| ORIGINAL ARTICLE | | | | | Year : 2002 | Volume : 20 | Issue : 4 | Page : 200-205 | | | Evaluation of an indigenous western blot kit for human immunodeficiency virus
V Lakshmi, S PD Ponamgi
Department of Microbiology, Nizam's Institute of Medical Sciences, Punjagutta, Hyderabad - 500 082, Andhra Pradesh, India
Correspondence Address:
Department of Microbiology, Nizam's Institute of Medical Sciences, Punjagutta, Hyderabad - 500 082, Andhra Pradesh, India
PURPOSE: The Western Blot test is considered a gold standard test for the confirmation of an ELISA and/or rapid assay screened reactive sample in the diagnosis of HIV infection, especially in the low risk population. In this study, an indigenously developed HIV W. Blot kit (J.Mitra & Co., New Delhi, India) was compared for its performance characteristics with a widely used Western Blot kit, HIV Blot 2.2 (Genelabs, Singapore). Antigens of both HIV-1 and the indicator antigen gp36 of HIV-2 are included in the strips. METHODS: A panel of 150 clinical serum samples was used in the evaluation. All the sera were tested simultaneously by both the kits. RESULTS: The HIV W. Blot kit had high performance characteristics (100% sensitivity and 100% specificity), like the HIV Blot 2.2. The test procedure was easy to perform. There was clear delineation of the bands. CONCLUSIONS: The interpretation of the results on the HIV W. Blot was less prone to subjective errors. The test gave positive bands at even very high serum dilutions in the test kit. This fact indicates that HIV W. Blot probably has a potential application in early phases of infection, when the antibody concentrations are still very low.
How to cite this article:
Lakshmi V, Ponamgi S P. Evaluation of an indigenous western blot kit for human immunodeficiency virus. Indian J Med Microbiol 2002;20:200-5 |
How to cite this URL:
Lakshmi V, Ponamgi S P. Evaluation of an indigenous western blot kit for human immunodeficiency virus. Indian J Med Microbiol [serial online] 2002 [cited 2014 Mar 6];20:200-5. Available from: http://www.ijmm.org/text.asp?2002/20/4/200/6956 |
Laboratory assessment and serological diagnosis and confirmation is the only method of identifying Human Immunodeficiency Virus (HIV) infected patient. A well established, reference gold standard test must be available to the laboratory, the results of which should correlate with the actual status of the patient.[1]
Essentially, any repeatedly positive result by Enzyme Linked Immuno Sorbent Assay (ELISA) or another rapid screening method for HIV must be confirmed by a more specific assay such as Western Blot (WB) test. The current strategy for a serological confirmation of infection with HIV is the WB assay[2] especially in low risk groups.
The WB detects antibodies to specific denatured HIV-1 proteins including core (p17, p24, p39 and p55), polymerase (p31, p51, p66), and envelope (gp41, gp120 and gp160) proteins using the principle of an enzyme immunoassay.[3],[4],[5] At present, there are several commer-cially available WB kits in the international market, that can confirm an HIV infection (HIV 1 &/or 2).
The objective of the present study was to evaluate the performance characteristics of an indigenously developed WB assay for HIV-1 (HIV W. Blot) from J.Mitra and Co. Ltd., New Delhi with the FDA approved kit (HIV Blot 2.2) of Genelabs, Singapore. The authors have no financial interest in either of the kits.
| ~ Materials and Methods | |  |
150-serum specimens (serum panel) from the Department of Microbiology, Nizam's Institute of Medical Sciences were included in the study. The serum panel included is shown in [Table - 1]. The sera were stored at - 200 C until tested.
Differentiation between HIV-1 & 2 was done by DOT blot assay (HIV TRIDOT, J.Mitra & Co. Ltd., New Delhi).
The WB kits used in the study were essentially manufactured by a similar process, using partially purified HIV-1 viral antigen. The antigen used in the W. Blot was produced from HIV-1 cell line, which was developed from an Indian strain, HIV-1 subtype C. The antigens were then electrophoresed and electrotransblotted on to a nitrocellulose paper.
