Albumin in Hepatic Disease in Small Animals
Albumin is synthesized exclusively by the liver and has an estimated half-life of ~8 days in dogs free of hepatic disease. Extensive studies in healthy humans estimate albumin half-life at ~17 days. In health, hepatic albumin synthesis functions at ~33% maximal capacity.
The liver maintains a large dynamic on-demand reserve for albumin synthesis. Degradation of albumin occurs in a variety of tissues, especially vasculature, where it ultimately undergoes lysosomal digestion.
Albumin's essential functions include its roles in providing the following:
~80% of the plasma colloidal effect
transport and distribution of endogenous and exogenous ligands influencing drug-receptor interactions
a buffer for plasma pH
essential antioxidant protection (~> 50% of plasma antioxidant protection)
The impact of albumin on intravascular osmotic pressure reflects its low molecular weight and its higher intravascular concentrations compared to other plasma proteins. Intravascular albumin remains in constant flux with extravascular albumin. Thus, disorders compromising vascular or lymphatic integrity (ie, inflammation, lymphatic obstruction, vasculitis) increase extravascular albumin dispersal that can lead to hypoalbuminemia.
Albumin concentration is influenced by numerous variables complicating the simple interpretation of hypoalbuminemia. Total plasma clearance of albumin and albumin synthesis inversely correlate with plasma albumin concentrations. Serum albumin concentration is not strongly influenced by nutritional status except in the most severe cachexia. For example, during severe protein and calorie restriction, albumin synthesis remains nearly normal due to availability of amino acids released from tissue catabolism.
A negative acute phase response (eg, tumor necrosis factor, IL-6) as a cause of decreased albumin synthesis is an often-cited cause of hypoalbuminemia. However, rather than decreased albumin synthesis, this phenomenon often is associated with disorders causing extravascular albumin dispersal. Numerous studies in humans document accelerated albumin synthesis during a negative acute phase response.
Other variables influencing serum albumin concentration include synthetic inhibition during acidemia and accelerated synthesis during release of stress hormones (eg, epinephrine, cortisol, glucagon), insulin, and somatotropin, and during acute endotoxemia.
Despite the complexity of confirming the cause of hypoalbuminemia, marginal decline in albumin concentration is a strong indication of increased risk for surgical and postsurgical morbidity. This effect is not negated by anticipatory nutritional supplementation, as often suggested.
The best characterized cause of hypoalbuminemia is liver disease, in which decreased synthesis is the dominant cause. However, albumin distribution into ascitic effusion is an often-encountered contributing factor. In liver disease, decline in albumin concentration and alteration in its molecular configuration compromise its essential transport functions. This increases risk for adverse drug reactions (more free or unbound drug) and may impair drug elimination. Any disease promoting oxidative injury (eg, diabetes mellitus, renal disease, hepatic insufficiency, hyperthyroidism) can irreparably damage the albumin molecule, accelerating its turnover and negating its transport, antioxidant, and acid-base–related attributes.
Both inflammatory disorders and malnutrition increase extravascular albumin dispersal by transcapillary leakage. This phenomenon hastens onset of hypoalbuminemia in necroinflammatory liver disease, long before development of ascites and before the impact of decreased albumin synthesis. Ascites in liver disease is largely dependent on development of hepatic presinusoidal, sinusoidal, or postsinusoidal hypertension.
Considering that albumin is freely permeable across hepatic sinusoids, albumin exerts no osmotic effect on trans-sinusoidal fluid movement and thus is not a predominant cause of ascites. However, distribution of albumin into ascitic effusion decreases circulating albumin concentrations, amplifying the impact of decreased hepatic albumin synthesis. In this circumstance, repeat large-volume abdominocentesis (ascitic effusion removal by paracentesis) can escalate hypoalbuminemia. Concurrent hypoalbuminemia and development of hepatic presinusoidal, sinusoidal, or postsinusoidal hypertension commonly is associated with ascites in animals with severe chronic diffuse liver disease.
In summary, in many animals with liver disease, an early trend toward hypoalbuminemia often reflects systemic inflammation. Only in severe hepatic insufficiency (eg, chronic progressive hepatitis) is synthetic failure a driving cause. Protein-losing nephropathy (glomerular disease), protein-losing enteropathy, and chronic enteric hemorrhage must be excluded as underlying causes of hypoalbuminemia in these patients. Glomerular causes are associated with a urine protein:urine creatinine ratio > 3 and hypercholesterolemia, whereas protein-losing enteropathy usually is associated with panhypoproteinemia and hypocholesterolemia. Finding hypoalbuminemia associated with ascites in a patient with liver disease generally portends a guarded prognosis.
