Question

An ELISA (Enzyme-Linked Immunosorbent Assay) for Hepatitis C has 95 percent sensitivity and 90 percent specificity. What is an ELISA? Write a statement and use numbers to say what the stated sensitivity and specificity numbers mean in terms of the assay’s detection of true positive samples and what the false-positive results will likely be.

Answer 1

ELISA- Enzyme Linked Immunosorbent Assay or EIA

ELISA is highly sensitive (0.01 ng) serological assay used to detect presence of antibodies or antigens in the blood. The method makes use of antibodies labelled with a suitable enzyme (eg; horse radish peroxidase; HRP). Thereafter there is specific interaction of labelled antibody to their counterparts (antigen/ antibody). On adding substrate of the enzyme (HRP- ABTS+H2O2), there is production of a colored product which can further be quantified using Elisa reader.

Sensitivity and specificity

The probability that the Elisa screening test will be positive among the individuals that are diseased is the sensitivity of a test.

While, specificity is the probability that the Elisa test would be negative among the individuals that are not having the disease.

False positive- when an individual is not diseased but still the test reports are positive for a particular disease, then the results are said to be false positive.

Given- An ELISA (Enzyme-Linked Immunosorbent Assay) for Hepatitis C has 95 percent sensitivity and 90 percent specificity.

True positive = no. of total individuals screened * sensitivity % divided by 100

                        = 100 * 95 /100 = 95

Therefore, 95 individuals are true positive for hepatitis C.

This means that 95 percent of the samples are true positive

90 % specificity means 90% patients are without the disease and are true negative (TN).

Specificity = TN/TN+FP

90/100 = 90/90+FP

0.9 = 90/90 +FP

90+FP = 90/0.9

90 + FP = 100

FP = 100-90 = 10

False positive = 10%

10% would be false positive.