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The number of patients in England being readmitted to hospital within 30 days of discharge has risen by nearly a quarter in the last four years, a watchdog has said.

Figures compiled by Healthwatch suggest one in five of these potentially distressing readmissions happen 48 hours after being sent home.

The watchdog warns patients may be being rushed out early.

NHS England says it is impossible to know the reasons for the readmissions.

An emergency readmission occurs when a patient needs to go back into hospital unexpectedly for further treatment, within 30 days of having been discharged.

Readmissions in such a short space of time raise concerns about patients being discharged unsafely, and about the pressure on hospitals to free up beds.

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Healthwatch was prompted to look at the issue as none of the official national NHS bodies collects data on readmissions.

It asked hospital trusts to provide information under the Freedom of Information Act.

There were some differences in the way the data was collected, but it received comparable data from 72 hospital trusts, just over half the total in England.

The results showed:

In 2016/17 there were 529,318 emergency readmissions reported by 84 hospital trusts

The number of emergency readmissions had risen by 23% over the past four years to 457,880

One in five of the readmissions were with 48 hours of discharge

There was a 29% rise in people being readmitted within 24 hours

The rise in the readmission rate outstrips the overall rise in hospital admissions, which stands at 9% over the period.

In total, there were 16.5 million admissions last year, suggesting that close to one in 20 patients in hospital may be a readmission.

Healthwatch speculated there could be a number of reasons for this, including patients being rushed out too soon or when there is insufficient support in the community to care for them.

Hospitals are under pressure to bring down the number of delays in freeing up hospital beds because of rising waiting lists for operations and increasing delays in A&E.

Imelda Redmond from Healthwatch England said the analysis "raises some big questions":

"With health and care services being expected to each free up 1,250 beds through reducing delayed discharges, it is vital that these services also watch what is happening to the people being sent home from hospital.

"It's clear that health and care leaders need to take a closer look at why people are returning to hospital so quickly."

But an NHS England spokesperson said: "Current data makes it impossible to distinguish between genuinely unforeseeable emergency readmissions and readmissions that would not be surprising to clinicians.

"We agree that it would be useful for NHS Digital to look these data more routinely - and will be asking them to consider doing so."

Nigel Edwards of Nuffield Trust said it's a sign of the pressure the NHS is under, and called for improved national data to better analyse emergency readmissions.