The bilirubin levels for initiative of phototherapy varies depends on the age and health status of the newborn. However, any newborn with a total serum bilirubin greater than 359 μmol/l ( 21 mg/dL) should receive phototherapy.

Diagnosis is often by measuring the serum bilirubin level in the blood. In those who are born after 35 weeks and are more than a day old transcutaneous bilirubinometer may also be used. The use of an icterometer, a piece of transparent plastic painted in five transverse strips of graded yellow lines, is not recommended.

Most patients presenting with jaundice will have various predictable patterns of liver panel abnormalities, though significant variation does exist. The typical liver panel will include blood levels of enzymes found primarily from the liver, such as the aminotransferases (ALT, AST), and alkaline phosphatase (ALP); bilirubin (which causes the jaundice); and protein levels, specifically, total protein and albumin. Other primary lab tests for liver function include gamma glutamyl transpeptidase (GGT) and prothrombin time (PT).

Some bone and heart disorders can lead to an increase in ALP and the aminotransferases, so the first step in differentiating these from liver problems is to compare the levels of GGT, which will only be elevated in liver-specific conditions. The second step is distinguishing from biliary (cholestatic) or liver (hepatic) causes of jaundice and altered laboratory results. The former typically indicates a surgical response, while the latter typically leans toward a medical response. ALP and GGT levels will typically rise with one pattern while aspartate aminotransferase (AST) and alanine aminotransferase (ALT) rise in a separate pattern. If the ALP (10–45 IU/L) and GGT (18–85) levels rise proportionately about as high as the AST (12–38 IU/L) and ALT (10–45 IU/L) levels, this indicates a cholestatic problem. On the other hand, if the AST and ALT rise is significantly higher than the ALP and GGT rise, this indicates an hepatic problem. Finally, distinguishing between hepatic causes of jaundice, comparing levels of AST and ALT can prove useful. AST levels will typically be higher than ALT. This remains the case in most hepatic disorders except for hepatitis (viral or hepatotoxic). Alcoholic liver damage may see fairly normal ALT levels, with AST 10x higher than ALT. On the other hand, if ALT is higher than AST, this is indicative of hepatitis. Levels of ALT and AST are not well correlated to the extent of liver damage, although rapid drops in these levels from very high levels can indicate severe necrosis. Low levels of albumin tend to indicate a chronic condition, while it is normal in hepatitis and cholestasis.

Lab results for liver panels are frequently compared by the magnitude of their differences, not the pure number, as well as by their ratios. The AST:ALT ratio can be a good indicator of whether the disorder is alcoholic liver damage (above 10), some other form of liver damage (above 1), or hepatitis (less than 1). Bilirubin levels greater than 10x normal could indicate neoplastic or intrahepatic cholestasis. Levels lower than this tend to indicate hepatocellular causes. AST levels greater than 15x tends to indicate acute hepatocellular damage. Less than this tend to indicate obstructive causes. ALP levels greater than 5x normal tend to indicate obstruction, while levels greater than 10x normal can indicate drug (toxic) induced cholestatic hepatitis or Cytomegalovirus. Both of these conditions can also have ALT and AST greater than 20× normal. GGT levels greater than 10x normal typically indicate cholestasis. Levels 5–10× tend to indicate viral hepatitis. Levels less than 5× normal tend to indicate drug toxicity. Acute hepatitis will typically have ALT and AST levels rising 20–30× normal (above 1000), and may remain significantly elevated for several weeks. Acetaminophen toxicity can result in ALT and AST levels greater than 50x normal.

"Neonatal jaundice" is usually harmless: this condition is often seen in infants around the second day after birth, lasting until day 8 in normal births, or to around day 14 in premature births. Typical causes for neonatal jaundice include normal physiologic jaundice, jaundice due to formula supplementation, and hemolytic disorders that include hereditary spherocytosis, glucose-6-phosphate dehydrogenase deficiency, pyruvate kinase deficiency, ABO/Rh blood type autoantibodies, or infantile pyknocytosis. Serum bilirubin normally drops to a low level without any intervention required. In cases where bilirubin rises higher, a brain-damaging condition known as kernicterus can occur, leading to significant disability. This condition has been rising in recent years due to less time spent outdoors. A Bili light is often the tool used for early treatment, which often consists of exposing the baby to intensive phototherapy. Sunbathing is effective treatment, and has the advantage of ultra-violet-B, which promotes Vitamin D production. Bilirubin count is lowered through bowel movements and urination, so frequent and effective feedings are especially important.