The Synthetic antigen of HIV-2 (gp36) was applied as a line blot on the strip.[6],[7] The antigen band patterns in each type of kit is shown in [Table - 2]. An internal or inbuilt control band (serum control) was also incorporated in each of the strips. This was to ensure quality of the test procedure. The conjugate used in both kits is an antihuman IgG (raised in goat in HIV Blot 2.2 and rabbit in HIV W. Blot) conjugated with alkaline phosphatase, with its specific substrate 5 bromo, 4 chloro, 3 indolyl phosphate + nitroblue tetrazolium (BCIP / NBT). The test duration for the kits is either 1 hour (HIV Blot 2.2) or 2 hours (HIV W. Blot) or overnight (both kits) for sample incubation, 1-hour conjugate and 5 - 15 minutes of substrate incubation in both the test kits. Depending on color development of the bands the last step should be performed.[6],[7]
Both kits provide for the detection of HIV Sub group 'O'
All the 150 sera were tested by both the WB kits. The tests were performed as per the manufacturers' instructions[6],[7] and were performed under identical conditions by the same investigator, using the overnight procedure. The readings were taken visually and the reactive indices were calculated as given in the kit literature of HIV Blot 2.2.
| ~ Results | | |
All the tests were validated by the presence of the serum control band.
Positive WB
Positive WB was considered by the presence of atleast two of the three bands representing the envelope bands (gp160, gp120, and gp41 of HIV-1 and gp36 of HIV-2) and gag gene p24.[8]
The WB results of 47/50 of the ELISA HIV-1 reactive samples were similar by both the kits. Three sera had discrepant results at the minor band positions of p31, p24, and p51/55. Since the reactive indices for these sera were >2.5 in both the strips, they were considered positive.
The HIV W. Blot was able to clearly detect the HIV-2 indicator band in all the 10 sera as detected by HIV BLOT 2.2.
HIV W. Blot detected 10 HIV 1 + 2 (dual reactive) samples detected by Blot 2.2. The antigen band patterns were correlating in both the kits in all these samples.
Negative WB
Absence of all the bands except control band was considered as negative test. Presence of any non-specific band or isolated p17 in HIV Blot 2.2 and p55 in HIV W. Blot was considered negative.[6],[7]
Fifty out of 63 ELISA HIV-1 & 2 non-reactive samples were negative by both the WB kits. These blots had no bands corresponding to the HIV antigens.
Indeterminate WB
Westernblot results that could not be classified as negative or positive were categorized as indeterminate.[8]
The HIV W. Blot interpreted 10 ELISA HIV-1 and 2 reactive sera that gave indeterminate result in HIV Blot 2.2 also as indeterminate. The bands obtained were also the same.
WB patterns of the remaining 13/63 ELISA HIV-1 and 2 non-reactive panel showed an indeterminate result [Table - 3]. Eight of these samples were from patients with autoimmune disorders and the rest five were from patients with other viral infections. While 4/13 of the ELISA non-reactive sera were indeterminate in HIV W. Blot, all (13/13) these sera showed indeterminate results with HIV Blot 2.2 (reactive indices in between 0.25 and 2.5) in our study. Isolated positive band at antigen sites like gp160 (n=2), gp120 (n=1), gp41 (n=2), p31 (n=3) and p24 (n=3) and gp41 + p31 bands were observed (n=2) in HIV Blot 2.2. Further the three samples that showed a p24 band were tested for p24 antigen by ELISA (J&J) and were found negative.
Staining Intensity
To evaluate the intensity of the staining of the antigen bands, seven reactive serum samples that gave a weak OD (within the gray zone of the test run) in the ELISA at 1:100 dilution were used. The two WBs were then performed using 1:100 and 1:1000 dilutions of these sera. In the WB procedure these sera underwent a further 1000 fold dilution. Thus at a final dilution of 1:10[5] and 1:10[6] in the WB, all the seven sera gave a positive result by the HIV W. Blot while in the HIV Blot 2.2 they gave an indeterminate result [figure].
