Still's disease does not affect children under 6 months old.

Hyperimmunoglobulin D syndrome in 50% of cases is associated with mevalonate kinase deficiency which can be measured in the leukocytes.

The diagnosis is based on observing the patient and finding the constellation of symptoms and signs described above. A few blood tests help, by showing signs of long standing inflammation. There is no specific test for the disease, though now that the gene that causes the disease is known, that may change.

Routine laboratory investigations are non specific: anaemia, increased numbers of polymorphs, an elevated erythrocyte sedimentation rate and elevated concentrations of C-reactive protein are typically all the abnormalities found. Lumbar puncture shows elevated levels of polymorphs (20-70% of cases) and occasionally raised eosinophil counts (0-30% of cases). CSF neopterin may be elevated.

The X ray changes are unique and charactistic of this syndrome. These changes include bony overgrowth due to premature ossification of the patella and the long bone epiphyses in very young children and bowing of long bones with widening and shortening periosteal reaction in older ones.

Audiometry shows a progressive sensineural deafness. Visual examination shows optic atrophy and an increase in the blind spot. CT is usually normal but may show enlargement of the ventricles. MRI with contrast may show enhancement of leptomeninges and cochlea consistent with chronic meningitis. EEG shows is non specific with slow waves and spike discharges.

Polymorphs tend to show increased expression of CD10.

MDDS is diagnosed based on systemic symptoms presenting in infants, followed by a clinical examination and laboratory tests (for example, high lactate levels are common) medical imaging, and usually is finally confirmed and formally identified by genetic testing.

Because CAPS is extremely rare and has a broad clinical presentation, it is difficult to diagnose, and a significant delay exists between symptom onset and definitive diagnosis. There are currently no clinical or diagnostic criteria for CAPS based solely on clinical presentation. Instead, diagnosis is made by genetic testing for "NLRP3" mutations. Acute phase reactants and white blood cell count are usually persistently elevated, but this is aspecific for CAPS.

DGUOK, POLG, and MPV17 related forms result in defects to the liver. Liver dysfunction is progressive in the majority of individuals with both forms of DGUOK-related MDS and is the most common cause of death. For children with the multi-organ form, liver transplantation provides no survival benefit.

Liver disease typically progresses to liver failure in affected children with MPV17-related MDS and liver transplantation remains the only treatment option for liver failure. Approximately half of affected children reported did not undergo liver transplantation and died because of progressive liver failure – the majority during infancy or early childhood. A few children were reported to survive without liver transplantation.

All patients with clinical or laboratory evidence of moderate to severe acute hepatitis should have an immediate measurement of prothrombin time and careful evaluation of mental status. If the prothrombin time is prolonged by ≈ 4–6 seconds or more (INR ≥ 1.5),

and there is any evidence of altered sensorium, the diagnosis of ALF should be strongly suspected, and hospital admission is mandatory. Initial laboratory examination must be extensive in order to evaluate both the etiology and severity.

- Initial laboratory analysis

- Prothrombin time/INR

- Complete blood count

- Chemistries

- Liver function test: AST, ALT, alkaline phosphatase, GGT, total bilirubin, albumin

- Creatinine, urea/blood urea nitrogen, sodium, potassium, chloride, bicarbonate, calcium, magnesium, phosphate

- Glucose

- Amylase and lipase

- Arterial blood gas, lactate

- Blood type and screen

- Paracetamol (acetaminophen) level, toxicology screen

- Viral hepatitis serologies: anti-HAV IgM, HBSAg, anti-HBc IgM, anti-HCV

- Autoimmune markers: ANA, ASMA, LKMA, immunoglobulin levels

- Ceruloplasmin level (when Wilson's disease suspected)

- Pregnancy test (females)

- Ammonia (arterial if possible)

- HIV status (has implication for transplantation)

History taking should include a careful review of possible exposures to viral infection and drugs or other toxins. From history and clinical examination, the possibility of underlying chronic disease should be ruled out as it may require different management.

A liver biopsy done via the transjugular route because of coagulopathy is not usually necessary, other than in occasional malignancies. As the evaluation continues, several important decisions have to be made; such as whether to admit the patient to an ICU, or whether to transfer the patient to a transplant facility. Consultation with the transplant center as early as possible is critical due to the possibility of rapid progression of ALF.

