Manual of basic techniques for a health laboratory - libdoc.who.int

342 Manual of basic techniques for a health laboratoryA test run is invalid if the positive control values are less than the calculated cutoffvalues. In such cases, the test run must be repeated.Note: Criteria for testing samples using a western blot vary from laboratory to laboratory.Some laboratories test all samples that give a positive reaction in the ELISA.In some cases a specific request may be made for a western blot even if the ELISAwas non-reactive.11.7.2 Dipstick testPrincipleDipstickIncubateTestsampleDipstick tests were developed for the rapid detection of antigens and antibodiesin human serum. These tests are generally employed in situations wherequick decisions may need to be taken and often require no equipment otherthan that provided in the kits.In the dipstick test for HIV antibody, which is commercially available, a polystyrenestrip is coated with HIV antigen and allowed to react with serum. AnyHIV antibody present will then bind to the HIV antigen. After subsequentincubation with a substrate solution, a coloured spot that indicates the presenceof HIV antibody will develop (Fig. 11.13).WashIncubateWashEnzyme-conjugatedantiserumMaterials and reagents● Timer● Absorbent towels or filter-paper●●Commercially available test kit containing dipsticks, reagents and positiveand negative controlsWeak-positive in-house control serum.MethodFollow the instructions provided in the kit.A positive result is indicated by any colour development on the antibodycoatedspot. A spot should be visible on the positive control and no colourshould be seen on the negative control. Weak-positive in-house controls shouldbe included to help in reading results. The test run is invalid if the resultsobtained using the controls are not as described above.IncubateWash, dry andevaluate visuallySpot = positive reactionNo spot = negative reactionSubstrateFig. 11.13 Dipstick assay for HIVantibody11.8 Tests for hepatitis virus infectionRoutine tests for hepatitis include the use of markers for hepatitis A, B and Cviruses. Hepatitis A is most common in children, especially in nurseries; however,it is not routinely tested for, except in cases of epidemics.Hepatitis B and C viruses are transmitted through blood products, body fluids,contaminated needles and other contaminated materials.Hepatitis B virus has several markers which include:— surface antigen (HBsAg)— antibody to surface antigen (anti-HBs)— envelope antigen (HBeAg)— antibody to envelope antigen (anti-HBe)— antibody to core antigen (anti-HBc).The concentrations of these markers vary during the course of an infection.The antigen markers appear first or earlier on after exposure to the virus.