Patients admitted in an emergency to hospital at weekends are older and more disabled than those admitted during the week, new research suggests.
The study could add to the row between health workers and the Government over the use of data suggesting people are more likely to die at the weekend in NHS hospitals.
Previous studies have found that patients admitted at weekends are older and more sick, although some have shown that death rates are still higher at weekends despite this.
The new study, of a major hospital in Belfast and published in Emergency Medicine Journal, examined data for 536 patients.
Researchers looked at factors affecting emergency admission between 5pm on a Friday and 9am on a Monday.
They found that patients admitted at the weekend as medical emergencies were significantly older than those admitted at other times of the week.
They were typically aged 68 compared with 65 during the week.
They also ranked higher on scores of disability and were more "functionally dependent" than those admitted during a weekday.
The research was led by Dr Paul Hamilton from the Royal Victoria Hospital in Belfast.
The team said: "These findings illustrate major differences in the age and functional dependence of patients admitted to hospital at weekends.
"This may fully or partially explain the increased mortality that has been publicised."
The experts also questioned whether increasing the numbers of staff at the weekend would make any difference to the survival of patients.
In November, a study published in the British Medical Journal (BMJ), found that babies born in NHS hospitals at weekends are more likely to be stillborn or die in the first week of life than those delivered Monday to Friday.
A different study in the BMJ in September showed that around 11,000 more people die every year within 30 days of admission to hospital on Friday, Saturday, Sunday or Monday compared with other days of the week.
Researchers behind that study said a lack of consultant cover and "reduced" services could be contributing to higher death rates.
But they argued it would be "rash and misleading" to conclude that an exact number of deaths could have been avoided.
Health Secretary Jeremy Hunt has used the figures as part of his drive to create a seven-day NHS.