injury, cholestasis, altered function or blood flow) are derived from all screening clinical pathologic tests - the hemogram, biochemical profile, and urinalysis - as outlined below. Of course, history, signalment, clinical signs and results of other diagnostic testing (ultrasound examination, abdominal radiographs) all help consolidate evidence for liver disease. Results from all this testing then dictate further procedures (e.g. aspiration or biopsy of the liver, peritoneal fluid analysis), which can provide the specific diagnosis.
HemogramThe hemogram may be absolutely normal in animals with liver disease, but sometimes there are findings that should make us think about underlying liver disease (which should then be confirmed through other diagnostic testing).
- Erythrogram: This can be normal in animals with liver disease, but some animals have a normocytic normochromic anemia due to inflammatory disease (even if the disease process in the liver is not inflammatory in nature).
- RBC indices (MCV, MCHC): As indicated above, these are usually normal in liver disease, however some dogs with portosystemic shunts (acquired or congenital) can have microcytosis (low MCV) which may or may not be accompanied by hypochromic RBCs (low MCHC). The red blood cell distribution width (RDW) can also be increased. The cause of these RBC changes is unknown but has been attributed to a "functional" iron deficiency due to abnormal iron metabolism (body iron stores may be normal). In a surgical model of shunt formation, normal dogs developed anemia (Hct mean of 33%) and microcytosis (mean of 56 fL) within 18 weeks of creation of the shunt. Even though serum iron concentrations decreased, liver iron stores were increased suggesting a defect in iron metabolism (Laflamme et al 1994). Another study in 25 dogs with congenital shunts showed that MCV and MCHC were abnormal in around 60% of the dogs. Serum iron was low in 56% of dogs, whereas % saturation was low in only 20% (which is inconsistent with a true or absolute iron deficiency) and 62% of dogs had increased hepatic iron. Surgical correction of the shunt resulted in resolution of the MCV changes in some dogs (Simpson et al 1997). Note that even though dogs with shunts can have microcytic and hypochromic RBCs, hypochromasia is typically not evident in RBCs on blood smear examinations (in our experience). If we observe hypochromasia in RBCs, then absolute iron deficiency (low body iron stores) is likely occurring, which we attribute to gastrointestinal bleeding secondary to the shunt. A syndrome of idiopathic noncirrhotic portal hypertension has also been associated with microcytosis in dogs (Bunch et al 2001). In contrast, microcytosis has not been reported in cats with shunts (or other species, but shunts are very rare in species other than the dog).
- RBC morphologic changes: Changes in RBC morphologic features occur in liver disease, but are not specific as to the cause of the disease. These changes are poikilocytosis, which includes acanthocytes and target cell formation. In both dogs and cats, acanthocytes can be a clue as to the presence of liver disease, but as for any RBC morphologic change, are not specific for liver disease. Cats with liver disease, particularly hepatic lipidosis, can develop many different shape changes, some of which defy naming, hence we often use the generic term of "poikilocytosis" in these animals. Again, poikilocytosis can be seen in cats without liver disease. Siderocytes may be seen in some dogs with shunts, supporting abnormal RBC turnover (which fits with increased hepatic iron stores). If dogs with liver disease (acute hepatitis, necrotizing hepatitis, cancer), secondary DIC can result in RBC changes, including acanthocytes, schistocytes and keratocytes. Some dogs with shunts can have RBC fragments (this is attributed to turbulent blood flow through the liver).
- Leukogram: This is generally normal in liver disease, unless there is inflammation or necrosis that recruits inflammatory cells, generating a systemic inflammatory response.
- Thrombogram: Even though thrombopoietin is produced in the liver, the platelet count is usually normal in animals with liver insufficiency, unless there is concurrent DIC.
Biochemistry panelNaturally, the biochemical panel is key to identifying underlying liver disease, but results within reference intervals do not exclude the possibility of disease. Generally, we divide up the tests on a screening chemistry panel into "mini-panels" which allow us to detect the 4 main pathophysiologic processes:
- Hepatocellular injury: Increased activity of "leakage" enzymes - ALT, AST, SDH, GLDH. Some animals with hepatic injury may increased concentrations of unconjugated bilirubin and may develop cholestasis (with increased conjugated bilirubin) depending on the cause of the injury.
- Cholestasis: Increased activity of "inducible" enzymes - ALP, GGT - and concentrations of bilirubin (total and conjugated, primarily). Cholestasis may secondarily result in hepatocellular injury due to toxic effects of bile or extension of the disease process from the portal region into the hepatic parenchyma.
- Hepatic insufficiency: Clues to this are obtained from the panel as none of the screening tests are definitive for hepatic insufficiency. Decreased concentrations of urea nitrogen, glucose, albumin, and cholesterol may be seen with liver insufficiency, but are not specific or sensitive to this condition. In fact, measurement of bile acids, ammonia and other proteins synthesized by the liver (antithrombin, protein C) are more sensitive to liver insufficiency than those tests on screening biochemistry panels.
- Abnormal blood flow: We often see mildly increased leakage enzyme (ALT, AST) and ALP activities in dogs with shunts, along with hemogram abnormalities (see above). Otherwise, we need to perform more specific tests of portal blood bypassing the liver, such as bile acids.
UrinalysisWhenever possible, the urine should be examined in sick animals with suspected liver disease or to screen animals with an unknown condition. This is more readily accomplished in dogs and cats, where it is far easier to obtain urine than larger domestic species. The following abnormalities can be seen in urine with liver disease (but again are not specific for liver disease):
- Defects in concentrating ability: In hepatic insufficiency, decreased production of urea will affect establishment of medullary hypertonicity and affect the ability of the kidney to concentrate urine, in the absence of renal disease.
- Dipstick abnormalities: The most important abnormality is a positive reaction for bilirubin on the dipstick (that is confirmed by tablet versions of the test if the color reaction is weak or the urine is strongly colored). This is because, in all species other than the dog, bilirubinuria is abnormal and indicates cholestasis. Dogs, particularly male dogs, can have a 1+ reaction for bilirubin (and rarely up to 2+) in concentrated urine, but a 1+ bilirubinuria in dilute urine (USG < 1.008) would be clearly abnormal and indicate excessive bilirubinuria and underlying cholestasis.
- Crystals: Ammonium biurate crystals can form in the urine in dogs and cats with portosystemic shunts due to decreased metabolism with subsequent high serum concentrations of ammonia. They can also be seen in certain dog breeds, e.g. Dalmations, with inherited defects in urate metabolism.