This remains a challenge in clinical practice due to a lack of reliable markers. Many other conditions lead to similar clinical as well as pathological pictures. To diagnose hepatotoxicity, a causal relationship between the use of the toxin or drug and subsequent liver damage has to be established, but might be difficult, especially when idiosyncratic reaction is suspected. Simultaneous use of multiple drugs may add to the complexity. As in acetaminophen toxicity, well established, dose-dependent, pharmacological hepatotoxicity is easier to spot. Several clinical scales such as CIOMS/RUCAM scale and Maria and Victorino criteria have been proposed to establish causal relationship between offending drug and liver damage. CIOMS/RUCAM scale involves a scoring system that categorizes the suspicion into "definite or highly probable" (score > 8), “probable” (score 6-8), “possible” (score 3-5), “unlikely” (score 1-2) and “excluded” (score ≤ 0). In clinical practice, physicians put more emphasis on the presence or absence of similarity between the biochemical profile of the patient and known biochemical profile of the suspected toxicity (e.g., cholestatic damage in amoxycillin-clauvonic acid ).

Mutation analysis of the "ATP7B" gene, as well as other genes linked to copper accumulation in the liver, may be performed. Once a mutation is confirmed, it is possible to screen family members for the disease as part of clinical genetics family counseling. Regional distributions of genes associated with Wilson's disease are important to follow, as this can help clinicians design appropriate screening strategies. Since mutations of the WD gene vary between populations, research and genetic testing done in countries like the USA or United Kingdom can pose problems as they tend to have more mixed populations.

Chronic liver disease takes several years to develop and the condition may not be recognised unless there is clinical awareness of subtle signs and investigation of abnormal liver function tests.

Testing for chronic liver disease involves blood tests, imaging including ultrasound and a biopsy of the liver. The liver biopsy is a simple procedure done with a fine thin needle under local anaesthesia. The tissue sample is sent to a laboratory where it is examined underneath a microscope.

Acute liver failure is defined as "the rapid development of hepatocellular dysfunction, specifically coagulopathy and mental status changes (encephalopathy) in a patient without known prior liver disease".

The diagnosis of acute liver failure is based on physical exam, laboratory findings, patient history, and past medical history to establish mental status changes, coagulopathy, rapidity of onset, and absence of known prior liver disease respectively.

The exact definition of "rapid" is somewhat questionable, and different sub-divisions exist which are based on the time from onset of first hepatic symptoms to onset of encephalopathy. One scheme defines "acute hepatic failure" as the development of encephalopathy within 26 weeks of the onset of any hepatic symptoms. This is sub-divided into "fulminant hepatic failure", which requires onset of encephalopathy within 8 weeks, and "subfulminant", which describes onset of encephalopathy after 8 weeks but before 26 weeks. Another scheme defines "hyperacute" as onset within 7 days, "acute" as onset between 7 and 28 days, and "subacute" as onset between 28 days and 24 weeks.

The diagnosis of nephronophthisis can be obtained via a renal ultrasound, family history and clinical history of the affected individual according to Stockman, et al.

The purpose of screening for viral hepatitis is to identify people infected with the disease as early as possible. This allows for early treatment, which can prevent disease progression, and decreases transmission to others.

Since interleukin 1β plays a central role in the pathogenesis of the disease, therapy typically targets this cytokine in the form of monoclonal antibodies (such as canakinumab), binding proteins/traps (such as rilonacept), or interleukin 1 receptor antagonists (such as anakinra). These therapies are generally effective in alleviating symptoms and substantially reducing levels of inflammatory indices. Case reports suggest that thalidomide and the anti-IL-6 receptor antibody tocilizumab may also be effective.

When Budd–Chiari syndrome is suspected, measurements are made of liver enzyme levels and other organ markers (creatinine, urea, electrolytes, LDH).

Budd–Chiari syndrome is most commonly diagnosed using ultrasound studies of the abdomen and retrograde angiography. Ultrasound may show obliteration of hepatic veins, thrombosis or stenosis, spiderweb vessels, large collateral vessels, or a hyperechoic cord replacing a normal vein. Computed tomography (CT) or magnetic resonance imaging (MRI) is sometimes employed although these methods are generally not as sensitive. Liver biopsy is nonspecific but sometimes necessary to differentiate between Budd–Chiari syndrome and other causes of hepatomegaly and ascites, such as galactosemia or Reye's syndrome.

