"What if the doctor missed something?"

Suppose you do what this book keeps recommending. You see the GP about the calf. She asks her questions, tests the strength in both legs, taps your knees and ankles, and tells you it's a benign twitch, very common, likely to settle. You walk out feeling light. By the following evening you're searching again. This time you're searching for how often doctors miss the disease, and what tests she should have ordered.

If you recognise this, you're in well-charted territory. The NHS list of signs of health anxiety includes worrying that a doctor or medical tests may have missed something. And Lucock's gastroscopy patients in chapter 5 are the same case. The ones with high health anxiety were reassured by a specialist with a camera, and a day later their worry was back.

Doctors do sometimes miss things, and I won't pretend otherwise. If a symptom changes, gets worse, or goes on past the point the doctor said it would, going back is sensible, and good doctors want you to.

But look at what the evening-after search is doing. You had an examination, which is the evidence chapter 4 said you needed, and it was clear. The search has no new evidence to set against it. All it can do is raise the general possibility of error, which applies to every consultation anyone has ever had and can never be removed. So the worry the search is feeding now is about certainty, as in chapter 6, and it isn't really about your leg any more.

There's something practical you can do about this, and it happens in the consulting room. Before you leave, ask two questions. "What should I look out for?" and "When should I come back if it hasn't settled?" Doctors call this safety netting, and many will do it without being asked. Write the answers down, there and then, in your phone or on paper. You now have a rule that came from someone who examined you. If the twitch spreads or the leg feels weak, go back. If it's still there in a month, go back. Otherwise, leave it.

You can use what you wrote down the next evening. When the thought comes that she might have missed something, there's a specific answer to it. You can ask whether any of the things she told you to watch for has happened. If it has, ring the surgery in the morning. If it hasn't, the search has nothing to add, and you can tell yourself so.

If you find that no examination reassures you for longer than a day or two, whatever the result and whoever gives it, I'd take that seriously. It suggests that reassurance doesn't last for you, and that more of it, from pages or from people, won't fix that. Chapter 18 covers what does help.