Showing papers on "False positive paradox published in 1991"

Theoretical analysis of library screening using a N-dimensional pooling strategy

Abstract: A solution to the problem of library screening is analysed. We examine how to retrieve those clones that are positive for a single copy landmark from a whole library while performing only a minimum number of laboratory tests: the clones are arranged on a matrix (i.e in 2 dimensions) and pooled according to the rows and columns. A fingerprint is determined for each pool and an analysis allows selection of a list containing all the positive clones, plus a few false positives. These false positives are eliminated by using another (or several other) matrix which has to be reconfigured in a way as different as possible from the previous one. We examine the use of cubes (3 dimensions) or hypercubes of any dimension instead of matrices and analyse how to reconfigure them in order to eliminate the false positives as efficiently as possible. The advantage of the method proposed is the low number of tests required and the low number of pools that require to be prepared [only 258 pools and 282 tests (258 + 24 verifications) are needed to screen the 72,000 clones of the CEPH YAC library (1) with a sequence-tagged site]. Furthermore, this method allows easy and systematic screenings and can be applied to a large physical mapping project, which will lead to an interesting map with a low, precisely known, rate of error: when fingerprinting a 150 Mb chromosome with the CEPH YAC library and 1750 sequence-tagged sites, 903,000 tests would be necessary to obtain about 20 contigs of an average length of 6.7 Mb, while only about one false positive would be expected in the resultant map. Finally, STSs can be ordered by dividing a clone library into sublibraries (corresponding to groups of microplates for example) and testing each STS on pooled clones from each sublibrary. This allows to dedicate to each STSs a fingerprint that consists in the list of the positive pools. In many cases these fingerprints will be enough to order the STSs. Indeed if large YACs (greater than 1 Mb) can be obtained, the combined screening of DNA families and YAC DNA pools would allow an integrated construction of both genetic and physical maps of the human genome, that will also reduce the optimal number of meioses needed for a 1 centimorgan linkage map.

Value of positive technetium-99m leucocyte scans in predicting intestinal inflammation.

Abstract: The value of positive scans of autologous 99mTc-labelled leucocytes in the detection of intestinal inflammation was assessed in 29 patients with known inflammatory bowel disease and a high probability of intestinal inflammation and 58 with a low probability of intestinal inflammation comprising 37 with intestinal symptoms and 21 with suspected abscess. Autologous leucocytes were labelled with 99mTc using either of the established methods. In patients with inflammatory bowel disease, 3/13 with ileocaecal and 1/11 with colonic accumulation of 99mTc were incorrectly diagnosed in that there was no histological evidence of inflammation in those regions. In the low probability patients, all 26 positive images were false positives: there were 19 in the ileocaecal and 7 in colonic regions and no differences between the two subgroups. False positives were more common in low probability than high probability patients (p = 0.011). The results were independent of the 99mTc cell labelling technique used. Three false positive patients also had normal scans for 111In-labelled granulocytes. The time of first detection of 99mTc in the bowel was earlier in true than in false positive patients (92% v 33% within 1 h, p = 0.008), the false positive rates in the first hour of imaging being 5% in high and 15% in low probability patients. In 99mTc scintigraphy, the appearance of the isotope in the intestine occurs commonly in the absence of inflammation irrespective of the cell labelling technique used. In high probability patients, imaging at 1 h can more reliably discriminate true from false positives and retain clinical usefulness. In low probability patients specificity remains unacceptable and 99mTc-labelled leucocyte scanning in these patients is of little value as a positive predictor of intestinal inflammation.

Optimal Scheduling of Inspections: A Delayed Markov Model with False Positives and Negatives

Abstract: A system subject to catastrophic failure deteriorates according to a delayed Markov process and is subjected to a series of binary tests that may yield false negative and false positive outcomes. A corrective action is carried out when a true positive is observed, thereby reducing the chance of system failure. Costs of inspections, false positives, the corrective action, and failure are incurred, and dynamic programming is used to compute the optimal inspection schedule. Two tractable computational methods are developed. The model, which is suited for medical screening, is applied to the problems of post-operative periumbilical pruritis and breast cancer.

Pitfalls in the diagnosis of drug smuggler's abdomen.

Abstract: Narcotics "body packing" can be detected in abdominal X-rays by the ring shadow caused by air trapped in the packs. In a series of 82 cases admitted for abdominal X-ray in Helsinki, Finland, in 1982 through 1988, we encountered 9 (11.0%) true positives, 3 (3.6%) false positives, and 1 (1.2%) false negative. The false positives were due to the constipation often associated with the narcotics abuse. The false negative X-ray diagnosis was attributable to an inexperienced radiologist. False negatives may also be associated with packets containing marijuana, packs with few wrappings, aluminum-foil coated packs, and machine-packed narcotics. Searching for trapped air in radiographs, repeated X-raying by an experienced radiologist, use of computed tomography, or combined urinary drug screening may be applied to diminish false findings and to avoid unnecessary arrest for the purpose of fecal screening over several days.

An analysis of false positive reactions occurring with the Captia Syph G EIA.

Abstract: AIM--The Captia Syph G enzyme immuno assay (EAI) offers the potential for the rapid automated detection of syphilis antibodies. This study was designed to assess the role of other sexually transmitted diseases (STDs) in producing false positive reactions in the Captia Syph G EIA. The role of rheumatoid factor (RF) as a potential source of false positives was also analysed. METHODS--Patients who attended a genitourinary medicine (GUM) department and gave a false positive reaction with the EIA between 1988 and 1990 were compared with women undergoing antenatal testing and with the control clinic population (EIA negative) over the same time period. The incidence of sexually transmitted disease (STD) in the clinic population and the false positive reactors was measured in relation to gonorrhoea, chlamydia, genital warts, candidiasis, "other conditions not requiring treatment" and "other conditions requiring treatment." Male: female sex ratios were also compared. Ninety two RF positive sera were analysed with the EIA. RESULTS--The rate of false positive reactions did not differ with respect to the diagnosis within the GUM clinic population. The antenatal group of women, however, had a lower incidence of false positive reactions than the GUM clinic group. No RF positive sera were positive on Captia Syph G EIA testing. CONCLUSIONS--There is no cross reaction between Captia Syph G EIA and any specific STD or with RF positive sera. The lower incidence of false positive reactions in antenatal women is unexplained but may be related to physiological changes associated with pregnancy.

Investigation and diagnosis of hypertrophic pyloric stenosis.

Abstract: Forty-nine infants who underwent investigation or surgery for hypertrophic pyloric stenosis (HPS) over a 30-month period were reviewed. Significant weight loss was present in 18 infants, of whom 16 had HPS. A test feed was performed in 46 infants with 1 of 27 false positive and 6 of 29 false negatives. An ultrasound examination was performed in 34 infants with 4 of 23 false positives and 1 of 11 false negatives. It is recommended that if the test feed is positive then surgery should be performed, otherwise an ultrasound examination can be used as a screening test but a positive result should be confirmed by other means before surgery.

Mediational screening: is the benefit worth the cost?

Abstract: Argued that Pillow et al. (1991) may have overestimated the benefits of mediational screening and underestimated the costs involved in the procedure. The three benefits they suggest: increased statistical power to detect prevention effects, increased cost-effectiveness, and decreased iatrogenic effects are shown to be modest at best. Costs associated with misclassifying people as either false positives or false negatives are considered in the total cost/benefit analysis of mediational screening. Because we cannot accurately predict who will react to the occurrence of life stress by developing the conditions we are interested in preventing, universal interventions are advocated.