Public trust in NHS waiting times has been put at risk by falsified figures and must be restored by the Scottish Government and health boards, according to a critical report.
Auditors found problems across the country in the way patient waiting times targets are approached by health boards, more than a year after the problem was identified in NHS Lothian.
An investigation by Audit Scotland, which scrutinises public spending, said its work was hampered by a lack of information and because of inadequately controlled management systems.
Caroline Gardner, auditor general for Scotland, said: "Waiting times are very important to patients and are a major performance target for the health service. The management and scrutiny of the waiting list systems have not been good enough. NHS boards and the Scottish Government must improve the monitoring of boards' use of waiting list codes if they are to retain public trust and assure patients they are being treated fairly."
The problem surfaced in 2011 when NHS Lothian was found to have manipulated waiting time codes, marking patients as unavailable for "social reasons" such as failing to get time off work or being on holiday.
Facing extra pressure to meet shorter 18-week guarantees, it was discovered managers were putting pressure on staff to find ways around the system. This included marking patients as unavailable if they refused to travel to Northumberland in England.
A wider internal investigation was ordered across health boards, which reported shortly before Christmas.
Audit Scotland said it had been hard to trace changes on patient records and identify reasons for the use of waiting time codes. They also found that the use of "social unavailability" increased from 11% in 2008 to just over 30% in 2011. The levels then dropped off around the time fiddled waiting figures were discovered at NHS Lothian.
Health Secretary Alex Neil said changes are already under way. The social unavailability code has been replaced and hospital IT systems are being improved, he said.
"However, one area where we recognise that improvements can still be made is in communicating clearly to patients what their rights are and what they can expect, and work is under way to improve this," he said. "Let us not forget the most important thing: that waiting times are amongst their lowest-ever levels in Scotland and they continue to improve."