OSCE answer 8

OSCE question 8

Marks

A forty year old nulliparous patient attends the antenatal clinic at 16 weeks.

She wishes to discuss biochemical screening for Down’s syndrome.

This will feature somewhere in the examination. It could be an MCQ, but makes an ideal OSCE, either question and answer, like this example, or a role-play. In a roleplay, counselling will be the big issue -these stations give most or all of their marks for communication skills, but you will be much more fluent and convincing if you know what you are talking about. It is vital in the examination, as in real life encounters with patients, that you are scrupulously honest. If you don't know the facts, say so. If a patient thinks you are trying to pull the wool over their eyes, they will treat you with the contempt a liar deserves.

1. What does “sensitivity” mean in relation to this test?

“Sensitivity” means the accuracy of the test in identifying the problem. In this case, the percentage of affected pregnancies that will be found by the test. This varies according to the number of substances assayed and the age of the woman and ranges from about 60% in younger women to >80% in women of 40 or more. The reason for the higher sensitivity for older women is that the risk computation is based on the age related risk, which is adjusted according to the biochemical findings. Women in their early twenties have a low age related risk of 1:1000 to 1:1500. Women of forty have a higher (about 1:100) age-related risk, so many of them will fall into the “screen positive” category, which is usually defined as having a risk greater that 1:250 or 1:300. As most will fall into the “screen positive” category, fewer affected pregnancies will be missed. The downside, obviously, is that many of them will end up being offered amniocentesis. Screening is still worth offering as some will fall into a low risk category and be spared the risks of amniocentesis. (See also MCQ 6, question 24.)

2. What sensitivity does biochemical screening have for this patient?

See answer to 1.

3. What are the 3 main substances measured?

The origin of DS screening was the realisation that patients having very low AFP results on screening for neural tube defects were at increased risk of having a baby with DS. The low AFP only increased the age-related risk by a factor of two or so, so it wasn’t especially helpful. Additional tests have been added to make the test more potent. The basic one is ß-HCG, making up the "double" test. Add oestriol and it becomes the "triple" test. Additional ones are inhibin and pregnancy associated plasma protein A. The more components the test has, the greater its sensitivity, but the additions only increase sensitivity marginally. (See also MCQ1, question 1.)

4. List 4 important issues to include in counselling, other than “sensitivity”.

Counselling should ensure that the patient understands:

what Down’s syndrome is. (Once you have explained a term such as “Down’s syndrome” you can then use it freely. Up to that point the use of such a term in a role-play will constitute a black mark for the use of medical jargon.)

what screening is: that it gives the risk of the problem’s existence; that it is not diagnostic and does not guarantee that the baby does not have Down's syndrome.

what a “risk” is – this is particularly important in role-play stations. Explain it along the lines of: “the test says the risk is 1:100. This means that if we had one hundred women exactly like you, one would have a baby with DS. The other ninety nine would not”. As above, once the concept has been explained, the term can be used.

that a “screen positive” result does not mean that the diagnosis has been made, merely that the patient falls into the risk category of patients for whom amniocentesis is offered.

that amniocentesis is the diagnostic test and would be offered if she wished to proceed with it and not screening.

what is involved in amniocentesis – a needle inserted, fluid aspirated, a 0.5 – 1% risk of procedure induced miscarriage, etc. For the gold medal, the tiny risk that amniocentesis gives a wrong result, mainly due to culture of maternal cells.

that there is not a lot of point in having amniocentesis with its attendant risks unless she would wish to have termination.

that the risk assessed by the screening test will be further reduced if no “soft markers” are seen on the 20 week scan.

that anti-D may be necessary with amniocentesis.

5. Name two other methods of screening for Down’s syndrome risk.

Nuchal fold thickness. “Soft markers”. (See MCQ2, question 41.)

6. Name two other conditions that may be suggested by the test.

Neural tube defect. (See MCQ 1, question 1 and MCQ2, question 13.) Trisomy 18. (See MCQ 2, question 35.)

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