Once a patient is frail, comorbidity counts stop predicting in-hospital decline

Design

Retrospective multicentre cohort

Population

1,299 adults ≥65 admitted to acute hospitals in Japan

Primary outcome

Hospital-associated disability (Barthel Index fall ≥5)

Effect

Charlson OR 1.44 (95% CI 1.20–1.73) non-frail; 0.98 (0.90–1.06) frail

This retrospective multicentre cohort studied 1,299 adults aged 65 or over admitted to acute hospitals in Japan between 2019 and 2025. It asked whether the Charlson Comorbidity Index predicted hospital-associated disability — a fall of 5 or more points in the Barthel Index during the stay — and whether that depended on frailty measured with the Clinical Frailty Scale.

In non-frail patients (scale 3 or less), each point of comorbidity raised the odds of hospital-associated disability (OR 1.44, 95% CI 1.20 to 1.73). In mildly frail patients the link weakened (OR 1.11) and in frail patients it disappeared (OR 0.98). The interaction was strongly significant.

In practice, a comorbidity count is a reasonable risk signal for a robust older patient, but in frail patients it adds little: they are at high risk of functional decline regardless of how many diagnoses they carry. That supports recording a frailty score on admission and starting mobility, nutrition and delirium prevention in frail patients without waiting for a comorbidity tally to justify it.

• Record a Clinical Frailty Scale score for every older patient on admission.
• Start mobilisation, nutrition and delirium prevention early in frail patients, whatever their comorbidity count.
• Use comorbidity burden as a risk marker mainly in non-frail older patients.
• Measure function (for example the Barthel Index) on admission and before discharge to detect hospital-associated decline.

Why it matters

It shows that adding up diagnoses misses the patients at greatest risk of leaving hospital less able.

Don't overread it

This is retrospective and Japanese; it shows changing association, not that targeting frailty prevents disability.

The statistics, in plain English

An odds ratio of 1.44 per Charlson point in non-frail patients means each extra point raised the odds of losing function by about 44%. In frail patients the ratio of 0.98 means no measurable link. A significant interaction means the difference between these groups is unlikely to be chance.

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