Accuracy Precision ISO 5725 1 Accuracy consists of

Διαγνωστικοί Έλεγχοι Accuracy - Precision ISO 5725 -1: Accuracy consists of Trueness (proximity of measurement results to the true value) and Precision (repeatability or reproducibility of the measurement) By Pekaje at English Wikipedia - Transferred from en. wikipedia to Commons. , GFDL, https: //commons. wikimedia. org/w/index. php? curid=1862863 By SV 1 XV - Own work, CC BY-SA 3. 0, https: //commons. wikimedia. org/w/index. php? curid=25587770 BS ISO 5725 -1: "Accuracy (trueness and precision) of measurement methods and results - Part 1: General principles and definitions. ", p. 1 (1994)

Διαγνωστικοί Έλεγχοι Accuracy - Precision Low accuracy, poor precision, good trueness Low accuracy, good precision, poor trueness By Dark. Evil - Dark. Evil, based on Image: Accuracy and precision-highprecisionlowaccuracy. gif which is free. , Public Domain, https: //commons. wikimedia. org/w/index. php? curid=1420117

Διαγνωστικοί Έλεγχοι Accuracy - Precision Good accuracy, precision and trueness

Διαγνωστικοί Έλεγχοι Ευαισθησία vs. Εξειδίκευση Ελέγχου True positive: the patient has the disease and the test is positive. False positive: the patient does not have the disease but the test is positive. True negative: the patient does not have the disease and the test is negative. False negative: the patient has the disease but the test is negative.

Διαγνωστικοί Έλεγχοι The sensitivity of a clinical test refers to the ability of the test to correctly identify those patients with the disease. A test with 100% sensitivity correctly identifies all patients with the disease. A test with 80% sensitivity detects 80% of patients with the disease (true positives) but 20% with the disease go undetected (false negatives). A high sensitivity is clearly important where the test is used to identify a serious but treatable disease (e. g. cervical cancer). Screening the female population by cervical smear testing is a sensitive test. However, it is not very specific and a high proportion of women with a positive cervical smear who go on to have a colposcopy are ultimately found to have no underlying pathology.

Διαγνωστικοί Έλεγχοι The specificity of a clinical test refers to the ability of the test to correctly identify those patients without the disease. a test with 100% specificity correctly identifies all patients without the disease. a test with 80% specificity correctly reports 80% of patients without the disease as test negative (true negatives) but 20% patients without the disease are incorrectly identified as test positive (false positives).

Διαγνωστικοί Έλεγχοι Θετική διαγνωστική αξία (Positive predictive value, PPV) ‘How likely is it that this patient has the disease given that the test result is positive? ’ Αρνητική διαγνωστική αξία (Negative predictive value, NPV) ‘How likely is it that this patient does not have the disease given that the test result is negative? ’

Διαγνωστικοί Έλεγχοι Λόγος πιθανοφάνειας (Likelihood ratio) how much more likely is it that a patient who tests positive has the disease compared with one who tests negative. Receiver operator characteristic curves a plot of (1−specificity) of a test on the x-axis against its sensitivity on the y-axis for all possible cut-off points. or the false positive rate of a test is shown on the x-axis against the true positive rate on the y-axis

Διαγνωστικοί Έλεγχοι Fig 1: Receiver operator curves: (A) line of zero discrimination (AUC=0. 5); (B) typical clinical test (AUC=0. 5– 1. 0); perfect test (AUC=1. 0). Clinical tests: sensitivity and specificity Abdul Ghaaliq Lalkhen, MB Ch. B FRCA Anthony Mc. Cluskey, BSc MB Ch. B FRCA Contin Educ Anaesth Crit Care Pain (2008) 8 (6): 221 -223.

Διαγνωστικοί Έλεγχοι Ευαισθησία vs. Εξειδίκευση Ελέγχου the higher the sensitivity, the lower the specificity, and vice versa.

Διαγνωστικοί Έλεγχοι Patients with bowel cancer (as confirmed on endoscopy) Condition positive Fecal occult blood screen test outcome Test outcome positive Test outcome negative Condition negative False positive (FP) = 180 Positive predictive value= TP / (TP + FP) = 20 / (20 + 180) = 10% False negative (FN) = 10 True negative (TN) = 1820 Negative predictive value= TN / (FN + TN) = 1820 / (10 + 1820) ≈ 99. 5% Sensitivity= TP / (TP + FN) = 20 / (20 + 10) ≈ 67% Specificity= TN / (FP + TN) = 1820 / (180 + 1820) = 91% True positive (TP) = 20

Διαγνωστικοί Έλεγχοι -Εφαρμογή Suppose that patients with T 4 values of 5 or less are considered to be hypothyroid: T 4 value Hypothyroid Euthyroid 5 or less 18 1 > 5 14 92 32 Totals: the sensivity is 0. 56 and the specificity is 0. 99 93 Consider patients with T 4 values of 7 or less to be hypothyroid: T 4 value Hypothyroid Euthyroid 7 or less 25 18 7 75 32 93 > 7 Totals: the sensivity is 0. 78 and the specificity is 0. 81

Διαγνωστικοί Έλεγχοι -Εφαρμογή Consider patients with T 4 values of 9 or less to be hypothyroid: T 4 value Hypothyroid Euthyroid 29 54 3 39 32 93 < 9 9 or more Totals: the sensivity is 0. 91 and the specificity is 0. 42. Cutpoint Sensitivity Specificity 5 0. 56 0. 99 7 0. 78 0. 81 9 0. 91 0. 42

Διαγνωστικοί Έλεγχοι -Εφαρμογή True Positives False Positives 5 0. 56 0. 01 7 0. 78 0. 19 9 0. 91 0. 58 Cutpoint The area under the curve is a measure of text accuracy.