While Gilbert's syndrome is considered harmless, it is clinically important because it may give rise to a concern about a blood or liver condition, which could be more dangerous. However, these conditions have additional indicators:

- Hemolysis can be excluded by a full blood count, haptoglobin, lactate dehydrogenase levels, and the absence of reticulocytosis (elevated reticulocytes in the blood would usually be observed in haemolytic anaemia).

- Viral hepatitis can be excluded by negative blood samples for antigens specific to the different hepatitis viruses.

- Cholestasis can be excluded by normal levels of bile acids in plasma, the absence of lactate dehydrogenase, low levels of conjugated bilirubin, and ultrasound scan of the bile ducts.

- More severe types of glucuronyl transferase disorders such as Crigler–Najjar syndrome (types I and II) are much more severe, with 0–10% UGT1A1 activity, with sufferers at risk of brain damage in infancy (type I) and teenage years (type II).

- Dubin–Johnson syndrome and Rotor syndrome are rarer autosomal recessive disorders characterized by an increase of conjugated bilirubin.

- In GS, unless another disease of the liver is also present, the liver enzymes ALT/SGPT and AST/SGOT, as well as albumin, are within normal ranges.

Neonatal cholestasis defines persisting conjugated hyperbilirubinemia in the newborn with conjugated bilirubin levels exceeding 15% (5.0 mg/dL) of total bilirubin level. The disease is either due to defects in bile excretion from hepatocytes or impaired bile flow.

General presentations in neonates include abdominal pain and general GI upset. Physical examination may show palpable liver and enlarged spleen. Differential diagnosis typically presents with a host of possibilities, many of them not treatable. Histopathology shows dilated bile duct system at all levels and bile duct proliferation in response to back pressure. The incidence has been found to be about 1:2,500 live births.

People with GS predominantly have elevated unconjugated bilirubin, while conjugated bilirubin is usually within the normal range and is less than 20% of the total. Levels of bilirubin in GS patients are reported to be from 20 μM to 90 μM (1.2 to 5.3 mg/dl) compared to the normal amount of < 20 μM. GS patients have a ratio of unconjugated/conjugated (indirect/direct) bilirubin commensurately higher than those without GS.

The level of total bilirubin is often further increased if the blood sample is taken after fasting for two days, and a fast can, therefore, be useful diagnostically. A further conceptual step that is rarely necessary or appropriate is to give a low dose of phenobarbital: the bilirubin will decrease substantially.

Tests can also detect DNA mutations of "UGT1A1" by polymerase chain reaction or DNA fragment sequencing.

Blood testing for the mother is called an Indirect Coombs Test (ICT) or an Indirect Agglutination Test (IAT). This test tells whether there are antibodies in the maternal plasma. If positive, the antibody is identified and given a titer. Critical titers are associated with significant risk of fetal anemia and hydrops. Titers of 1:8 or higher is considered critical for Kell. Titers of 1:16 or higher are considered critical for all other antibodies. After critical titer is reached, care is based on MCA scans. If antibodies are low and have a sudden jump later in pregnancy, an MCA scan is warranted. If the titer undergoes a 4 fold increase, it should be considered significant regardless of if the critical value has been reached. It should be noted that maternal titers are not useful in predicting fetal anemia after the first affected gestation and should not be used for the basis of care. Titers are tested monthly until 24 weeks, after which they are done every 2 weeks.

"In only 2 situations are patients not monitored identically to patients who are Rh sensitized. The first is that of alloimmunization to the c, E, or, C antigens. Some concern exists that hemolysis may occur in these patients with a lower than 1:16 titer. Thus, if the initial titer is 1:4 and stable but increases at 26 weeks' gestation to 1:8, assessment with MCA Doppler velocity at that point is reasonable. However, if the patient presents in the first trimester with a 1:8 titer that remains stable at 1:8 throughout the second trimester, continued serial antibody titers are appropriate.

The second situation in which patients should not be treated identically to patients who are Rh D sensitized is that of Kell isoimmunization because several cases of severe fetal hemolysis with anti-Kell antibodies have occurred in the setting of low titers."

In the case of a positive ICT, the woman must carry a medical alert card or bracelet for life because of the risk of a transfusion reaction.

There are several intervention options available in early, mid and late pregnancies.

The only effective way at preventing kernicterus is to lower the serum bilirubin levels either by phototherapy or exchange transfusion. Visual inspection is never sufficient; therefore, it is best to use a bilimeter or blood test to determine a baby's risk for developing kernicterus. These numbers can then be plotted on the Bhutani nomogram.

There are 3 possible ways to test the fetal antigen status. Free Cell DNA, Amniocentesis, and Chorionic Villus Sampling. Of the three, CVS is no longer used due to risk of worsening the maternal antibody response. Once antigen status has been determined, assessment may be done with MCA scans.

