Hepatitis B: How Serology Decides When Chronic Infection Is Treated

Chronic hepatitis B is a disease that lab work stages before any drug enters the conversation, and four numbers do most of that work: HBeAg, HBsAg, the liver enzyme ALT, and the amount of virus in blood measured as HBV DNA. Read together, they place a person into a phase, and the phase, not the diagnosis alone, decides whether the standard of care is a daily antiviral or scheduled monitoring. The American Association for the Study of Liver Diseases (AASLD) treats infection as a moving picture rather than a single verdict, which is why two people who both carry the virus can receive opposite advice on the same day. Not everyone with chronic hepatitis B is treated because some phases carry little active liver injury, and the guidance reserves therapy for the states where the immune system and the virus are damaging tissue.

What the serology actually names

Before any threshold matters, serology establishes the situation. The CDC's interpretation framework uses a small panel to separate states that look similar on paper. HBsAg, the surface antigen, marks current infection; when it persists beyond six months, the infection is chronic. Total anti-HBc reflects exposure and stays positive for life, while IgM anti-HBc points to recent, acute infection, and anti-HBs signals recovery or vaccine immunity. So a person who is HBsAg negative, anti-HBc positive, and anti-HBs positive has resolved a past infection, whereas someone HBsAg positive with negative IgM anti-HBc has established chronic disease.

HBeAg adds a second layer. It is a viral protein that loosely tracks with high replication. HBeAg-positive disease and HBeAg-negative disease are graded on different numeric scales, which is why the antigen is not a treat-or-not switch but a coordinate that changes where the thresholds sit.

The phases and their numbers

AASLD anchors staging on two measurements read against reference values: an ALT upper limit of normal of 35 U/L for men and 25 U/L for women, and HBV DNA reported in international units per milliliter.

The immune-tolerant phase is HBeAg positive with very high viral load, often above 1 million and frequently above 10 million IU/mL, but with normal ALT and little inflammation on biopsy. Large virus number, quiet liver.

The immune-active phase is where treatment enters. AASLD defines it as ALT at least twice the upper limit of normal, or clear histologic injury, together with HBV DNA above 20,000 IU/mL when HBeAg is positive, or above 2,000 IU/mL when HBeAg is negative. Both a replication signal and a tissue-injury signal are required.

The inactive phase is HBeAg negative, anti-HBe positive, HBV DNA below 2,000 IU/mL, and persistently normal ALT. Here the guidance recommends scheduled monitoring rather than drugs, because the liver is not under active attack.

Why a positive test is not a prescription

The logic that keeps many people off therapy is that the approved first-line agents, PEG-interferon, entecavir, and tenofovir, suppress the virus well but rarely clear it. Treatment is usually long term, and in the quiet phases the expected benefit is small while the commitment is real. The guidance therefore asks two questions at once: is the virus replicating meaningfully, and is the liver being injured. When the answer to both is yes, therapy lowers the risk of cirrhosis and liver cancer enough to justify itself. When only the viral load is high and ALT stays normal, as in immune tolerance, the historical calculus favored watching.

Two situations override the phase table. Cirrhosis is one: if scarring is present and HBV DNA is detectable, AASLD recommends antiviral therapy regardless of ALT, because the reserve to absorb further injury is already gone. Pregnancy is another: to reduce mother-to-child transmission, the guidance supports starting tenofovir in the third trimester when maternal HBV DNA exceeds 200,000 IU/mL, a threshold aimed at the newborn rather than the mother's staging.

Where the line moved in 2025

The framework is not frozen. The 2025 AASLD/IDSA practice guideline, released in November 2025, widened who should at least be offered treatment. For immune-tolerant patients, it now conditionally suggests therapy for those over age 40, or with significant inflammation (grade 2 or higher) or fibrosis (F2 or greater) on biopsy or noninvasive testing, recognizing that a "tolerant" liver can accumulate quiet damage over decades. AASLD rates that suggestion conditional and the underlying certainty of evidence as very low.

The update also gave weight to the indeterminate, or grey-zone, phase: patients who fit none of the classic boxes and who make up a large share of real clinics. Rather than defaulting to observation, the guideline supports shared decision-making, citing a systematic review in which antiviral treatment in this group was associated with a lower incidence of liver cancer. Quantitative HBsAg and noninvasive fibrosis assessment are folded in as additional tools that inform the same staging question. The direction of travel is toward treating more of the ambiguous middle, while the core principle holds: serology and enzymes define the phase, and the phase, not the label of infection, sets the plan.

For readers, the practical translation is that a single positive hepatitis B result is the beginning of staging, not the end of it, and the follow-up numbers are what a clinician reads to decide between a prescription and a calendar. This article is educational and is not medical advice.