When Numbers Start Making Decisions · Season Three, “The People the Average Does Not See” · Article 3
1. What changes when a calculated ratio crosses 35?
Two people can have similar metabolic disease, mobility limits and histories of unsuccessful treatment. One has a body mass index of 34.9 and the other 35.1. In some referral pathways, that small numerical difference helps determine whether a person can enter assessment for publicly funded bariatric surgery.
Queensland’s Statewide Bariatric Surgery Service illustrates the structure. Its published eligibility includes a BMI above 35 alongside other conditions: applicants must be aged 18 to 65, weigh less than 185 kilograms, have type 2 diabetes treated with at least two diabetes medicines, have an HbA1c above 6.5 per cent, have stopped smoking for at least six months and be referred by an appropriate specialist service. Queensland Health: Statewide Bariatric Surgery Service
BMI is not acting alone, and crossing 35 does not guarantee surgery. It opens a route to multidisciplinary assessment within a resource-constrained program. Yet for the person on the other side, the distinction can still be decisive.
The philosophical issue is not whether thresholds are unavoidable. It is whether a population-level ratio has enough information to control entry into an individual clinical process, and what safeguards should surround it.
2. BMI is a useful population measure with limited individual resolution
BMI divides weight in kilograms by height in metres squared. It is inexpensive, repeatable and widely understood. At population level, higher BMI categories are associated with increased prevalence of diabetes, cardiovascular disease and other health problems. This makes BMI useful for surveillance, broad risk communication and initial screening.
But the formula does not directly measure body fat, its distribution, muscle, metabolic health or the reason for a person’s weight. The Australian Department of Health notes that BMI is only one way of assessing whether weight may affect health and that waist measurement and clinical context also matter. Department of Health: BMI and waist circumference
People with the same BMI can have different amounts of visceral fat, muscle and organ risk. Age, sex, ancestry, disability, medication and previous illness affect interpretation. A muscular person and a person with substantial central adiposity may share the ratio. Conversely, someone below a threshold can carry serious metabolic risk.
BMI’s strength—reducing height and weight to one comparable number—is also its blindness. It estimates a dimension of body size that correlates with risk across groups. It does not contain the complete clinical judgement about a particular person.
3. Why a public program uses a threshold
Bariatric surgery requires operating capacity, specialist staff, preoperative assessment and long-term follow-up. Public supply is limited. A statewide program needs rules that identify people likely to experience substantial benefit and for whom other treatment has not been enough.
The threshold serves several administrative purposes. It creates a consistent referral rule, reduces arbitrary variation, makes demand more predictable and directs scarce assessment capacity towards a population with elevated risk. Combined with diabetes control, medication, smoking and other requirements, it defines the program’s intended clinical group.
The number 35 is therefore neither a natural law nor a stand-alone diagnosis. It is a policy-clinical boundary embedded in a larger pathway. Its defensibility depends on that pathway: whether the other criteria are relevant, whether the assessment is genuinely individual, whether exceptions exist and whether the evidence is reviewed as treatment and population knowledge change.
Calling the cut-off “evidence based” should not end the explanation. Evidence can support higher average benefit above a range without proving that every person at 35.1 benefits more than every person at 34.9. The continuous relationship has been converted into a discrete administrative gate.
4. Measurement error becomes important near the line
Height and weight look objective, but their measurement still varies. Clothing, time of day, fluid retention, equipment calibration and posture can shift weight or height enough to move a calculated BMI around a boundary. Rounding can turn a continuous value into an apparently exact category.
Near 35, a responsible process should specify how measurements are taken, whether values are repeated and which date controls. A single self-reported or rounded result should not silently bear a major consequence. If the purpose is clinical triage rather than a one-time statistical report, reliable measurement is part of procedural fairness.
Even a perfectly measured BMI may not validly represent the intended clinical condition. The question is not only “is this number correct?” but “does it provide the evidence this decision requires?” Measurement accuracy and decision relevance are different.
