'The biggest black box in the NHS' was how my fellow columnist Simon Stevens last month described the £10bn or more spent by English PCTs on district nursing, therapies and other mainstream community healthcare.

He says they are "subject to almost no meaningful measurement or commissioning" and is sceptical of the ability of "payment by results technocrats" to do much about it.

He would address the second deficit via "specialised health and social care commissioning organisations" - which do sound remarkably like PCTs, though PCTs were not quite what he had in mind - to help people with long-term conditions spend their new individual budgets. But that still leaves the question of measurement. Whoever the commissioners are, they will need a currency.

And there is perhaps a second question: what is the hurry? Why the recent stirring of interest in the doings of PCT provider arms and the current pressure for them to mutate into arm's-length trading organisations by next spring?

First, though, measurement. Here is the core dilemma. When you go to the fruit market you need three pieces of information. You need to know whether you want apples, pears or oranges. (The payment by results equivalent is knowing which healthcare resource groups you are buying, and let's assume only specialist regional commissioners - sorry, big city shoppers - can afford pineapples.) You need to know whether they are sold singly or by the half-kilo bag. (Payment by results uses the "spell" as its standard currency.) And you need to know today's price. Then you are ready to shop. Lack any one of them and you are lost.

So where are the equivalents of the healthcare resource group and the "spell" in district nursing? Or among allied health professions? The reason payment by results has not yet reached effectively into community care is that a price based on a spell of care does not really mean much for long-term illness, palliative care, or any condition that does not come in neat episodes.

This is not to suggest community services are inefficient. Many therapy and community nurse managers are only too familiar with pressures to increase productivity. Within the professions, there is often a sound enough understanding of what a standard unit of a clinician's time - such as 10 minutes of physiotherapy - should be able to contribute to clinical outcomes.

These are, however, input measures. Commissioners (and accountants) expect to be able to measure outputs and they grow frustrated and suspicious and start muttering about "contestability" when they can't. So there may be an accommodation that involves counting something meaningless: "contacts" have been a favourite in the past. Or there may be an undue focus on episodic aspects of community care - the treatment of leg ulcers, for example - while non-episodic work, such as falls management, receives less attention.

What may be tempting for use in meeting "challenging" deadlines can become poor practice when used for moving real money around the NHS. If community providers are paid by the contact, you can be sure they will record more contacts. Yet the complexity of community care is well understood. Last year's consultation on payment by results showed prudent caution about extending the system beyond acute care. This renders all the more intriguing recent reports, that the DH and strategic health authorities are busy working on a tariff for community services. Why now? What is the urgency?

It is nearly three years since the Department of Health published Commissioning a Patient-led NHS, with its proposal that PCTs should lose their provider functions and concentrate on commissioning. The logic of eliminating conflicts of interest was impeccable and the vision of competition between NHS, commercial and third sector providers coherent enough; but where were the new providers? Who would employ PCT nurses, therapists and others?

A tense autumn saw the timetable for separation rapidly extended and the Royal College of Nursing taking Patricia Hewitt to judicial review and ultimately the departure of the then NHS chief Nigel Crisp.

But three years on, those would-be providers exist. Indeed, there is emerging competition. Some PCTs have been busy promoting the case for community foundation trusts; others designing innovative solutions based around "horizontal" (with social care) or "vertical" (with acute hospitals) integration.

Meanwhile the environment has changed. Foundation trusts have learned how to grow by acquisition and merger. The "credit crunch" has made relatively secure government-backed income streams - such as NHS contracts - attractive to private hospitals and their venture-capital owners.

Where is the growth potential for foundation trusts and the commercial healthcare sector, now NHS growth is down to 4 per cent and may fall further and the enthusiasm for independent sector treatment centres has cooled?

Some will look to devouring those trusts that fail to jump the Monitor hurdles, some to the promised 250 new primary care centres. Some, no doubt, are sizing up£10bn-plus of PCT provider arms with enthusiasm, warming to the scope for taking commissioners for a ride.

This may partly explain the new-found enthusiasm for a tariff in the community care sector. However, two significant obstacles remain. The first is access to the NHS pension scheme, without which clinicians - understandably - may see red.

And the second is the very real possibility of commissioners being faced with local monopolies. This is why a small team at the DH is understood to be working on an updated version of a old friend, one that might serve as an alternative. It is called the "community trust".