Bilirubin in Hepatic Disease in Small Animals
Hyperbilirubinemia can reflect prehepatic (eg, hemolysis), hepatic (impaired uptake, intracellular transport, glucuronide conjugation, or canalicular elimination), or posthepatic or extrahepatic causes (EHBDO, biliary tree rupture).
Total bilirubin concentrations vary markedly with different disease processes. Concentrations are higher in dogs with hemolytic disorders compared to liver causes and in cats with HL and EHBDO.
Clinical icterus results when total bilirubin is > 2.5–3 mg/dL.
Bilirubinuria can be detected in healthy dogs because of their ability to conjugate bilirubin in renal tubules (low renal threshold). However, bilirubinuria in cats is always abnormal and should be investigated.
Fractionation of total bilirubin into direct (conjugated) and indirect (unconjugated) moieties (van den Bergh fractionation) offers little diagnostic utility. Conjugated bilirubin can spontaneously form covalent bonds with albumin (biliprotein complexes or delta bilirubin) and can remain in the circulation for sustained intervals.
Delta-bilirubin is not eliminated by liver or kidneys but rather is slowly catabolized along with serum albumin. During the convalescent stage of effectively treated chronic cholestatic disorders, delta-bilirubin can represent 90% of total bilirubin. Its slow elimination explains sustained jaundice (7–14 days) observed in some animals after definitive treatment of liver disease causing jaundice (eg, animals with EHBDO or dogs with gallbladder mucocele chronically jaundiced before cholecystectomy). Comparatively, the normal half-life of conjugated bilirubin is only a matter of hours.
Neonatal jaundice in dogs and cats is not a recognized disorder as encountered in human infants.
Common causes of hyperbilirubinemia include the following:
increased hemoprotein liberation (eg, hemolytic anemia, ineffective erythropoiesis, body cavity hemorrhage)
bile duct occlusion
ruptured biliary tract
impaired hepatobiliary bilirubin processing
Jaundiced dogs and cats with regenerative anemia should be tested for hemolytic disorders, including immune-mediated hemolytic anemia, Heinz body hemolysis, zinc toxicity, and erythroparasites (including hemotropic Mycoplasma [cats, dogs] and Babesia [dogs]). Sepsis-related jaundice can evolve from gram-negative (endotoxin) and gram-positive bacteria and reflects downregulation and translocation of bilirubin membrane transporters in hepatocytes and on cholangiocytes (biliary ductules). Persistent invariant jaundice in chronic liver disease portends a guarded prognosis.
Clinical Utility of Bilirubin Fractionation
Traditionally, measured bilirubin is categorized according to its conjugation status. This historical conceptual approach was enabled by the van den Bergh reaction that segregates CBr from UnBr based on the reaction rate with diazo reagents. The CBr is denoted as direct-reacting bilirubin while UnBr is denoted as indirect-reacting bilirubin.
After measuring fast-reacting CBr, the addition of a reaction accelerator hastens the diazo reaction with UnBr, allowing quantification of the total bilirubin concentration. UnBr is subsequently calculated by subtracting the measured CBr from the total bilirubin concentration.
In animals with acquired portosystemic shunting, bilirubin produced by the splenic RES can bypass the liver in portosystemic collaterals. Increases in circulating UnBr coordinate with increases in total bilirubin concentration, and UnBr is subsequently calculated by subtracting the measured CBr from the total bilirubin concentration. Notably, categorization of bilirubin moieties by this method is largely a pedantic exercise because of notorious physiologic exceptions. For example, patients with advanced-stage liver disease (eg, characterized by the presence of regenerative nodules, parenchymal remodeling, sinusoidal capillarization, development of acquired portosystemic shunts) lack normal sinusoidal-to-hepatocyte exchange processes (solute exposure and substrate uptake).
Studies comparing bilirubin fractionation in healthy dogs, dogs with prehepatic hemolytic jaundice, and dogs with hepatic jaundice confirmed no difference between plasma UnBr concentration between groups and no diagnostic utility for calculation of a fractionation ratio (ie, ratio of UBr concentration to total bilirubin concentration) in the diagnostic discrimination of hemolytic from hepatobiliary jaundice. While this has not been formally studied in cats, clinical observations in feline patients support a similar observation.