There are several methods available for diagnosing and monitoring hemosiderosis including:

- Serum ferritin

- Liver biopsy

- MRI

Serum ferritin is a low cost, readily available, and minimally invasive method for assessing body iron stores. However, the major problem with using it as an indicator of hemosiderosis is that it can be elevated in a range of other medical conditions unrelated to iron levels including infection, inflammation, fever, liver disease, renal disease and cancer.

While liver biopsies provide a direct measure of liver iron concentration, the small sample size relative to the size of the liver can lead to sampling errors given the heterogeneity of iron concentration within the liver. Furthermore, the invasive nature of liver biopsy and the associated risks of complications (which can range from pain, haemorrhage, gallbladder perforation and other morbidities through to death in approx 1 in 10,000 cases) prevent it being used as a regular monitoring tool.

MRI is emerging as an alternative method for measuring liver iron loading because it is non-invasive, safer and generally cheaper to perform than liver biopsy; does not suffer from problems with sampling variability; and can be used more frequently than performing liver biopsies.

In most cases, liver function will return to normal if the offending drug is stopped early. Additionally, the patient may require supportive treatment. In acetaminophen toxicity, however, the initial insult can be fatal. Fulminant hepatic failure from drug-induced hepatotoxicity may require liver transplantation. In the past, glucocorticoids in allergic features and ursodeoxycholic acid in cholestatic cases had been used, but there is no good evidence to support their effectiveness.

An elevation in serum bilirubin level of more than 2 times ULN with associated transaminase rise is an ominous sign. This indicates severe hepatotoxicity and is likely to lead to mortality in 10% to 15% of patients, especially if the offending drug is not stopped (Hy's Law). This is because it requires significant damage to the liver to impair bilirubin excretion, hence minor impairment (in the absence of biliary obstruction or Gilbert syndrome) would not lead to jaundice. Other poor predictors of outcome are old age, female sex, high AST.

Serum copper is low, which may seem paradoxical given that Wilson's disease is a disease of copper excess. However, 95% of plasma copper is carried by ceruloplasmin which is often low in Wilson's disease. Urine copper is elevated in Wilson's disease and is collected for 24 hours in a bottle with a copper-free liner. Levels above 100 μg/24h (1.6 μmol/24h) confirm Wilson's disease, and levels above 40 μg/24h (0.6 μmol/24h) are strongly indicative. High urine copper levels are not unique to Wilson's disease; they are sometimes observed in autoimmune hepatitis and in cholestasis (any disease obstructing the flow of bile from the liver to the small bowel).

In children, the penicillamine test may be used. A 500 mg oral dose of penicillamine is administered, and urine collected for 24 hours. If this contains more than 1600 μg (25 μmol), it is a reliable indicator of Wilson's disease. This test has not been validated in adults.

Several studies have attempted to predict the survival of patients with Budd–Chiari syndrome. In general, nearly 2/3 of patients with Budd–Chiari are alive at 10 years. Important negative prognostic indicators include ascites, encephalopathy, elevated Child-Pugh scores, elevated prothrombin time, and altered serum levels of various substances (sodium, creatinine, albumin, and bilirubin). Survival is also highly dependent on the underlying cause of the Budd–Chiari syndrome. For example, a patient with an underlying myeloproliferative disorder may progress to acute leukemia, independently of Budd–Chiari syndrome.

A diagnosis can be made by measuring cultured tissue samples for increased levels of free sialic acid. Prenatal testing is also available for known carriers of this disorder.

Hepatitis A causes an acute illness that does not progress to chronic liver disease. Therefore, the role of screening is to assess immune status in people who are at high risk of contracting the virus, as well as in people with known liver disease for whom hepatitis A infection could lead to liver failure. People in these groups who are not already immune can receive the hepatitis A vaccine.

Those at high risk and in need of screening include:

- People with poor sanitary habits such as not washing hands after using the restroom or changing diapers

- People who do not have access to clean water

- People in close contact (either living with or having sexual contact) with someone who has hepatitis A

- Illicit drug users

- People with liver disease

- People traveling to an area with endemic hepatitis A

The presence of anti-hepatitis A IgG in the blood indicates past infection with the virus or prior vaccination.

