What is the initial test for diagnosing Hepatitis B (Hep B)?

Initial Testing for Hepatitis B Diagnosis

The initial test for diagnosing Hepatitis B infection is the Hepatitis B Surface Antigen (HBsAg) test, which is the cornerstone of hepatitis B screening and should be performed using an FDA-approved test with confirmatory testing for initially reactive results. 1

Understanding HBsAg Testing

HBsAg is the hallmark of HBV infection and is the first serological marker to appear in acute hepatitis B. Its presence indicates active HBV infection, which could be either acute or chronic:

• A positive HBsAg result indicates the presence of active hepatitis B virus infection 1
• HBsAg is detectable in serum 30 days on average after infection (range: 6-60 days) 2
• Persistence of HBsAg for more than 6 months suggests chronic HBV infection 2

Additional Testing After Positive HBsAg

If HBsAg is positive, additional testing should be performed to determine the status of infection:

1. IgM anti-HBc (Hepatitis B core IgM antibody):

   • Helps distinguish between acute and chronic infection 1
   • Positive in acute infection and typically persists for up to 6 months 2
   • May be positive during flares of chronic infection 2

2. Additional serologic markers:

   • HBeAg and anti-HBe: Indicate level of viral replication 2
   • HBV DNA: Quantifies viral load 2
   • Anti-HBs: Indicates recovery or immunity from vaccination 2
   • Total anti-HBc: Indicates previous or ongoing infection 2

Interpretation of Serologic Patterns

• Acute HBV infection: Positive HBsAg and IgM anti-HBc 2
• Chronic HBV infection: Positive HBsAg for >6 months, negative IgM anti-HBc (or positive total anti-HBc) 2
• Resolved infection: Negative HBsAg, positive anti-HBs, positive total anti-HBc 2
• Vaccine-induced immunity: Negative HBsAg, positive anti-HBs, negative anti-HBc 2
• "Window period": Negative HBsAg, negative anti-HBs, positive IgM anti-HBc 2

Laboratory Assessment of Disease Activity

After confirming HBV infection with HBsAg, assessment of disease activity should include:

• Liver function tests: AST, ALT, bilirubin, albumin, prothrombin time 2
• Viral replication markers: HBeAg/anti-HBe, HBV DNA quantification 2
• Tests for coinfection: anti-HCV, anti-HDV (in high-risk individuals), anti-HIV 2

Common Pitfalls to Avoid

• Failing to perform confirmatory testing for initially reactive HBsAg results 1
• Not testing high-risk individuals regardless of vaccination history 1
• Misinterpreting isolated anti-HBc positivity, which may indicate occult HBV infection 1
• Relying solely on ALT levels without viral markers, as normal ALT doesn't exclude significant disease 2
• Overlooking the need for HBV DNA testing in HBeAg-negative patients 2

Populations That Should Be Screened

• Persons born in regions with HBsAg prevalence >2% 2, 1
• Men who have sex with men and injection drug users 2, 1
• Household contacts and sexual partners of HBV-infected individuals 2
• Persons with HIV infection 2, 1
• Pregnant women 1
• Persons with persistently elevated liver enzymes 1
• Hemodialysis patients 1

By following this testing algorithm starting with HBsAg, clinicians can accurately diagnose hepatitis B infection, determine its chronicity, assess disease activity, and guide appropriate management to reduce morbidity and mortality from this potentially serious infection.