The cleanest recent count: an adverse event in 23.6 percent of hospital admissions, one in four of them preventable
2,809 random admissions to 11 Massachusetts hospitals in 2018, every record reviewed. At least one adverse event in 23.6 percent; 22.7 percent of events preventable. Seven deaths in the sample, one judged preventable.
Reasonable evidence with real limitations. how we score evidence
Caveat on this rating: Eleven hospitals in one state in one year, funded by the malpractice insurer of the hospitals studied. Preventability and severity are reviewer judgements made from the record after the fact. The other direction: this is a count, not an extrapolation, and its death finding (one preventable death in 2,809 admissions) is far closer to the low end of the published range than to the 251,454 headline, while its harm finding (nearly one admission in four) is far higher than what hospital reporting systems capture.
The best current answer to "how often does hospital care hurt people" is the 2023 SafeCare study, because it counted rather than extrapolated.
Significant findings
A random sample of 2,809 admissions to 11 Massachusetts hospitals in 2018. Trained nurses screened each record with a trigger method and physicians adjudicated every flagged event. "We identified at least one adverse event in 23.6%. Among 978 adverse events, 222 (22.7%) were judged to be preventable and 316 (32.3%) had a severity level of serious" or higher. "A preventable adverse event occurred in 191 (6.8%) of all admissions, and a preventable adverse event with a severity level of serious or higher occurred in 29 (1.0%)."
Drugs were the most common category. "Adverse drug events were the most common adverse events (accounting for 39.0% of all events), followed by surgical or other procedural events (30.4%), patient-care events" such as falls and pressure injuries "(15.0%), and health care-associated infections (11.9%)."
And the deaths: "There were seven deaths, one of which was deemed to be preventable." That is one preventable death in 2,809 admissions. Scale that the way the big estimates are scaled and you land nearer the critics' 25,000 than the 251,454.
What this does not show
It is one state and one year. The study was paid for by the malpractice insurer of the Harvard hospitals, which is worth knowing and does not change the arithmetic. "Preventable" is a reviewer's judgement after the fact. The hospital's own count is not the count: when three hospitals had the same patients checked three ways, the voluntary incident reports and the government's patient safety indicators "missed 90 percent of the adverse events" that a systematic chart review found. So 23.6 percent is what careful review finds, not what hospitals report.
Worth asking
If nearly one admission in four involves a harm from care, and drugs are the most common kind, which of my medicines is being checked against the others on this stay?
Source
The safety of inpatient health care — Bates DW, Levine DM, Salmasian H, et al. (2023)
Top-tier peer-reviewed journal
Read the source: https://doi.org/10.1056/NEJMsa2206117
DOI: 10.1056/NEJMsa2206117
How this was scored
- Study design
- Cohort study
- Funding
- Independently funded
- Published in
- Top-tier peer-reviewed journal
- Sample size
- 2,809
- Preregistered
- not recorded
- Conflicts disclosed
- not recorded
- Independent of proponent
- not recorded
- Retracted
- No
Read the full scoring rubric, including what it can't tell you.
Published September 29, 2026.
Questions
How strong is the evidence behind this?
veisund rates this source "moderate evidence". Reasonable evidence with real limitations. It scores 68 out of 100 on our published rubric. One caveat travels with that badge: Eleven hospitals in one state in one year, funded by the malpractice insurer of the hospitals studied. Preventability and severity are reviewer judgements made from the record after the fact. The other direction: this is a count, not an extrapolation, and its death finding (one preventable death in 2,809 admissions) is far closer to the low end of the published range than to the 251,454 headline, while its harm finding (nearly one admission in four) is far higher than what hospital reporting systems capture. The score is calculated from recorded facts about the source — study design, funding, publication venue, sample size, preregistration — not typed in by an editor.
What is the source for this?
The safety of inpatient health care — Bates DW, Levine DM, Salmasian H, et al. (2023). Published in: Top-tier peer-reviewed journal. DOI: 10.1056/NEJMsa2206117. The full source is linked on this page so you can read it yourself.
Who paid for this research, and does that matter?
Study design: Cohort study. Funding: Independently funded. Independence from the proponent is not recorded. Industry sponsorship is one of the most reliably measured biases in medicine, which is why funding carries real weight in the score rather than sitting in a footnote.
Is this medical advice?
This is information to bring to your prescriber, not medical advice and not a reason to change anything on your own. Nothing here is an instruction to stop or reduce a medication. If you are in crisis, call or text 988.
This is information to bring to your prescriber, not medical advice and not a reason to change anything on your own. Nothing here is an instruction to stop or reduce a medication. If you are in crisis, call or text 988.