Describing the performance of a diagnostic test

Dr. Amany R.AboseoudProf. of Community Medicine

Why we need diagnostic tests? To differentiate between healthy and diseased persons

What are the types of diagnostic tests? Qualitative : +ve / -ve as presence of abnormality or fracture Quantitative: the cut-off level above or below which we consider disease. (positivity criterion, critical value, referent value)

Cultural, (subjective opinion)

percentiles

Presence of risk factors

Therapeutic method

Predictive value

Normal distribution

curve

How to select a cut-off level (or critical value)?

o Not scientific, depends on culture and beliefs of citizens.

o Depends on subjective opinion of people: as, disfigurement, old age, obesity etc..

o In some countries, wasting of children is normal while cachexia is abnormal

o In rural areas of Egypt in the thirties, presence of blood in urine was normal(because bilharsiasis was very common)

Cultural or subjective method

Any value below or above: mean +/- 1.96 SDis considered abnormal provided that there is population mean & SD

values (obtained by examination of large group of normal individuals). Advantage: It is simple method Disadvantages: If the diagnostic test results do not fit Gaussian distribution, so normals

not constitutes 95% Normal persons in the test under study may be abnormal in other tests

(no 100% normals) Individual readings may change within the normal range which denotes

pathology(SBP from 90 to 110) Abnormal to that range may be normal in other subgroup(normal height

for a rural group may be short in another urban subgroup)

Normal distribution curve

The same as normal distribution but depending on 5 or 3 percentile values as the lowest permissible limits for normal. 95 or 97 percentile values as the highest permissible limits.

Simple method Disadvantages: same as NDC

Percentile method

To define who will take treatment and who will be under survillance. Treatment of hypertension is considered if BP >120\80 which was in the past 140\90

Needs many experimental studies to know the benefit of early treatment or changing the cut-off level.

Therapeutic method

To categorize individuals into diseased or not according to the risk factors they have

Individuals with +ve family history of diabetes, obese, live sedentary life (all are risk factors for developing diabetes) can be put in one group against those who have no risk factors.

Smoking is a risk factor for cancer lung. But not all cancer cases are smokers or not all smokers develop cancer lung so smoking can not be diagnostic test.

Risk factor method

The most clinically applied method There must be gold standard to assess the predictive

value of the diagnostic test (knowing patients & healthy)

Make 2 x 2 table to calculate sensitivity, specificity and predictive values of test

Predictive value can be done for risk factors, new test for diagnosis or choosing a cut-off level

Changes done in these values depend on the aim of test and its use in detection of normal or diseased persons

Predictive value method

Disadvantages: Sometimes there is no gold standard The gold standard available is worse than the new test

so the validity will be misleading There is relation between predictive value of test and

the prevalence of disease. The higher the prevalence the higher will be the PVP (if sensitivity & specificity are the same for that test). Use back calculation method to know the PVP for a test in another population who has lower prevalence rate of the disease.

Measures that should be taken in selection of a new test: 1- to compare its results to a gold standard 2- to be blind testing (investigator does not know that

this result if for patient or healthy) 3- to be reliable (repeat the test 3 times and take the

mean value. 4- standardize the measuring scale before testing

(apparatus or ruler) 5- take accurate reading (not approximate) 6-test large samples with different reading scales

For being more accurate use more than one test for diagnosis (to overcome low sensitivity, predictive values).

1- Parallel testing: to order more than one test from a blood sample (as in check up, emergency test). Any test positive means a disease. (Sensitivity increases) used if tests are cheap, no serious condition if false positive

2- Serial testing: if the initial test is +ve do next and so on (Low sensitivity). Used in no emergency, expensive or risky tests. Begin with the most specific test

Multiple testing

If 2 tests were performed for diagnosis of heart disease

Sensitivity of parallel tests= 15+20+60=95\100 Sensitivity of series tests=60/100 Specificity of parallel=60/100 Specificity of series=20+15+60=95/100

Example:

Test result serial parallel +ve cases -ve cases

T1+ve,T2 -ve -ve +ve 15 20

T1-ve,T2+ve -ve +ve 20 15

T1+ve,T2+ve +ve +ve 60 5

T1-ve,T2-ve -ve -ve 5 60

Total 100 100

It is graphic presentation of the relation between sensitivity & specificity of diagnostic test.

Uses: 1- Choose the best cut-off level for a test 2- Comparison between two tests

Receiver operator characteristic (ROC) curve

specificity

Sensitivity

The best cut-off level is that close to upper left corner

ROC curve