Seeing things clearly

By BBC Doctor Colin Thomas

We all secretly pride ourselves in being good communicators don't we? And doctors are no exception.

We all try to put ourselves in the place of the patient, but inevitably although we as professionals feel we are explaining things properly, there is a danger that we still can assume too much prior knowledge on the patient's behalf.

Last week I was performing a medical which was necessary for a patient to qualify for a car racing licence.

Part of the medical was to ensure that his vision was normal including perfect colour vision.

There are two standard accepted methods for testing this. One is the Ishihara chart. That's the one where you see numbers composed of different coloured dots.

Those who are colour blind won't see the number, or will see a different one.

On this occasion I used the City University test which is a less well known test, but equally as effective.

I sat the patient in front of me and explained that on the page were five coloured dots. One in the middle, and one respectively above, below, to the left, and to the right of it.

And I asked him to tell me which colour was closest to the middle one. I even asked him if he understood and he said he did.

He carefully examined the first page, and seemed to me to be taking an inordinate amount of time deciding which spot matched. In view of the obvious struggle he was having telling a green from a bright purple spot, this gave me concern that here was someone with previously undiscovered colour blindness.

I even reiterated - "you have to find the colour closest to the one in the middle".

He nodded again, but when I turned over to the next page on the test he continued with another lengthy deliberation before he chose the wrong one yet again.

Blind to the truth

I stopped him. "I'm sorry" I said "I think this shows that you're colour blind". He was visibly shocked at first, but when I went back to the first page and pointed out which spot was the right colour it dawned on him that he had been trying to tell which spot was physically closest to the middle coloured spot rather than which one most closely matched the centre one in colour.

What he was trying to do was indeed impossible because all four spots were equidistant from the middle one!

We both had a good laugh, and when he repeated the test with the correct information he passed with flying colours.

If that can happen with something as simple as a colour blindness test goodness knows what other tangles one can get in!

Oh, and by the way, next time for colour vision testing I shall be mostly using the Ishihara test.