Rahi and colleagues have provided helpful data enabling the annual
incidence of congenital cataracts in the UK to be estimated.(1) Allowing
for their assessment of the non-completeness of the data the incidence is
approximately 3 per 10000 live births. While this rarity does not make the
condition unimportant it does raise very considerable difficulties for a
screening programme. This is especially so when the apparently simple
screening test relies on the
interpretation of a clinical sign which to elicit from many infants is far
from straightforward in the circumstances in which the test is usually
performed. The challenge for the clinician is to retain high sensitivity
for detecting the abnormality without unduly reducing the specificity. In
addition the clinician has to be prepared to resist the inevitable
pressure to reduce their sensitivity in the light of the large number of
false positives they detect. To put this in perspective only one in every
180 infants referred for further assessment would have the diagnosis
confirmed even if the clinician achieves the very impressive 93%
sensitivity and 95% specificity achieved in the small study of
ophthalmoscopic diagnosis in 3 to 30 year old subjects to which the
authors refer.(2) Thus a hospital paediatrician doing 20 assessments a
week would see one case in 4
years but would have referred one case each week throughout that time. In
the community a full time GP would refer one case each year but it would
take 6 working lifetimes to see a true case. Faced with this level of
false positives most clinicians are likely to increase their specificity
substantially with the almost inevitable result that their diagnostic
sensitivity falls. That the current screening
programme achieves about 50% sensitivity may be a cause for modest
celebration. Unless an even more specific screening test is developed
attempts to reduce the number of cases missed by the current system will
require the screening doctors and ophthalmologists to accept many more
false positive referrals than might usually be judged as acceptable
clinical practice for other diagnoses.
1. Rahi JS, Dezateux C. National cross sectional study of detection
of congenital and infantile cataract in the United Kingdom: role of
childhood screening and surveillance. BMJ 1999;318:362-365
2. Ruttum, M.S., Nelson, D.B., Wamser, M.J., Balliff, M. Detection of
congenital cataracts and other ocular media opacities. Pediatrics
1987;79:814-817.
Stephen Morgan
Primary Medical Care
University of Southampton
Aldermoor Health Centre
Aldermoor Close
Southampton SO16 5ST
Rapid Response:
An even more specific screening test is needed
Rahi and colleagues have provided helpful data enabling the annual
incidence of congenital cataracts in the UK to be estimated.(1) Allowing
for their assessment of the non-completeness of the data the incidence is
approximately 3 per 10000 live births. While this rarity does not make the
condition unimportant it does raise very considerable difficulties for a
screening programme. This is especially so when the apparently simple
screening test relies on the
interpretation of a clinical sign which to elicit from many infants is far
from straightforward in the circumstances in which the test is usually
performed. The challenge for the clinician is to retain high sensitivity
for detecting the abnormality without unduly reducing the specificity. In
addition the clinician has to be prepared to resist the inevitable
pressure to reduce their sensitivity in the light of the large number of
false positives they detect. To put this in perspective only one in every
180 infants referred for further assessment would have the diagnosis
confirmed even if the clinician achieves the very impressive 93%
sensitivity and 95% specificity achieved in the small study of
ophthalmoscopic diagnosis in 3 to 30 year old subjects to which the
authors refer.(2) Thus a hospital paediatrician doing 20 assessments a
week would see one case in 4
years but would have referred one case each week throughout that time. In
the community a full time GP would refer one case each year but it would
take 6 working lifetimes to see a true case. Faced with this level of
false positives most clinicians are likely to increase their specificity
substantially with the almost inevitable result that their diagnostic
sensitivity falls. That the current screening
programme achieves about 50% sensitivity may be a cause for modest
celebration. Unless an even more specific screening test is developed
attempts to reduce the number of cases missed by the current system will
require the screening doctors and ophthalmologists to accept many more
false positive referrals than might usually be judged as acceptable
clinical practice for other diagnoses.
1. Rahi JS, Dezateux C. National cross sectional study of detection
of congenital and infantile cataract in the United Kingdom: role of
childhood screening and surveillance. BMJ 1999;318:362-365
2. Ruttum, M.S., Nelson, D.B., Wamser, M.J., Balliff, M. Detection of
congenital cataracts and other ocular media opacities. Pediatrics
1987;79:814-817.
Stephen Morgan
Primary Medical Care
University of Southampton
Aldermoor Health Centre
Aldermoor Close
Southampton SO16 5ST
Competing interests: No competing interests