Modern imaging techniques allow the diagnosis to be made more easily and without invasive imaging of the biliary tree. Commonly, the disease is limited to the left lobe of the liver. Images taken by CT scan, X-ray, or MRI show enlarged intrahepatic (in the liver) bile ducts due to ectasia. Using an ultrasound, tubular dilation of the bile ducts can be seen. On a CT scan, Caroli disease can be observed by noting the many fluid-filled, tubular structures extending to the liver. A high-contrast CT must be used to distinguish the difference between stones and widened ducts. Bowel gas and digestive habits make it difficult to obtain a clear sonogram, so a CT scan is a good substitution. When the intrahepatic bile duct wall has protrusions, it is clearly seen as central dots or a linear streak. Caroli disease is commonly diagnosed after this “central dot sign” is detected on a CT scan or ultrasound. However, cholangiography is the best, and final, approach to show the enlarged bile ducts as a result of Caroli disease.

The diagnosis is made in a patient with history of significant alcohol intake who develops worsening liver function tests, including elevated bilirubin and aminotransferases. The ratio of aspartate aminotransferase to alanine aminotransferase is usually 2 or more. In most cases, the liver enzymes do not exceed 500. The changes on liver biopsy are important in confirming a clinical diagnosis.

The usual initial investigations include chest X ray, electrocardiogram and echocardiography. Typical findings are those of an enlarged heart with non specific conduction defects. Biochemical investigations include serum creatine kinase (typically increased 10 fold) with lesser elevations of the serum aldolase, aspartate transaminase, alanine transaminase and lactic dehydrogenase. Diagnosis is made by estimating the acid alpha glucosidase activity in either skin biopsy (fibroblasts), muscle biopsy (muscle cells) or in white blood cells. The choice of sample depends on the facilities available at the diagnostic laboratory.

In the late onset form, the findings on investigation are similar to those of the infantile form with the caveat that the creatinine kinases may be normal in some cases. The diagnosis is by estimation of the enzyme activity in a suitable sample.

On May 17, 2013 the Secretary's Discretionary Advisory Committee on Heritable Diseases in Newborns and Children (DACHDNC) approved a recommendation to the Secretary of Health and Human Services to add Pompe to the Recommended Uniform Screening Panel (RUSP). The HHS secretary must first approve the recommendation before the disease is formally added to the panel.

In addition to genetic tests involving "PEX" genes, biochemical tests have proven highly effective for the diagnosis of infantile Refsum disease and other peroxisomal disorders. Typically, IRD patients show elevated very long chain fatty acids in their blood plasma. Cultured primarily skin fibroblasts obtained from patients show elevated very long chain fatty acids, impaired very long chain fatty acid beta-oxidation, phytanic acid alpha-oxidation, pristanic acid alpha-oxidation, and plasmalogen biosynthesis.

There is an association between taking aspirin for viral illnesses and the development of Reye syndrome, but no animal model of Reye syndrome has been developed in which aspirin causes the condition.

The serious symptoms of Reye syndrome appear to result from damage to cellular mitochondria, at least in the liver, and there are a number of ways that aspirin could cause or exacerbate mitochondrial damage. A potential increased risk of developing Reye syndrome is one of the main reasons that aspirin has not been recommended for use in children and teenagers, the age group for which the risk of lasting serious effects is highest.

No research has found a definitive cause of Reye syndrome, and association with aspirin has been shown through epidemiological studies. The diagnosis of "Reye Syndrome" greatly decreased in the 1980s, when genetic testing for inborn errors of metabolism was becoming available in developed countries. A retrospective study of 49 survivors of cases diagnosed as "Reye's Syndrome" showed that the majority of the surviving patients had various metabolic disorders, particularly a fatty-acid oxidation disorder medium-chain acyl-CoA dehydrogenase deficiency.

In some countries, oral mouthcare product Bonjela (not the form specifically designed for teething) has labeling cautioning against its use in children, given its salicylate content. There have been no cases of Reye syndrome following its use, and the measure is a precaution. Other medications containing salicylates are often similarly labeled as a precaution.

The Centers for Disease Control and Prevention (CDC), the U.S. Surgeon General, the American Academy of Pediatrics (AAP) and the Food and Drug Administration (FDA) recommend that aspirin and combination products containing aspirin not be given to children under 19 years of age during episodes of fever-causing illnesses. Hence, in the United States, it is advised that the opinion of a doctor or pharmacist should be obtained before anyone under 19 years of age is given any medication containing aspirin (also known on some medicine labels as acetylsalicylate, salicylate, acetylsalicylic acid, ASA, or salicylic acid).

Current advice in the United Kingdom by the Committee on Safety of Medicines is that aspirin should not be given to those under the age of 16 years, unless specifically indicated in Kawasaki disease or in the prevention of blood clot formation.