Hospitals in Scotland could save £8m a year if more patients were sent home the same day as their operation, official auditors said today.

A review found the number of procedures performed as day cases varied widely between Scotland's health boards - and the differences could not be explained by patients' circumstances or location.

Patients in Scotland are also more likely to be kept on wards overnight than their counterparts in England, according to the Audit Scotland report.

Opposition politicians called for an urgent investigation into why some health boards had such low rates of day surgery.

Sending people home the same day as their procedure is estimated to save £237 per person and, according to Audit Scotland, it can also be in their best interests.

Robert Black, auditor general for Scotland, said: "It's long been recognised that day surgery benefits patients through promoting a speedy recovery and minimising the disruption that an overnight stay in hospital can have on people's lives. It also promotes a more efficient use of NHS resources by reducing waiting times and freeing up hospital beds."

When his researchers compared how well health boards were performing with day-turn-around targets for 19 benchmarking surgical procedures, such as varicose vein and cataract removal, they found huge disparities.

While NHS Fife achieved the targets for 83% of the procedures in 2006-07, NHS Tayside hit the goals in just 29% and NHS Greater Glasgow and Clyde in 39%.

Nick Hex, health portfolio manager for Audit Scotland, said individual boards had room for improvement: "Some of the best performers are continuing to raise their performance, such as NHS Fife on varicose veins, whereas, if you look at NHS Forth Valley, they are at a very low base and are going down. There is scope for local level improvement."

Previous studies have found increasing in-a-day treatment was hindered by the poor management and organisation of day surgery.

Professor Brian Williams, president of the Royal College of Physicians and Surgeons of Glasgow, said staff had embraced the shift from inpatient treatment to more outpatient procedures.

Audit Scotland found rates had improved. In 2006-07, NHS Scotland met targets for 10 of the 19 bench marking procedures compared with seven in 2002-03.

The former Scottish Executive gave the health service a goal for one-quarter of all planned surgery to involve an overnight stay in hospital.

Audit Scotland estimates 34,000 cases where patients are detained in hospital would have to convert to day cases to hit this target.

A spokeswoman for NHS Greater Glasgow and Clyde said: "We recognise the benefits of day surgery and have increased the number and range of procedures carried out across Greater Glasgow and Clyde over the last 18 months. Our day surgery rates are set to increase further following the opening of the new Stobhill and Victoria hospitals in 2009."

Health Secretary Nicola Sturgeon said: "The current target for day surgery is for 83% of 160 types of surgical procedure to be carried out as day surgery by March 2011. Current performance is 68% and we are on track to deliver."