| ~ Discussion | | |
The development, introduction, and use of the newly arriving HIV antibody detection assays are a dynamic process. It is essential that these assays are thoroughly evaluated before use at different levels and conditions. The sensitivity, specificity, predictive values, false positive ratio, ease of performance and interpretation, suitability for use in small collection centers and the test efficiency need to be evaluated using different combinations of assays.[9]
The recommended HIV-1 testing algorithm comprises of an initial screening with an approved enzyme linked immunoassay (ELISA). The repeatedly reactive ELISAs are confirmed with a licensed supplemental test.[5]
Because of the distinct staining patterns of polypeptides, WB test is considered a gold standard test for the confirmation of an HIV reactive sample.[11] Although the overall sensitivity and specificity of the WB for detection of antibodies to the various viral proteins are high, there has been substantial debate regarding the interpretive criteria. This is because, the final result and the interpretation depend on the several types of antigen ligands (lysate, recombinant or synthetic) that are used for blotting. As a result, the definition of what should constitute a positive WB has been subjected to a considerable debate. Various regulatory bodies have given the specific criteria for the interpretation of a positive WB result. Majority of the centers interpret the results as per the recommendations of the CDC, ASTPHLD or WHO [Table - 4]. The same guidelines have been recommended by NACO, India.[11] We have applied the same recommendations in the interpretation of the WB results in this study. The results were also interpreted based on the reactive indices as indicated in the kit literature of Genelabs HIV Blot 2.2.
The sensitivity of the HIV W. Blot as compared with the ELISA or the HIV Blot 2.2 was 100%, as all the 50 HIV-1 ELISA reactives, were positive by both kits. A further measure of the sensitivity of the kit under evaluation would be to use a weak reactive serum. However, no sero-conversion panels were available with us nor did we get any opportunity to identify a weak reactive sample.
The criteria for a negative WB interpretation specify absence of antigen bands of diagnostic significance. This interpretation is essential because some observed bands may reflect the presence of antibodies to HIV regulatory proteins or may indicate partially processed or degraded viral structural proteins. Furthermore, different WBs (commercial, as well as “in-house” preparations) and different virus-antigen preparations used to prepare WBs may contain different numbers and concentrations of both viral-specific and contaminating cellular proteins that may have unpredictable molecular weights. Presence of such non-specific bands is not taken into consideration and the result is reported negative.
The indeterminate WB results are observed in healthy persons with antibodies that cross-react with specific HIV-1 peptides or recombinant antigens.[12],[13] The proportion of indeterminate results also varies according to the immunoblots used, the prevalence of HIV infection in the population tested, and the interpretive criteria used.[14] While 4/13 of the ELISA non-reactive sera were indeterminate in HIV W. Blot [Table - 3], all (13/13) these sera showed indeterminate results with HIV Blot 2.2 (reactive indices in between 0.25 and 2.5) in our study. The bands were probably due to cross-reactive antibodies[2] as all these samples were from patients with autoimmune diseases and other viral infections. As they were ELISA non reactive all these sera were interpreted as negative for HIV-1 and 2 antibodies.
Thus, the specificity of the HIV W. Blot against the HIV Blot 2.2 was found to be 100% with no false positives (All the 63 ELISA non reactive were interpreted as negative).
This strategy of using both ELISA and supplementary tests further increases the accuracy of results and diagnosis. In principle, any WB kit that gives a high frequency of indeterminate reactivity (the overwhelming preponderance of which represents non-specific binding) is not appropriate as a primary screening tool for the population at large. Its strength is only as a confirmatory assay in the setting of a positive or indeterminate HIV-1 ELISA or initial screening test.[15] The same has been shown in our results using ELISA non reactive serum samples where we obtained indeterminate results on WB. Hence, like any other HIV WB test, the HIV W. Blot is not to be used for primary screening for HIV antibodies.
The intensity of staining of the bands is a valuable measure of standardization of any WB.[16] Since in majority of the laboratories the results of the WB are read visually, there is always a high probability of subjective errors. Higher the intensity of the bands lesser is the visual error.[16] We attempted to evaluate the same feature in both the WB kits by using serial dilution of ELISA HIV-1 reactive serum samples. The results indicate that the HIV W. Blot has a higher staining intensity than the HIV Blot 2.2 at higher dilutions of the antibodies. Hence there is a lesser probability of false negative interpretation with the HIV W. Blot even at lower concentrations of the antibodies. Charles T Hardy, using a scanning densitometry in the reflectance mode, did objective analysis of such a finding.[16]
Based on the results in this study, we conclude that the indigenously developed HIV W. Blot kit had high performance characteristics like the widely used HIV Blot 2.2 of Genelabs. The test procedure is easy to adopt. With clear delineation of the bands, the interpretation of the results on the HIV W. Blot is less prone to subjective errors. The fact that the HIV W. Blot gave a positive result even at very high serum dilutions indicates that it probably has a potential application in early phases of infection, when the antibody concentrations are still very low.
| ~ References | | |
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