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In screening for congenital cataract, many false positive referrals will occur

BMJ 1999; 319 doi: (Published 10 July 1999) Cite this as: BMJ 1999;319:122

This article has a correction. Please see:

  1. Stephen Morgan, General practitioner (S.Morgan{at}
  1. Primary Medical Care, University of Southampton, Aldermoor Health Centre, Southampton SO16 5ST

    EDITOR—Rahi et al's data enable the annual incidence of congenital cataracts in the United Kingdom to be estimated.1 The incidence is roughly 3 per 10 000 live births after allowance is made for their assessment of the non-completeness of the data. This rarity does not make the condition unimportant but does raise considerable difficulties for a screening programme. This is especially so when the apparently simple screening test relies on the interpretation of a clinical sign that is difficult to elicit from many infants in the circumstances in which the test is usually performed.

    The challenge for clinicians is to retain high sensitivity for detecting the abnormality without unduly reducing the specificity. In addition, they must resist the inevitable pressure to reduce their sensitivity in the light of the large number of false positive results 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 achieved the impressive 93% sensitivity and 95% specificity achieved in the small study of ophthalmoscopic diagnosis in 3-30 year old subjects to which the authors refer.2 Thus a hospital paediatrician doing 20 assessments a week would see one case in four years but would have referred one case each week throughout that time. In the community a full time general practitioner would refer one case each year but it would take six working lifetimes to see a true case.

    Faced with this level of false positive results, most clinicians are likely to increase their specificity substantially, with the almost inevitable result that their diagnostic sensitivity will fall. 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 acceptable clinical practice for other diagnoses.


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