DiagnosticTesting讲义.pptVIP

如何确定截断值 如果疾病预后差，漏掉病人可能带来严重后果，且目前又有可靠的治疗方法，则临界点向左移，尽可能多地发现可疑病人，但会使假阳性增多； 如疾病的预后不严重，且现有的诊疗方法不理想，临界点可右移，以降低灵敏度，提高特异度，尽可能将非患者鉴别出来，但增加假阴性； 如果假阳性作进一步诊断的费用太高，为了节约经费，可将临界点向右移； 如果灵敏度和特异度同等重要，可将临界点定在非病人的分布曲线与病人的分布曲线的交界处。 ROC Curves The ROC (receiver operating characteristic curve,受试者工作特征曲线) curve is constructed with sensitivity (true-positive rate) on the vertical axis and 1-specificity (false-positive rate) on the horizontal axis. （横轴为假阳性，纵轴为真阳性） This curve derives its name from its first application---measur-ing the ability of radar operator to distinguish radar signals from noise. How to set the cutoff point How to set the cutoff point There are many more options about where to set the cutoff point between a positive and negative test result Toward the left-hand portion of the graph is a steep rise in sensitivity with the corresponding modest increase in 1-specificity. in other words, substantial gains can be made in sensitivity, with only modest reduction in specificity---a very favorable trade-off point . ROC 分析资料收集与整理 假设某诊断系统的对照组有4 个受试者其空腹血糖检验结果为6.29, 6.24, 4.61,3.77 ，病例组有5 个受试者其检验结果为15.90, 13.35, 12.87, 10.22, 6.01 。将这9 个数据从大到小排列,以前8个数据的每一个作为诊断阈值(或截断点) 。大于等于该阈值为阳性，小于该阈值为阴性。则对于每一个诊断阈值都可整理出以下8个ROC 工作点FPR,TPR =(0/4,1/5) (0/4,2/5) (0/4,3/5) (0/4,4/5) (1/4,4/5) (2/4,4/5) (2/4,5/5) (3/4,5/5)。 请根据上述数据，试划出ROC曲线。 Study question A four-item memory test is assessed as a screening tool for dementia（痴呆） in the elderly. In a sample of 483 persons, 50 had dementia as defined by extensive neurologic and cognitive（认知的） assessment. of those with dementia, the screening test was positive for 40 persons. Of those without dementia, the screening test was negative for 416 person. 1.what is the sensitivity of the test ? 2.what is the specificity of the test? 3.what is the positive predictive valve of the test? 4.what is the negative predictive valve of the test? 5.what is the likelihood ratio for a positive finding of the test? 6.What is the likelihood ration for a negative fi