Clinical Chemistry I 0202304 Lecture Notes PDF

Clinical Chemistry I 0202304 Instructor: Mohammad QABAJAH E-mail: [email protected] Week 05&06 Clinical Evaluation of Laboratory Tests 2 Objectives I. State the formulas used in calculating clinical sensitivity, clinical speci...

Clinical Chemistry I 0202304 Instructor: Mohammad QABAJAH E-mail: [email protected] Week 05&06 Clinical Evaluation of Laboratory Tests 2 Objectives I. State the formulas used in calculating clinical sensitivity, clinical specificity, and predictive value of a laboratory test. II. State how the predictive value of a laboratory test is affected by prevalence. III. Construct a receiver operating characteristic plot using data from a diagnostic test study. IV. Interpret the difference plot used in method comparison. Agreement of test results: Accurate diagnosis - Sensitivity and Specificity - Predictive Values - Prevalence - Likelihood Ratio (LR) - Odds Ratio (OR) Review: Gaussian Probability Distribution Sensitivity and Specificity Negative and Positive Source of the False  Reference Method (Gold/Reference Standard) “Best current practice for establishing the presence of a disorder” I. Screening; High sensitivity: (high FP, low FN) ̵ Pick up all positive results for further diagnosis ̵ Make sure nobody is missed II. Diagnostic; High specificity: (low FP, high FN) ̵ Exclude all the people who do not have the disease ̵ Make sure nobody is picked Screening Vs Diagnostic: Run the diagnostic test on all positive screened Sensitivity and Specificity # Patients with # Patients with Positive test result Negative test result # Patients with disease True Positive (TP) False Negative (FN) # Patients without disease False Positive (FP) True Negative (TN) Sensitivity and Specificity Sensitivity: The proportion of subjects with disease who have a positive laboratory test result 𝑇𝑃 𝑇𝑃 Sensitivity = 𝑇𝑃+𝐹𝑁 , % Sensitivity = 𝑇𝑃+𝐹𝑁 X 100 # Patients with # Patients with Positive test result Negative test result # Patients with disease True Positive (TP) False Negative (FN) # Patients without disease False Positive (FP) True Negative (TN) Sensitivity and Specificity Specificity: The proportion of subjects without disease who have a negative laboratory test result. 𝑇𝑁 𝑇𝑁 Specificity = 𝐹𝑃+𝑇𝑁 , % Specificity = 𝐹𝑃+𝑇𝑁 X100 # Patients with # Patients with Positive test result Negative test result # Patients with disease True Positive (TP) False Negative (FN) # Patients without disease False Positive (FP) True Negative (TN) IF IF Spec. = 95% Sen. = 90% # of tested -ve = 100 # of tested +ve = 100 TN ? TP ? FP ? FN ? Predictive Values Positive Predictive Value (PV+): Negative Predictive Value (PV-): The fraction with a positive test who The fraction with a negative test who have the disease don’t have the disease 𝑇𝑃 𝑇𝑁 𝑃𝑉+ = 𝑃𝑉− = 𝑇𝑃 + 𝐹𝑃 𝑇𝑁 + 𝐹𝑁 # Patients with # Patients with Positive test result Negative test result # Patients with disease True Positive (TP) False Negative (FN) # Patients without disease False Positive (FP) True Negative (TN)  Prevalence: The frequency of disease in the population examined Screening and prevalence Population 1,000,000 Prevalence 1 in 10 (0.1) People with disease 100,000 (0.1 X 1,000,000) Example: Sensitivity, Specificity, PV+, and PV- Calculate: Sensitivity, Specificity, PV+, and PV- Positive Result Negative Result Have disease 100 5 Don’t have disease 7 95 Likelihood Ratio (LR) The Likelihood Ratio (LR) is the likelihood that a given test result would be expected in a patient with the target disorder compared to the likelihood that that same result would be expected in a patient without the target disorder. Positive Likelihood Ratio (+LH) = Sensitivity/(1-Specificity) Negative Likelihood Ratio (-LH) = (1- Sensitivity)/Specificity Likelihood Ratio (LR) Test is GOOD if: +LH > 10; -LH < 0.1 19 20 21 Odds Ratio (OR) The probability of the presence of a specific disease divided by the probability of its absence (+LH) = Sensitivity/(1-Specificity) (-LH) = (1- Sensitivity)/Specificity 24 25 See YOU Next Lecture