Medical Diagnostic Tests: A Review of Test Anatomy, Phases, and Statistical Treatment of Data

Table 8

Standard statistic indicators used to evaluate diagnostic accuracy.

Statistic (Abb)	Formula	Remarks
Sensitivity (Se)	TP/(TP + FN)	(i) The highest the Se, the smallest the number of false negative results; (ii) High Se: (a) a negative result rules-out (SnNOUT); (b) suitable for screening (ruling-out)
Specificity (Sp)	TN/(TN + FP)	(i) The highest the Se, the smallest the number of false-positive results; (ii) High Sp: (a) a positive result rules-in (SpPIN); (b) It is suitable for diagnosis (ruling-in)
Accuracy index (AI)	(TP + TN)/(TP + FP + FN + TN)	(i) Give information regarding the cases with the right diagnosis; (ii) It is difficult to convert its value to a tangible clinical concept; (iii) It is affected by the prevalence of the disease
Youden’s index (J) [159]	Se + Sp − 1	(i) Sums the cases wrongly classified by the diagnostic test; (ii) Assess the overall performance of the test. J = 0, if the proportion of positive tests is the same in the group with/without the disease. J = 1, if no FPs or FNs exist; (iii) Misleading interpretation in comparison of the effectiveness of two tests; (iv) Used to identify the best cutoff on ROC analysis: its maximum value corresponds to the highest distance from diagonal
Positive predictive value (PPV)	TP/(TP + FP)	(i) Answer the question “what is the chance that a person with a positive test truly has the disease?”; (ii) Clinical applicability for a particular subject with a positive test result; (iii) It is affected by the prevalence of the disease
Negative predictive value (NPV)	TN/(TN + FN)	(i) Answer the question “what is the chance that a person with a negative test truly not to have the disease?”; (ii) Clinical applicability for a particular subject with a negative test result; (iii) It is affected by the prevalence of the disease
Positive likelihood ratio (PLR/LR+)	Se/(1 − Sp)	(i) Indicates how much the odds of the disease increase when a test is positive (indicator to rule-in); (ii) PLR (the higher, the better); (a) > 10 ⟶ convincing diagnostic evidence; (b) 5 < PLR < 10 ⟶ strong diagnostic evidence
Negative likelihood ratio (NLR/LR−)	(1 − Se)/Sp	(i) Indicates how much the odds of the disease decrease when a test is negative (indicator to rule-out); (ii) NLR (the lower, the better); (a)  < 0.1 ⟶ convincing diagnostic evidence; (b) 0.2 < PLR < 0.1 ⟶ strong diagnostic evidence
Diagnostic odds ratio (DOR) [160]	(TP/FN)/(FP/TN); [Se/(1 − Se)]/[(1 − Sp)/Sp]; [PPV/(1 − PPV)]/[(1 − NPV)/NPV]; PLR/NLR	(i) High DOR indicates a better diagnostic test performance (ranges from 0 to infinite). A value of 1 indicates a test not able to discriminate between those with and those without the disease; (ii) Combines the strengths of Se and Sp; (iii) Useful to compare different diagnostic tests; (iv) Not so useful when the aim is to rules-in or rules-out; (v) Convenient indicator in the meta-analysis
Posttest odds (PTO); Posttest probability (PTP)	Pretest odds (prevalence/(1 − prevalence)) × LR; PTO/(PTO + 1)	(i) Gives the odds that the patient has to the target disorder after the test is carried out; (ii) Gives the proportion of patients with that particular test result who have the target disorder

All indicators excepting J are reported with associated 95% confidence intervals; ROC = receiver-operating characteristic; patient-centered indicator; TP = true positive; FP = false positive; FN = false negative; TN = true negative; and PPV and NPV depend on the prevalence (to be used only if (no. of subjects with disease)/(no. of patients without disease) is equivalent with the prevalence of the disease in the studied population).