EQUALITY of care is a sacred pillar of the NHS.

How often is it the man that mantra that our healthcare system is "free at the point of delivery and based on need, not the ability to pay" touted as evidence of its moral superiority over other nations' models?

In theory then, all patients should be treated equally. But we know this is not the case, from "postcode lotteries" of drug access to how far cancer patients must travel for treatment.

But arguably the biggest gulf is found in primary care, where a long-standing failure to link investment with deprivation has been blamed for entrenching and exacerbating Scotland's health inequalities - the widest in Western Europe.

Here is a fairly shocking statistic: average GP funding per patient per year is only seven per cent higher for the most deprived tenth of Scotland's population than it is for the most affluent tenth. That is despite 20 per cent more GP visits among the poorest patients, a 120 per cent increase in the prevalence of patients with combined mental and physical illnesses, and a 142 per cent increase in mortality among under-75s.

In other words, despite the massive burdens of ill health facing general practice in Scotland's most deprived communities, their share of funding is only fraction higher than practices whose patients are the most affluent - and statistically healthiest - in the country.

This phenomenon is not new. It is known as the "inverse care law", and was first coined in 1971 to describe this maldistribution of good medical care.

A major report on the issue in late 2015 led First Minister Nicola Sturgeon to reassure MSPs that the new GP contract - due to be finalised in 2017 - offered "a good opportunity" to revise the Scottish Allocation Formula, the equation used to calculate funding quotas per practice.

More than a year on and with the new contract looming, Professor Graham Watt, co-author of the report, said it was still "unclear whether a suitably revised [formula] can be achieved".

Worryingly, there was also no mention of it in the Scottish Government's recent Health and Social Care Delivery Plan - a blueprint for the future of the NHS.

Prof Watt, a Glasgow University academic and prominent critic of the inverse care law, retires next week - but the fight will go on.

If anything, it is more pressing than ever amid evidence of the pressures facing A&E departments and growing demand for acute hospital beds. Much of this is linked to problems spilling out from deprived areas as these patients' needs go unmet by primary care. With the proportion of GPs in Scotland aged over 50 highest in deprived areas, this will only become worse.

To date, neither BMA Scotland nor the Royal College of GPs have campaigned to eradicate the inverse care law; the priority was to safeguard general practice as a whole during a decade when its total share of NHS funding steadily declined.

Prof Watt acknowledges that redistribution of reduced funding is "not a realistic option" when all general practices are under pressure.

However, with the Scottish Government now promising an extra £500 million for primary care by 2021, the challenge must be to target it to where it is most needed.