterology and Nutrition in 2019 tested interobserver reliability across 18 countries and 18 centers, with 2,462 participants — 1,181 parents, 624 nurses, and 657 physicians [3]. Participants rated seven photographs of diapers containing infant and toddler stool. Overall agreement rates ranged from 83 to 96%, and the weighted kappa was 0.72 (95% CI: 0.59–0.85), indicating good reliability across rater groups [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 evaluates infant stool along three axes.

1. Consistency: four levels — watery, soft, normal, hard
2. Amount: four levels — small, moderate, large, very large
3. Color: six categories — yellow, brown, green, black, red, white

The color categories map directly onto the clinical distinctions discussed 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 BTISS-based research is that the normal range for infant stool consistency differs substantially by feeding type.

• Breastfed infants: stool is generally soft to watery in consistency. A low BTISS consistency score (toward the soft end) is normal and expected. Bowel movement frequency is also higher.
• Formula-fed infants: stool tends to be firmer than in breastfed infants, with a higher BTISS consistency score. Frequency is typically lower.
• 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.

Research on exclusively breastfed infant stool confirms the degree of normal variation [5].

A large comparative study published in Neurogastroenterology and Motility demonstrated that BTISS is more sensitive than Bristol for detecting hard stool and functional constipation in non-toilet-trained children — BTISS identified 57.4% of cases compared with 25.3% for the Bristol scale [4]. Put plainly, using the wrong scale means missing constipation more than half the time.

Note on reference [4]: The Japanese source listed Teixeira-Cintra MA, Huysentruyt K, Vandenplas Y et al. as authors for PMID 33094889. Citation verification indicates this PMID corresponds to a study by Velasco-Benitez CA, Llanos-Chea A, and Saps M — different authors, though the finding described (BTISS sensitivity advantage over Bristol for functional constipation in non-toilet-trained children) appears consistent with that publication. This discrepancy is flagged for editorial resolution.

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.

BTISS-based data make the normal range explicit and nutrition-type-specific. Applying Bristol's Type 7 ("watery stool = severe 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 infants and toddlers incorporate frequency, consistency, and defecation difficulty — but 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 Ghent University and validated across 18 countries and more than 2,400 raters — addresses this gap directly. Its photograph-based, diaper-context format produces 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

1. Lewis SJ, Heaton KW. Stool form scale as a useful guide to intestinal transit time. Scand J Gastroenterol. 1997;32(9):920–924. doi:10.3109/00365529709011203. PMID: 9299672.
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. Teixeira-Cintra MA, Huysentruyt K, Vandenplas Y, et al. 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(4):e14015. doi:10.1111/nmo.14015. PMID: 33094889. [[unverified — author discrepancy]: Verification found PMID 33094889 corresponds to Velasco-Benitez CA, Llanos-Chea A, and Saps M, not Teixeira-Cintra MA et al. as listed. The finding described is consistent with the published abstract. Editor should confirm correct authorship and citation.]
5. Gustin J, Smolkin ME, Grollman K, Grossman NJ. Characterizing Exclusively Breastfed Infant Stool via a Novel Infant Stool Scale. JPEN J Parenter Enteral Nutr. 2019;43(5):673–680. doi:10.1002/jpen.1468. PMID: 30370924.
6. O'Donnell LJD, Virjee J, Heaton KW. Detection of pseudodiarrhoea by simple clinical assessment of intestinal transit rate. BMJ. 1990;300(6722):439–440. doi:10.1136/bmj.300.6722.439.