• Confirmed infection with hepatitis A is defined by:
• An acute illness with a discrete onset of symptoms (see Clinical features), and
• Jaundice or elevated serum aminotransferase levels (see Investigations), and
• Laboratory-confirmed IgM antibodies to hepatitis A virus (see Assay results).
• Probable infection is defined by:
• An acute illness with a discrete onset of symptoms, and
• Its occurrence in a person who has an epidemiological link with someone with laboratory-confirmed hepatitis A (for example a household or sexual contact during the 15–50 days before the onset of symptoms).

Recommendations on definitions of confirmed and probable infection are based on guidance for the prevention and control of hepatitis A infection produced by the Health Protection Agency [HPA, 2009a].

• The clinical features of acute hepatitis A are common to all forms of viral hepatitis. Distinguishing hepatitis A from hepatitis due to other causes is clinically impossible, but suspicion may be increased by a history of a specific exposure or risk.
• The clinical features of the prodromal phase of hepatitis include:
• Flu-like symptoms (such as general fatigue, malaise, joint and muscle pain, low-grade fever [up to 39°C]).
• Gastrointestinal symptoms (such as anorexia, nausea, vomiting, and abdominal or right upper quadrant discomfort).
• Sometimes, the person has headache, cough, pharyngitis, constipation, diarrhoea, itchiness, and urticaria.
• Usually, there are no specific signs on examination.
• The clinical features of the icteric phase of hepatitis include:
• Jaundice (yellowing of the skin and eyes, pale stools, and dark urine).
• Pruritus (present in 40% of those with jaundice).
• Fatigue, anorexia, nausea, and vomiting — although symptoms often improve once jaundice occurs.
• Hepatomegaly (85%), splenomegaly (15%), lymphadenopathy (5%) and hepatic tenderness are often present on examination.
• The clinical features of the convalescent phase of hepatitis include:
• Malaise.
• Hepatic tenderness.
• Abnormalities of liver function.

The clinical phases of hepatitis A are described in Guidance for the prevention and control of hepatitis A infection from the Health Protection Agency [HPA, 2009a], a review published by the National Travel Health Network and Centre [NaTHNaC, 2009], a review by the World Health Organization [WHO, 2009], the textbook Principles and practice of infectious diseases [Curry and Chopra, 2009; Wasley et al, 2009], a review of combined hepatitis A and hepatitis B vaccination for travellers [Spira, 2003], guidelines published by the British Association for Sexual Health and HIV [BASHH, 2008], a review article [Peyrin-Biroulet et al, 2008], draft guidance of National Standard Methods from the Health Protection Agency [HPA, 2010a], and in Department of Health guidance in the 'Green Book' [DH, 2009a].

• Hepatitis screen
• Screening for hepatitis usually tests for past or present infection with hepatitis A, B, or C.
• In some geographical areas, hepatitis A may not be routinely included during hepatitis screening for people above a certain age. Therefore, if hepatitis A is suspected, this should be stated on the request form.
• Liver function tests
• Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels are greatly increased (usually higher than 1000 IU/L).
• Bilirubin may be elevated (up to 500 micromoles/L).
• Test results return to normal within 12 months.
• Prothrombin time
• May be prolonged. Prolongation by 5 seconds or more suggests hepatic decompensation is developing, and requires admission.

Recommendations and information about serological tests are based on expert opinion in the National standard method on acute infective hepatitis [HPA, 2010b], the draft National standard method on hepatitis from the Health Protection Agency [HPA, 2010a], guidelines from the British Association for Sexual Health and HIV [BASHH, 2008], the textbook Principles and practice of infectious diseases [Curry and Chopra, 2009], a review by the World Health Organization into the global prevalence of hepatitis A virus infection and susceptibility [WHO, 2009], and the opinion of a CKS expert reviewer.

• The recommendation to state any suspicion of hepatitis A on the request form when ordering a hepatitis screen is based on expert opinion (in a personal communication from a local laboratory).

• Hepatitis A virus immunoglobulin M (HAV-IgM)
• If the test result is negative:
• Recent hepatitis A infection is unlikely.
• Repeat the test 1 week later if the person is in the first 7–10 days of symptoms, to exclude a false-negative result.
• If the test result is reactive:
• Acute hepatitis A infection is likely.
• HAV-IgM is detectable about 3 weeks after exposure, increases in titre over 4–6 weeks, then declines to non-detectable levels (generally within 6–12 months of infection).
• Interpret a reactive result with caution in elderly people, who are more likely to have had hepatitis A in childhood (false-positive IgM results are more common in elderly people).
• If the test result is reactive but probable non-specific IgM reactivity:
• Repeat the test.
• Hepatitis A virus immunoglobulin G (HAV-IgG)
• HAV-IgG is detectable 5–10 days after the onset of symptoms and persists, conferring lifelong immunity.
• The presence of HAV-IgG indicates current or past infection, or immunity from previous vaccination.

Recommendations for interpretation of hepatitis A virus antibody test results are mainly based on the National standard method for hepatitis A virus acute infection serology produced by the Health Protection Agency [HPA, 2007], expert opinion in a review article [WHO, 2000], and the UK National guideline on the management of the viral hepatitides A, B, and C, published by the Clinical Effectiveness Group of the British Association for Sexual Health and HIV [BASHH, 2008].

• The recommendation to repeat HAV-IgM levels a week later, if the person has had an initial negative result but is in the first 7–10 days of symptoms (to exclude a false-negative result) is based on the opinion of a CKS expert reviewer.

• The causes of acute hepatitis include viruses, bacteria, drugs, poisons, and pregnancy.
• Viruses
• Hepatitis virus A, B, C, D, and E.
• Epstein-Barr virus (infectious mononucleosis).
• Cytomegalovirus.
• Yellow fever virus.
• Ebola and Marburg viruses (African haemorrhagic fever).
• Other identified viruses.
• Bacteria
• Leptospira icterohaemorrhagiae (Leptospirosis).
• Coxiella burnetii.
• Drugs
• Analgesics
• Paracetamol.
• Aspirin.
• Nonsteroidal anti-inflammatory drugs.
• Cardiac drugs
• Methyldopa.
• Amiodarone.
• Anaesthetics
• Halothane (rare).
• Psychotropics
• Monoamine-oxidase inhibitors.
• Phenothiazines.
• Others
• Sodium valproate.
• Oestrogens.
• Rifampicin.
• Isoniazid.
• Flucloxacillin.
• Poisons
• Carbon tetrachloride.
• Amanita phalloides (a fungus that can be mistaken for the edible mushroom).
• Pregnancy

The differential diagnosis is based on information from a textbook, Color atlas and text of clinical medicine [Forbes and Jackson, 2003].