The larger the consequence and the smaller the numerical distance across the boundary, the greater the case for corroboration. Waist, metabolic markers, functional impairment, comorbidities, treatment history and specialist judgement can supply information that BMI omits.
5. Assessment should be opened by the screen, not replaced by it
A screening rule is most defensible when it identifies who should receive closer evaluation. It becomes more troubling when it prevents evaluation despite substantial evidence that the person falls within the program’s purpose.
Queensland’s pathway already recognises that surgery is not decided by BMI. Eligibility leads to assessment. Clinical teams then consider medical, nutritional, psychological and surgical factors, capacity for follow-up and likely benefits and harms. The published implementation standard describes a coordinated model rather than an automatic operation. Queensland Health: Statewide Bariatric Surgery Service Implementation Standard
The asymmetry is important. A false positive at screening consumes assessment resources but can be corrected before surgery. A false negative may prevent a person from reaching the clinicians able to conduct that correction. When a proxy controls the gateway, exclusion error deserves particular attention.
That does not mean every person below 35 must enter the same statewide service. It means systems should explain whether alternative pathways, local specialist assessment or exceptional review are available when clinical need is not represented by the ratio.
6. Different referral routes need visible differences
Statewide criteria, hospital referral criteria and individual clinical decisions may not be identical. A local service can use “BMI greater than or equal to 35” while another document says “greater than 35”, or combine BMI with different comorbidities because it serves a different population. Metro North Health: Assessment for metabolic surgery suitability
Such variation is not necessarily inconsistency. Programs can have different purposes and capacity. It becomes unfair when patients and referrers cannot tell which rule applies, or when an apparent technical wording difference decides access without explanation.
Documents should identify the service, effective date, comparator, measurement method and route for advice. Referrers should not have to infer whether 35.0 qualifies from casual language. Where local capacity drives a stricter boundary, that resource choice should not be disguised as a universal medical fact.
Transparency also allows evaluation. If many clinically serious cases cluster just below the threshold, the program should know what happens to them. If people above it rarely proceed after assessment, the screen may be too broad or the rest of the criteria misaligned.
7. Weight stigma makes careful use more important
BMI categories are often interpreted as identities rather than measurements. People living with obesity can encounter blame, dismissal and assumptions about behaviour. A threshold can intensify that stigma if staff treat the number as a moral verdict or a complete explanation of health.
Respectful practice distinguishes the person from the category, explains why the measure is used and acknowledges its limits. Requirements such as smoking cessation or diabetes treatment should be connected to surgical safety and expected benefit, not presented as tests of deservingness.
The Department of Health describes overweight and obesity as complex conditions influenced by biological, social and environmental factors. Department of Health: About overweight and obesity A fair service should therefore avoid turning a simplified risk marker into a simplified account of responsibility.
8. A practical test for BMI thresholds
When BMI affects entry to care, ask:
- Is BMI being used for surveillance, screening, diagnosis or final treatment selection?
- What evidence supports this threshold for this program and population?
- How are height and weight measured, repeated and rounded?
- Which clinical factors are considered alongside the ratio?
- What happens to people immediately below the line?
- Can exceptional evidence obtain review by a qualified clinician?
- Are alternative care pathways clear?
- Are outcomes and exclusions audited across age, sex, ancestry and disability?
The goal is not to abolish a workable gateway. It is to prevent a gateway from becoming a substitute for judgement.
Conclusion: 35 can organise access but cannot contain the clinical case
BMI offers public systems a low-cost, consistent signal associated with health risk and probable treatment benefit. In a constrained statewide service, a threshold can make allocation more predictable and less arbitrary than informal discretion.
But the difference between 34.9 and 35.1 is much smaller than the difference in administrative consequence. My judgement is that BMI 35 is legitimate as one component of a transparent screening rule, provided measurement is reliable, other clinical evidence remains decisive, routes differ visibly and exclusion near the line can be examined.
A ratio may open an assessment. It should never pretend that it has already assessed the person.
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