Screening I Flashcards
Be able to calculate and interpret the risk of being affected with a disorder for an individual based on a quantitative screening marker.
h
What is meant by a ‘detection rate’ e.g. a DR of 75%?
Using the example of spina bifida and the maternal serum AFP screening test, a DR of 75% means that 75% of pregnancies with open spina bifida have positive results.
How do you calculate detection rate?
number of affected with positive test results/total number of affected
How do you calculate false positive rate?
number of unaffected with positive test results/total number of unaffected
If the birth prevalence of OSB was 1 per 1000 instead of 2 per 1000. How would the OAPR be affected?
Lower (1:40, about 2.5%)
How does the positive predictive value differ from OAPR?
OAPR is a ratio e.g. 1:20. The positive predictive value is a fraction e.g. 1/21.
If the birth prevalence of OSB was 4 per 1000 instead of 2 per 1000. How would the OAPR be affected?
Higher (1:10, about 10%)
Amniotic fluid AFP measured in screen-positive women has a detection rate of 98% and a false positive rate of 0.5%.
What is the new OAPR?
10:1
about 90%
If the amniotic fluid AFP is positive, an AChE test is carried out on the same sample.
The amniotic fluid AChE in screen-positive women with a positive amniotic fluid AFP result has a detection rate of 99% and a false positive rate of 5%.
What is the new OAPR?
200:1
about 99%
What is detection rate also called?
Sensitivity
What is false positive rate also called?
1-specificity
What is the difference between a cervical smear test result and a maternal serum AFP test result?
One is qualitative (result is +ve or -ve), the other is quantitative (result is a number – a cut-off level is needed to separate results into +ve or -ve)
What is MoM?
Multiples of the Median
If the median (say of MS-AFP) is 30 IU/ml, what is 0.5 MoM?
What about 2 MoM?
15 IU/ml
60 IU/ml
Recognise the trade-off between higher detection rate but higher false-positive rate.
Again using the example of the MS-AFP test, the MoM curves for unaffected and OSB overlap. The test needs to set a cut-off level that has a high DR, but if it was 100% then the false positive rate would be very high. Therefore, you set it at a level that the DR is high enough but so you don’t have too many false positives.
Recognise the trade-off between higher detection rate but higher false-positive rate.
Again using the example of the MS-AFP test, the MoM curves for unaffected and OSB overlap. The test needs to set a cut-off level that has a high DR, but if it was 100% then the false positive rate would be extremely high. Therefore, you set it at a level that the DR is high enough but so you don’t have too many false positives.
This can be seen on the “receiver operating characteristic curve”.
How is the likelihood ratio calculated?
DR/FPR
How do you calculate the odds of being affected given a positive test result?
Use the prevalence to create a flow diagram.
E.g. 1000 –> 2 affected
–> 998 unaffected
Then use DR and FPR to calculate how many positive results for each group. Then do the ratio between the two.
OR more simply, the likelihood ratio x prevalence as an odds
Calculate the test specific risk of the being affected for an individual woman with an serum AFP of 2.5 MoM.
You calculate the likelihood ratio by looking at the height of the two curves of unaffected and OSB at 2.5 MoM, and dividing the height of OSB curve by the unaffected curve.
You then times the LR by prevalence as a ratio.
If a woman is told that her risk of having an open spina bifida pregnancy is quite ‘normal’, that is, the risk is no higher and no lower than the risk for women in general, what is her MS-AFP level?
This means the LR is 1/1 so the risk is unchanged i.e. the normal prevalence (2:1000).
The LR is 1/1 when the height of the curves is the same (where they intersect).
The MoM is just under 2, so the level is just under 60 IU/ml.
Define DR.
Proportion of affected individuals with positive results
OAPR =
Define FPR.
Proportion of unaffected individuals with positive results
Define DR.
Proportion of affected individuals with positive results
Which of the following depend on the prevalence of the disorder in the screened population?
- DR, FPR, OAPR
OAPR
What are the requirements for a worthwhile screening programme? (10)
- Well defined disorder
- Prevalence known
- Medically important disorder for which there is an effective remedy available
- Cost-effective
- Facilities available or easily installed
- Procedures following a positive result are generally agreed and acceptable both to the screening authorities and to the patients
- Equal access to screening services
- Test simple and safe
- Distributions of test values in affected and unaffected individuals known, extent of overlap sufficiently small, and a suitable cut-off level defined
- Avoid unnecessary complexity in the screening process
What are the requirements for a worthwhile screening programme? (10)
- Well defined disorder
- Prevalence known
- Medically important disorder for which there is an effective remedy available
- Cost-effective
- Facilities available or easily installed
- Procedures following a positive result are generally agreed and acceptable both to the screening authorities and to the patients
- Equal access to screening services
- Test simple and safe
* 9. Distributions of test values in affected and unaffected individuals known, extent of overlap sufficiently small, and a suitable cut-off level defined* - Avoid unnecessary complexity in the screening process
