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Chapter-14 All about diagnostic tests

BOOK TITLE: Research Methodology Simplified: Every Clinician a Researcher

Author
1. Parikh Mahendra N
2. Mukherjee Joydev
3. Hazra Avijit
4. Gogtay Nithya
ISBN
9789350250037
DOI
10.5005/jp/books/11435_14
Edition
1/e
Publishing Year
2010
Pages
10
Author Affiliations
1. Seth GS Medical College and Nowrosjee Wadia Maternity Hospital, Mumbai, Maharashtra, India, Seth GS Medical College and Nowrosjee Wadia Maternity Hospital, Mumbai, Seth Gordhandas Sunderdas Medical College, Nowrosjee Wadia Maternity Hospital, Mumbai, Maharashtra, India; Shushrusha Citizens’ Cooperative Hospital, Mumbai, Maharashtra, India; Fertility Sterility, India; The Journal of Obstetrics and Gynaecology of India, Nowrosjee Wadia Maternity Hospital, Mumbai, Mumbai, Maharashtra, India, Mumbai
2. RG Kar Medical College, Kolkata, West Bengal, India, RG Kar Medical College, Kolkata, North Bengal Medical College, West Bengal, India, RG Kar Medical College, Kolkata, India, RG Kar Medical College, Kolkota
3. Institute of Postgraduate Medical Education, and Research, Kolkata, Institute of Postgraduate Medical Education and Research, Kolkata, India, Institute of Postgraduate Medical Education and Research (IPGMER), Kolkata, West Bengal, India
4. Seth Gordhandas Sunderdas Medical College and King Edward Memorial Hospital, Mumbai, Maharashtra, India; Journal of Postgraduate Medicine, Seth GS Medical College and KEM Hospital, Mumbai, India, Department of Clinical Pharmacology, Seth GS Medical College and KEM Hospital, Mumbai, India
Chapter keywords

Abstract

Diagnostic tests are required to arrive at a diagnosis. Very few tests are 100% accurate and give positive or negative prediction of a disease. Since they vary in their accuracy and cost they must be evaluated before inducting them in clinical practice. Sensitivity gives probability of a positive test when the disease is present while specificity gives the probability of a negative test when the disease is absent. Positive and negative predictive values tell us how likely a person is in having or not having the disease given the test result; they are dependent on the prevalence of the disease. Likelihood ratios are alternate statistic very useful to the clinicians for summarizing diagnostic accuracy of the tests. Results of some diagnostic tests are on a numerical scale requiring a cut-off point to separate negative and positive result. ROC curve is used to select the optimum cut-off value. Cohen’s kappa statistic is a measure of inter-rater agreement for categorical variables; it takes into account the agreement occurring by chance. Diagnostic tests must be both accurate and precise. Reference range is a range of expected measurements obtained by repeatedly conducting the test in non-diseased individuals. Pretest probability is the probability of a patient having the disease, in clinician’s opinion, before he asks for diagnostic test. After the test result this probability changes and is now called the posttest probability. Bayes theorem combines the pretest probability with likelihood ratio of the test to arrive at the posttest probability of the disease. The calculations involved in arriving at posttest probability can be avoided by using Fagan’s nomogram. McGee has simplified calculations to give approximate values that serve the clinicians. Lastly, clinician should order a diagnostic test only if the test result is likely to affect the management of the patient.

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