Additional canine studies defined an ~20-fold greater hepatic clearance of UnBr in dogs as compared to humans. Those studies found that ~60%–70% of bilirubin pigment derives from erythrocyte degradation, ~5% from ineffective erythropoiesis, and 25%–35% from hepatic hemoprotein catabolism. The greatest contribution of bilirubin pigment in many dogs with liver disease was erythrocyte hemolysis, similar to dogs with hemolytic anemia. However, some dogs with liver disease had substantial amounts of bilirubin derived from hepatic catabolism of heme proteins.
Canine studies also document an ability of canine renal cortical tubules to conjugate bilirubin, albeit only samples from two dogs were studied. Tubular conjugation was notably absent in samples from three cats. These observations reconcile with the frequent detection of urine bilirubin in nonjaundiced dogs and the absence of urine bilirubin in most nonjaundiced cats.
Urine Bilirubin Detection
Because CBr is water soluble and less strongly bound to albumin than UnBr, it can undergo some degree of renal elimination and extravascular dissemination. Encountering bilirubinuria in the absence of albuminuria indicates conjugated hyperbilirubinemia.
Some clinicians advocate for testing with reagent tablets, especially for feline urine, for more sensitive detection of urine bilirubin compared to the urine dipstick methodology. The test employs a unique solid diazonium salt that reacts with bilirubin in an acid medium. A positive reaction yields a blue staining reaction on the reagent tablet. A negative dipstick urine test for bilirubin and a positive urine test with the reagent tablet might incriminate hyperbilirubinemia in an anicteric cat.
However, it is more cost-efficient simply to measure a plasma or serum total bilirubin concentration, if such has not yet been documented. This test is otherwise advocated for ruling out a false-positive bilirubin dipstick result caused by urine color interference; this problem may be more common with recently popularized automated urinalyses.
BUN and Creatinine in Hepatic Disease in Small Animals
There are no characteristic changes in BUN or creatinine concentrations in hepatic disorders except that low concentrations are associated with portosystemic shunting and feeding of a restricted protein diet (only BUN, not creatinine) formulated to decrease clinical signs of hepatic encephalopathy (HE).
The BUN concentration reflects numerous variables, including hydration status, nutritional support, increased hemoglobin or protein turnover (enteric or body cavity hemorrhage, blood transfusion, hemolysis, tissue catabolism), and the hepatic capacity to detoxify ammonia. Anorexia, a low-protein diet, fluid diuresis, or hepatic insufficiency can result in low-normal to subnormal concentrations of BUN, whereas increased BUN concentrations relative to creatinine (discordant BUN:creatinine ratio) reflect dehydration (intravascular volume contraction), enteric bleeding, body cavity hemorrhage, hemolysis, blood transfusion, or consumption of a high-protein diet.
Compared to BUN, serum creatinine concentrations are less influenced by dietary protein intake and are not influenced by enteric bleeding, body cavity hemorrhage, or blood transfusions unless there is intravascular volume contraction (dehydration). The subnormal to low-normal concentrations of BUN and creatinine encountered in animals with portosystemic shunting reflect the dual influence of decreased ammonia detoxification (BUN) and low body muscle mass due to juvenile status (creatinine). An additional factor is increased water turnover increasing glomerular filtration rate up to 2-fold, which contributes to polyuria/polydipsia (PU/PD).
Decreased hepatic synthesis of creatinine also contributes to low creatinine concentrations in animals with hepatic insufficiency, considering that creatinine depends on hepatic synthesis of creatine in the transmethylation pathway. Diagnostic utility of BUN and creatinine as markers of hepatic insufficiency or portosystemic shunting is abolished by concurrent renal insufficiency (acute or chronic kidney injury or obstructive uropathy).
Glucose in Hepatic Disease in Small Animals
The inability to store hepatic glycogen or convert glycogen to glucose is more common in neonates and juvenile small-breed dogs with congenital portosystemic shunts subjected to prolonged food withholding (> 12 hours) or that are anorectic.
Hypoglycemia is uncommon in acquired hepatic disease with the exceptions of end-stage cirrhosis, fulminant liver failure, and, in a subset of dogs, severe congenital portosystemic shunts. Other causes of hypoglycemia, including sepsis, insulinoma, iatrogenic insulin overdose, hypoadrenocorticism, rare glycogen storage disorders, or paraneoplastic causes (eg, large primary hepatic neoplasia [canine hepatocellular carcinoma] or other tumors), should be considered differential diagnoses.
Rarely, acquired adrenal insufficiency underlies persistent hypoglycemia in toy breed dogs with congenital portosystemic shunts. These fail to respond to simple dextrose supplementation and require glucocorticoid support.
