Flick through PR Week and you find endless stories about companies ditching their PR agencies. Constantly replacing retained agencies is a bizarre and excessively costly ritual, both for the companies and the agencies.

Sometimes it’s done for no other reason than that the client has a new head of communications and wants to make “an impact”. Sometimes the agency has lost interest in the client, or the client’s needs have changed.

But, in my view, the most significant cause of agency divorce stems from bad procurement. To get contracts, agencies have a tendency of being over-optimistic about results. And this can be combined with a client’s unrealistic views of what can be achieved on the budget.

If you’re a client, relying exclusively on agency pitches is, in my view misguided. It opens you up to all manner of wishful thinking by agencies – and let’s make no mistake, if the agency needs your business, there’s little hope that they will try to manage expectations down to a realistic level.

And if you’re an agency, a client who has no idea what’s realistic – but who thinks your business-to-business product will be reviewed on Radio 4’s Woman’s Hour – is a difficult client to pitch to.

These difficulties are why I recommend that clients do a PR audit before they seek proposals from agencies. An audit, by an external and independent consultant, will examine a range of issues including what a reasonable level of coverage based on other companies of a similar size and of others in the same sector. It has to be by someone who’s not in the running for the retainer, otherwise the recommendation will simply be “Hire me! Hire me! Hire me!”.

With an audit in hand, it becomes easier to work out if hiring an agency is likely to deliver the right return on investment, and whether prospective agencies are being realistic in what they propose.

Find out more about PR audits here.