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Jacob, There are two questions here. First, the false positive rate as it is commonly used is 1 - the specificity. In the first example, in order to calculate the false positive rate, you would need to know the number without disease. The number you have calculated is the probability that someone with a postive test is without disease. This is also called the False Alarm Rate. In the case of the Downs syndrome test, I would interpret the statistics as a sensitivity of 80% and a specificity of 95%, leaving a 5% false positive rate. In the population with a prevalence of Downs of 1 in 1000, the False Alarm rate can be calculated from the Positive Likelihood Ratio. In this case LR+ = 0.8 / 0.05 = 16 (not bad in general, but not great for a mass screening test). The pretest odds are 0.001 (1:999) and the post test odds of disease (Positive Predictive Value) is 0.001 x 16 = 0.016 or 1.6%. This means that 98.4% (1 - 0.016) of the positive tests will be False Alarms. We get the same answer (probably because the prevalence is so low), but the semantics are a bit different. Dan >>> Jacob Puliyel <[log in to unmask]> 03/05/02 01:54PM >>> Dear Mail-base This is a question I hope people familiar with population based screening tests can help me with. Ordinarily we say 'false positive' if test is positive but person is not diseased. Assume population of 100 persons. Suppose 10 tested positive to a screening test and of these only 5 had the disease then the false positive rate in 50% Why is it then, that when we look at a screening test like triple test or nuchal translucency in Downs syndrome, people talk of 80% detection for a 5% false positive rate meaning 5% of the population will be positive. (Wald NJ & Hackshaw AK, Prenat Diagn 1997; 17:821-9) Downs syndrome occurs 1 per 1000 deliveries. For purposes of simplicity let us assume 100 % pick up by the test. If there are 100,000 deliveries we can expect 5000 (5%) to be identified by the screening test. 100 among the 5000 identified by the test, will have Downs(1%). In other words 4900 of those identified by the test did not have disease and 100 had disease. The false positive rate is 4900/5000 multiplied by 100 = 98% false positive. It does not sound like a good test if we say it has 98% false positvity. Is that we say the test has a 5% false positive rate? Sincerely Jacob M. Puliyel _____________________________ Residence eMail: [log in to unmask] Residence Phone: 00-91-11-3946388 eFax UK 07092-124285 Fax 00-91-11-3932412 Jacob M. Puliyel MD MRCP MPhil Head of Department Department of Pediatrics and Neonatology St Stephens Hospital Tis Hazari Delhi-110054 INDIA Work Phone: 00-91-11-3966021 to 27 Sara Varughese FRCS Consulant Ophthalmologist Dr Shroff's Charity Eye Hospital Daryaganj Delhi-110054 Work Phone: 00-91-11-3251564 ------------- **************************************************************************** Dan Mayer, MD Professor of Emergency Medicine Albany Medical College 47 New Scotland Ave. Albany, NY, 12208 Ph; 518-262-6180 FAX; 518-262-5029 E-mail; [log in to unmask] ****************************************************************************