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Nick Raine-Fenning
Course Director
Joined: 27 May 2006
Posts: 1852
Location: Nottingham
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 Posted: Sat Oct 21, 2006 3:09 pm Post subject: Layterms: How do you explain sensitivity and specificity? |
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During this weekends Theoretical OSCE Course we discussed how to explain things to patients in layterms.
Several of you made some excellent suggestions so I thought I would start a series of "Layterm Posts" where I want you to explain in simple, non-medical words that are understandable to the general public.
Let's start with sensitivity and specificity! 
You are asked to discuss screening for Down syndrome. During the 'role play' you come to the point where you need to explain the sensitivity and specificity of nuchal translucency.
How do you explain this to the patient?
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chandra
Joined: 21 Oct 2006
Posts: 6
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| Posted: Sun Oct 22, 2006 5:45 pm Post subject: |
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Nuchal thickness is measurement of neck skin thickness in fetus between 11wks to 14 wks by ultrasound scan.
This is a screening test - that which identifies high risk fetuses for chromosomal abnormalities.
The test has a detection rate of 70% and false positive rate of 5%
sensitivity is the probability of a positive test among patients with disease specificity is the probability of a negative test among patients without disease
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Nick Raine-Fenning
Course Director
Joined: 27 May 2006
Posts: 1852
Location: Nottingham
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| Posted: Mon Oct 23, 2006 8:53 am Post subject: |
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No - how do you explain this to a patient? - that is what we mean by layterms.
Last edited by Nick Raine-Fenning on Mon Oct 23, 2006 9:12 am; edited 1 time in total |
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Nick Raine-Fenning
Course Director
Joined: 27 May 2006
Posts: 1852
Location: Nottingham
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| Posted: Mon Oct 23, 2006 9:11 am Post subject: |
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| chandra wrote: | | Nuchal thickness is measurement of neck skin thickness in fetus between 11wks to 14 wks by ultrasound scan. |
Measurement of the thickness of the skin at the back of the neck made between 11 weeks and 3 days and 13 completed weeks (i.e. 13 weeks 6 days)
| chandra wrote: | | This is a screening test - that which identifies high risk fetuses for chromosomal abnormalities. The test has a detection rate of 70% and false positive rate of 5%. Sensitivity is the probability of a positive test among patients with disease specificity is the probability of a negative test among patients without disease |
That's not entirely true as it also identifies many fetuses who are not high risk - be careful how you say this to patients.
A screening test is a test that alters an individuals risk to give a more accurate prediction of their risk. It is usually applied to the general population. A screening test does not, therefore, tell you if the individual (in this case a fetus) is affected. This requires further tests known as a 'diagnostic test'. The decision to go ahead with a diagnostic test will depend on the new calculated level of risk and the individuals wishes.
No screening test is 100% accurate. Some individuals who are affected (in this case, fetuses with Down syndrome) will be missed. The ability of a test to detect all affected individuals is referred to as its sensitivity. For NT measurements this may be as high as 80% meaning it should accurately indicate a higher risk in 80 of 100 women who have a fetus with Down syndrome. Unfortunately this means 20 women who have an affected fetus will be falsely reassured by a low risk result.
Furthermore screening tests can sometimes suggest an individual is at an increased risk of being affected when they are in fact entirely normal. This is know as the specificity of the test. In the cas of NT measurements, for every 100 women with a unaffected fetus, 5 will be incorrectly told that they are at increased risk.
In summary. it is important to always remember that the result of a screening test is just a estimate of risk. The result may be falsely reassuring or falsely worrying. Ultimately the interpretation of the test result depends on how the person feels about the condition being looked for and their own interpretation of risk. For some individuals a risk of 10% may seem acceptable whilst for others a risk of 1% may seem unacceptably high.
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