The number of Scottish patients being sent to other parts of the UK for specialist treatment has risen by almost 50% over the past four years, according to figures from health boards.
The NHS National Services Division (NSD) approved funding for 625 referrals outside Scotland in 2016/17, up from 427 in 2013/14.
Over the past four years the total cost of referrals reached £51.7 million, rising from £11.9 million in 2013/14 to £15.2 million in 2016/17.
NSD funds access for Scottish patients to highly specialist services south of the border, with access ensured through a service agreement.
It also manages a pool of funding on behalf of Scottish health boards to pay for individual patient referrals for a “prescribed” list of specialised services which are not included in the agreement.
However the Liberal Democrats have called for ministers to explain why there has been such a significant rise in recent years.
The party’s health spokesman Alex Cole-Hamilton said: “It’s important that patients in Scotland have access to the best possible care.
“If there is expertise elsewhere in the UK that they can benefit from then it is right that they can do so.
“However, we need to know whether the significant rise in patients being sent outside Scotland is a consequence of SNP ministers’ failures.
“For example, the failure to plan the NHS workforce properly has led to huge numbers of long-term vacancies. More than 250 consultant posts have been empty for six months or more.
“It is inevitable that patients will have to travel unnecessarily, within Scotland and further afield, if we don’t have the right balance of specialist staffing in each part of the country.
“As well as repairing the NHS workforce, concerns about the excessive centralisation of health services demand a better model of health provision that is fit for remote and rural areas too.”
A Scottish Government spokeswoman said: “NHS funding has increased to a record high under this Government, as has staffing, which has increased by more than 12,400.
“Spending on this very specialist care represents just 0.1% of the record £47.4 billion investment in the NHS over the last four years, and a tiny proportion of the total number of procedures carried out in the NHS, which reached a record one million inpatient procedures last year.
“As complexity of healthcare increases and costs rise, it is right that very specialised care for procedures such as lung transplants is occasionally provided at specialist centres outwith Scotland to allow expertise to be concentrated and patients and families to be treated in quality settings.”