Cholesterol in Hepatic Disease in Small Animals
Hepatic disease is associated with several disorders of cholesterol function.
All cells except nucleus-free erythrocytes synthesize cholesterol with the product destined for intracellular use. Cholesterol incorporated in plasma lipoproteins is synthesized only in the liver and distal small intestine. In health, de novo cholesterol synthesis exceeds the amount absorbed from food. Bile provides the major excretory route for cholesterol with both intact cholesterol and cholesterol catabolized during bile acid synthesis being eliminated.
Hypocholesterolemia and hypercholesterolemia can each be associated with different hepatobiliary disorders. Hypocholesterolemia is associated with numerous disease processes including endocrine, metabolic, inflammatory, neoplastic, and nutritional factors as well as hepatic insufficiency and portosystemic shunting.
Nonhepatic disorders causal to hypocholesterolemia include conditions associated with inflammatory acute phase responses:
infectious diseases and sepsis
maldigestion or malabsorption due to diffuse intestinal disease or pancreatic exocrine insufficiency
starvation associated with extreme cachexia
chronic regenerative anemias (increased cholesterol utilization by proliferating erythroid cells)
various types of neoplasia (eg, multiple myeloma, histiocytic sarcoma)
severe hemorrhage (usually enteric hemorrhage)
Hepatic causes of hypocholesterolemia, including portosystemic shunting (congenital or acquired) and severe hepatic insufficiency (eg, end-stage cirrhosis and fulminant hepatic failure), are associated with synthetic failure and often with the presence of APSSs. Portosystemic shunting leads to hypocholesterolemia via breakdown in the FXR feedback loop.
Discovery of hypercholesterolemia requires thoughtful consideration of potential nonhepatic disorders, including the following:
hyperadrenocorticism or treatment with glucocorticoids
treatment with cyclosporine
(rarely) postprandial effect
Among liver disorders, hypercholesterolemia is usually encountered in EHBDO and less commonly in animals with diffuse intrahepatic cholestasis associated with destructive cholangitis with progressive ductopenia and after acute injury during a marked regenerative response.
Urinalysis in Hepatic Disease in Small Animals
Urine concentration is dilute in liver patients with polyuria and polydipsia where urine specific gravity is < 1.020. Another feature associated with liver disease is discovery of ammonium biurate crystalluria in patients with portosystemic shunting (congenital or APSSs) or animals with acute fulminant hepatic failure.
Animals with inborn errors of uric acid transport (ie, Dalmatians, Bulldogs, Black Russian Terriers, other breeds occasionally, cats occasionally) also develop ammonium biurate crystalluria. In these, total serum bile acid testing or blood ammonia determinations are mandatory to differentiate the underlying cause.
Dogs with portosystemic shunting have increased total serum bile acid concentrations (evaluating paired sample analysis before and 2 hours after meal consumption) and, less dependably, hyperammonemia on blood ammonia measurements. Patients with uric acid transport mutations have neither of these abnormalities, with the exception of small-breed dogs with microvascular dysplasia where total serum bile acids (TSBAs) are increased but blood ammonia concentrations are not.
Bilirubinuria is common in dogs because of their ability to conjugate bilirubin in the renal tubules. However, bilirubinuria in cats is always abnormal and warrants further scrutiny of plasma bilirubin concentrations.
Finding ammonium biurate crystalluria in an animal with high TSBAs is nearly pathognomonic for hyperammonemia and portosystemic shunting. A minimum of three urine samples collected at separate daily intervals should be inspected to optimize surveillance for crystal discovery. In animals on restricted protein intake using diets specifically formulated for hepatic insufficiency, finding ammonium biurate crystals may be difficult because of the high efficacy of such diets to control hyperammonemia.
Some dogs with acute hepatotoxicity due to copper, xylitol, drug toxicity, and other toxins or with liver disease due to infectious agents (eg, leptospirosis, other causes of pyelonephritis) that develop concurrent renal injury may manifest acquired Fanconi syndrome. This is characterized by unconcentrated urine, euglycemic glucosuria, loss of other filtrates reabsorbed in the proximal renal tubule (ie, amino acids), and usually an active urine sediment (granular casts). Differential diagnosis must consider dogs with congenital Fanconi syndrome (eg, Basenjis, rare Siberian Huskies). While discovery of an acquired Fanconi syndrome in dogs with liver disease signifies a severe injury, recovery is possible with appropriate supportive care.