Division Analysis of results:X1

Using the software package print out the optical density (OD) values, and the percentage inhibition (PI) for test and control sera based on the mean value recorded in the antigen control wells. The date expressed as OD and PI values are used to determine whether the test has performed within acceptable limits. The upper control limits (UCL) and lower control limits (LCL) for the Mab control (antigen plus Mab in the absence of test sera) are between OD values 0,4 and 1.4. Any plate that fails to conform to the above criteria must be rejected.

If a computer software package is not available print out the OD values using the ELISA printer. Calculate the mean OD value for the antigen control wells, which is equivalent to the 100 % value. Determine the 50 % OD value and manually calculate the positivity and negativity of each sample.

Percentage inhibition (PI) value = 100 – (OD of each test control/Mean OD of Cm) × 100.

The duplicate negative control serum wells and the duplicate blank wells must record PI values between + 25 % and – 25 %, and between + 95 % and + 105 %, respectively. Failure to be within these limits does not invalidate the plate but does suggest that background colour is developing. The strong and weak positive control sera must record PI values between + 81 % and + 100 %, and between + 51 % and + 80 %, respectively.

The diagnostic threshold for test sera is 50 % (PI 50 % or OD 50 %). Samples recording PI values >50 % are recorded negative. Samples that record PI values above and below the threshold for the duplicate wells are considered doubtful; such samples may be re-tested in the spot test and/or titration. Positive samples may also be titrated to provide an indication of the degree of positivity.

Visual reading: Positive and negative samples are easily discernible by eye; weakly positive or strong negative samples may be more difficult to interpret by eye.