- Free Cell DNA can be run on certain antigens. Blood is taken from the mother, and using PCR, can detect the K, C, c, D, and E alleles of fetal DNA. This blood test is non-invasive to the fetus and is an easy way of checking antigen status and risk of HDN. Testing has proven very accurate and is routinely done in the UK at the International Blood Group Reference Laboratory in Bristol. Sanequin laboratory in Amsterdam, Netherlands also performs this test. For US patients, blood may be sent to either of the labs. In the US, Sensigene is done by Sequenome to determine fetal D status. Sequenome does not accept insurance in the US, but US and Canadian patients have had insurance cover the testing done overseas.

- Amniocentesis is another recommended method for testing antigen status and risk for HDN. Fetal antigen status can be tested as early as 15 weeks by PCR of fetal cells.

- CVS is possible as well to test fetal antigen status but is not recommended. CVS carries a higher risk of fetal maternal hemorrhage and can raise antibody titers, potentially worsening the antibody effect.

There are 3 possible ways to test the fetal antigen status. Free Cell DNA, Amniocentesis, and Chorionic Villus Sampling. Of the three, CVS is no longer used due to risk of worsening the maternal antibody response. Once antigen status has been determined, assessment may be done with MCA scans.

- Free Cell DNA can be run on certain antigens. Blood is taken from the mother, and using PCR, can detect the K, C, c, D, and E alleles of fetal DNA. This blood test is non-invasive to the fetus and is an easy way of checking antigen status and risk of HDN. Testing has proven very accurate and is routinely done in the UK at the International Blood Group Reference Laboratory in Bristol. Sanequin laboratory in Amsterdam, Netherlands also performs this test. For US patients, blood may be sent to either of the labs. In the US, Sensigene is done by Sequenome to determine fetal D status. Sequenome does not accept insurance in the US, but US and Canadian patients have had insurance cover the testing done overseas.

- Amniocentesis is another recommended method for testing antigen status and risk for HDN. Fetal antigen status can be tested as early as 15 weeks by PCR of fetal cells.

- CVS is possible as well to test fetal antigen status but is not recommended. CVS carries a higher risk of fetal maternal hemorrhage and can raise antibody titers, potentially worsening the antibody effect.

There are several intervention options available in early, mid and late pregnancies.

Cholestatic pruritus is the sensation of itch due to nearly any liver disease, but the most commonly associated entities are primary biliary cirrhosis, primary sclerosing cholangitis, obstructive choledocholithiasis, carcinoma of the bile duct, cholestasis (also see drug-induced pruritus), and chronic hepatitis C viral infection and other forms of viral hepatitis.

There are several intervention options available in early, mid and late pregnancies.

Routine antenatal antibody screening blood tests (indirect Coombs test) do not screen for ABO HDN. If IgG anti-A or IgG anti-B antibodies are found in the pregnant woman's blood, they are not reported with the test results, because they do not correlate well with ABO HDN. Diagnosis is usually made by investigation of a newborn baby who has developed jaundice during the first week of life.

Testing

- Coombs - after birth baby will have a direct coombs test run to confirm antibodies attached to the infant’s red blood cells. This test is run from cord blood. In some cases, the direct coombs will be negative but severe, even fatal HDN can occur. An indirect coombs needs to be run in cases of anti-C, anti-c, and anti-M. Anti-M also recommends antigen testing to rule out the presence of HDN.

- Hgb - the infant’s hemoglobin should be tested from cord blood.

- Reticulocyte count - Reticulocytes are elevated when the infant is producing more blood to combat anemia. A rise in the retic count can mean that an infant may not need additional transfusions. Low retic is observed in infants treated with IUT and in those with HDN from anti-Kell

- Neutrophils - as Neutropenia is one of the complications of HDN, the neutrophil count should be checked.

- Thrombocytes - as thrombocytopenia is one of the complications of HDN, the thrombocyte count should be checked.

- Bilirubin should be tested from cord blood.

- Ferritin - because most infants affected by HDN have iron overload, a ferritin must be run before giving the infant any additional iron.

- Newborn Screening Tests - Transfusion with donor blood during pregnancy or shortly after birth can affect the results of the Newborn Screening Tests. It is recommended to wait and retest 10–12 months after last transfusion. In some cases, DNA testing from saliva can be used to rule out certain conditions.

The diagnosis of any porphyria is often delayed due to the rarity of the disease as well as the varied and non-specific findings that patients present with. Bedside measurement of urine porphobilinogen is recommended as a screening test for patients suspected of having an acute porphyria. Elevated porphobilinogen is indicative of an acute porphyria, and additional testing can be done to narrow down the specific type.

