orientation and perform dilutions down the columns instead. Similarly, the starting dilution, serial dilution factor, incubation times, and choice of U or V-bottom plate can depend on the specific laboratory. Advantages HA and HI have the advantages that the assays are simple, use relatively inexpensive and available instruments and supplies, and provide results within a few hours. The assays are also well established in many laboratories around the world, allowing some measure of credibility, comparison, and standardization. Limitations Optimal and reliable results require controlling several variables, such as incubation times, red blood cell concentration, and type of red blood cell. Non-specific factors in the sample can lead to interference and incorrect titer values. For example, molecules in the sample other than virus-specific antibodies can inhibit agglutination between virus and RBCs, as well as potentially blocking antibody from binding to virus. Receptor-destroying enzymes (RDE) are commonly used to treat samples prior to analysis to prevent non-specific inhibition. Analysis of the HA or HI results relies on a qualified individual to read the plate and determine the titer values. The manual interpretation method introduces more opportunities for discrepancies in the assay because results can be subjective and the agreement between human readers is inconsistent. Also, there is no digital record of the plate or titer determinations so the initial interpretation is tedious and commonly done in replicates. The range of potential variables and differences between expert readers can make comparing inter-laboratory results difficult. See also Hemagglutination Virus quantification References Diagnostic virology Microbiology