Laboratory-based reflex HDV testing refers to a testing algorithm in which individuals have only a single clinical encounter and one blood draw or specimen for an initial laboratory-based HDV antibody test. If the individual’s sample for HBsAg screening in the laboratory is positive, then the same or a duplicate specimen is automatically used for a prompt reflex laboratory-based antiHDV test. The results returned to the individual and health-care worker are therefore for both the HBsAg result and, if positive, the anti-HDV result. Further reflex testing for the presence of HDV RNA may also be performed at the laboratory level for individuals testing positive for anti-HDV. Currently, reflex testing for anti-HDV can only be implemented through a laboratory-based testing strategy since HDV RDTs are not commercially available to enable clinic-based reflex testing. The successful implementation of a universal or risk-based HDV antibody testing approach and testing strategy is underpinned by the following considerations.

• 1.Development of HDV testing policy and guidelines. Countries should develop national testing policy and guidelines that incorporate their selected HDV testing approach and routine reflex HDV antibody testing. These can be integrated into HBV testing guidelines and/or in a stand-alone HDV clinical management guideline.
• 2.Procurement and supply of quality-assured assays. Access to low-cost and well-performing quality-assured HDV serological and molecular assays is critical in ensuring the reliability of HDV diagnosis and the success of HDV testing strategies. Assays should meet minimum quality, safety and performance standards. 3. Enabling testing environment and enhanced laboratory capacity. Implementing a universal HDV antibody testing approach, including routine reflex testing, requires expanded laboratory capacity, which may be challenging in resource-limited settings. Testing environments should operate in accordance with quality management systems and have access to qualified, proficient laboratory personnel trained specifically in performing the various assays with adequate and supervisory support. Updating electronic laboratory order forms for HBsAg and HDV serology to list reflex-only testing options should be considered, and laboratory guidance for HDV antibody (and, if possible, HDV RNA) reflex testing should be developed. Planning for additional costs should be considered as needed, such as additional tubes, transport and storage.
• 4.Education and training of health-care providers and laboratory personnel. Implementation of HDV testing will require training clinic phlebotomy and laboratory personnel on new procedures for collecting and processing specimens for anti-HDV serology and HDV RNA. Health-care workers should be able to counsel individuals who are screened and advise appropriate action on the results, both positive and negative.
• 5.Assuring accurate diagnosis. The variability in diagnostic performance of various HDV RNA assays must be considered when performing HDV testing and in clinical practice. a. Commercial assays must be performed in accordance with the manufacturer’s instructions, and any modification of the original protocols – including in-house protocols

• must be further validated against the reference standard. b. Potential non-detection or underestimation of HDV RNA levels among populations or individuals with certain genotypes – especially African subgenotype 1 and African genotypes 5 to 8 – must be considered in clinical practice and individuals adequately counselled on their test result and linked to care. c. Standardized real-time NATs for HDV RNA are recommended to assure accurate diagnosis of active (viraemic) HDV infection and to monitor antiviral therapy. HDV RNA should be quantified by a reference laboratory using well-standardized validated assays, and the results should be given in IU/mL to improve precision and comparability across laboratory test systems (15).

• 6.Counselling and linkage to care. Individuals must be provided with adequate pre- and post-test counselling and linkage to care, and clinical pathways should be established for individuals diagnosed with HDV infection for linkage to care and treatment and monitoring for liver-disease complications. Additionally, individuals should be counselled on preventing transmission to household members, sexual partners and in the case of people who inject drugs, injecting partners, by promoting preventive interventions, such as HBV vaccination and harm reduction.
• 7.Retesting for anti-HDV antibodies for HBsAg-positive individuals. This should be considered where clinically indicated, such as in cases of aminotransferase flares or acute decompensation of chronic liver disease and for those remaining at risk of infection (15). Evidence also suggests that HDV RNA levels for individuals with HDV infection may fluctuate over time, and this therefore needs to be considered in clinical practice and retesting considered to confirm ongoing active infection or possible clearance of HDV RNA (16,17).
• 8.Monitoring and evaluation systems. Monitoring and evaluating HDV testing approaches (universal and risk-based) and testing strategies (including laboratory-based HDV reflex testing) and providing reports to providers and laboratory managers are essential for improving quality. A database of people with HDV can be established based on simple individual clinical or laboratory records and integrated into HBV national reporting systems to monitor testing, diagnosis and treatment indicators.