THE idea that private patients might be admitted into an NHS bed, undergo surgery in an NHS theatre, or attend consultations in an NHS hospital might come as a surprise, but it is not necessarily a bad thing - as long as it is rigorously monitored.

If private patient numbers are too low to impact on waiting times or theatre space, it could even be a net benefit for the health service in terms of boosting coffers.

Read more: Call for investigation as thousands of private patients treated on the NHS in Scotland 

The problem is that there are no routinely published, national statistics on the practice in Scotland.

The figures obtained by the Herald were done on a board-by-board basis through freedom of information, and it is up to the boards to vouch that they are adhering to the rules as far as ensuring no NHS patient is disadvantaged. But how can that be measured without independent scrutiny?

The Herald recently reported on concerns that NHS Ayrshire and Arran was earning £100,000 a year providing cataract treatment to private patients.

At the same time, it has restricted NHS referrals so that patients cannot automatically have treatment on a second eye and in any case, their eyesight must deteriorate below the legal threshold for driving to qualify.

Read more: Call for investigation as thousands of private patients treated on the NHS in Scotland 

Since this increases the waiting time, it is inevitable some patients who can afford it will give up and go private - only to end up back in an NHS Ayrshire hospital.

NHS Ayrshire denies their restrictions have contributed to increased private activity - but it is easy to see how cash-strapped health boards can end up with a perverse incentive to ration certain procedures if they can turn it into a money-spinner instead.

Why not do the same with hip and knee replacements, biopsies and specialist scans?

In the context of health boards' overall annual budget of £11.2 billion in 2017/18, the fact that they earned an extra £2.1 million (£1.35m of it in NHS Lothian) is not going to make a seismic impact on their finances.

But the enormous regional variation in private activity - NHS Grampian handled 31 private patients last year compare to 502 in Lothian - does beg a little more scrutiny.

Read more: Call for investigation as thousands of private patients treated on the NHS in Scotland 

In England, the controversial 2012 'Health and Social Care Act' means hospital trusts are now entitled to make 49% of their annual income from private activity (compared to 2% previously). In some hospitals, especially in London, beds are ring-fenced specifically for private activity.

We are very far from that in Scotland - but we should not be complacent either.