The identification of a specific porphyria is based on the results of laboratory findings, including blood, urine and stool tests. HCP can be distinguished from most other acute porphyrias by the cutaneous findings. VP presents similarly, but can be distinguished based on urine and stool porphyrin analysis, typically done using high performance liquid chromatography with fluorescence detection. The results of biochemical testing for porphyrias are most informative when samples are collected during an acute attack. Typically, the distinguishing metabolite for HCP and VP is the presence of protoporphyrin in the plasma and feces of individuals affected with VP.

Elevated coproporphyrin is a common finding in urine, known as coproporphyrinuria as it is the predominant porphyrin species in urine. This is a non-specific finding that is not necessarily due to an acute porphyria. Coproporphyrinuria can be caused by other stressors to the heme biosynthetic pathway, such as liver disease, lead poisoning and certain bone marrow disorders.

The diagnosis of HDN is based on history and laboratory findings:

"Blood tests done on the newborn baby"

- Biochemistry tests for jaundice

- Peripheral blood morphology shows increased reticulocytes. Erythroblasts (also known as nucleated red blood cells) occur in moderate and severe disease.

- Positive direct Coombs test (might be negative after fetal interuterine blood transfusion)

"Blood tests done on the mother"

- Positive indirect Coombs test

In some cases, the direct coombs will be negative but severe, even fatal HDN can occur. An indirect coombs needs to be run in cases of anti-C, anti-c, and anti-M. Anti-M also recommends antigen testing to rule out the presence of HDN.

Cholestasis means "the slowing or stopping of bile flow" which can be caused by any number of diseases of the liver (which produces the bile), the gallbladder (which stores the bile), or biliary tract (also known as the biliary tree, the conduit that allows the bile to leave the liver and gallbladder and enter the small intestine). When this occurs, conjugated bilirubin and the waste products that usually would be cleared in bile reflux back into the bloodstream. This causes a primarily conjugated hyperbilirubinemia and jaundice; the liver conjugates the bile to make it water-soluble and because the bile has already been processed by the liver, when it gets backed up because of a blockage and is refluxed into the blood, the blood will have high levels of conjugated bilirubin. This is in contrast to primarily unconjugated hyperbilirubinemia which is the water-insoluble form that is bound to serum albumin; the liver has not had a chance to conjugate the bilirubin yet and can be caused either because too much unconjugated bilirubin is made (such as in massive hemolysis or ineffective erythropoiesis) or because too little is conjugated (Gilbert's disease or Crigler-Najjar syndrome). Unconjugated hyperbilirubinemia does not typically cause pruritus.

It is thought that bile salts that deposit into the skin are responsible for the pruritus (itching) but the levels of bilirubin in the bloodstream and the severity of the pruritus does not appear to be highly correlated. Patients that have been administered bile salt chelating agents do report some relief, however, and patients that have complete liver cell failure (and therefore cannot make these products to begin with) do not have pruritus. This suggests that products made by the liver must have some role in pruritus although it is not known exactly which product is responsible.

Gilbert's syndrome and G6PD deficiency occurring together especially increases the risk for kernicterus.

The most characteristic feature are elevated levels of gamma glutamyl transferase (100–300 IU/L), aspartate transaminase (>1000 IU/L) and sorbitol dehydrogenase, with AST levels > 4000 IU/L indicating a poor prognosis. High levels of unconjugated and total bilirubin, and serum bile acids, can be seen. Moderate to severe acidosis, leukocytosis, polycythaemia, increased creatine kinase and hyperammonemia may be present, and hemolysis can occur at the end stage. The prothrombin time (PT) and partial thromboplastin time (PTT) is often prolonged. Subclinical horses may only show elevated liver enzymes without any other clinical signs. Horses are rarely hypoglycemic, but blood glucose monitoring is ideal to indicate which horses may be benefited by glucose treatment.

At present this can only be made definitively by liver biopsy or post mortem examination. Given the isolation of a causative virus it should soon be possible to diagnose this by serology, polymerase chain reaction or viral culture. On necropsy, the liver will be small, flaccid, and "dish-rag" in appearance. It has a mottled and bile stained surface. On microscopy there is marked centrilobular to midzonal hepatocellular necrosis and a mild to moderate mononuclear infiltrate. Mild to moderate bile duct proliferation may also be present. On radiology, the liver may be shrunken and difficult to visualize on ultrasound. Ascites may be present.

Dubin–Johnson syndrome is similar to Rotor syndrome, but can be differentiated by:

Prognosis is good, and treatment of this syndrome is usually unnecessary. Most patients are asymptomatic and have normal lifespans. Some neonates present with cholestasis. Hormonal contraceptives and pregnancy may lead to overt jaundice and icterus (yellowing of the eyes and skin).