12 Immunologic and Plasma Protein Disorders

Serum Total Protein and Albumin

	DOGS	CATS
Plasma total protein	6.0–7.8	6.0–7.5
Serum total protein	5.5–7.5	5.5–7.8
Serum albumin*	2.5–4.0	2.5–4.0

* Serum globulin concentration should mirror serum albumin concentration.

Note

Lower values are normal for perinates and very young animals. Values gradually increase until adulthood; higher values in Table 12-1 are average normal values for adults. Both albumin and globulin tend to decline with advancing old age.

Danger Values

Albumin less than or equal to 1.0 g/dl can be associated with major fluid shifts, but animals with concurrent increased portal hypertension can be at risk for abdominal effusion formation at higher values (i.e., >1.5 g/dl). Patients with severe hypoalbuminemia may also have decreased antithrombin III (AT III) activity (see under Causes of Hypoalbuminemia), thus also putting them at risk for thromboembolism of pulmonic, mesenteric, or portal vasculature.

Artifacts

Falsely increased values (refractometer) can result from lipemia, severe hyperglycemia, azotemia, and significant hypernatremia and hyperchloremia. Hyperbilirubinemia is historically listed as a cause for falsely increased serum total protein, but this likely occurs only with marked elevations.¹⁵ Likewise, hemolysis interferes with visual interpretation of total protein reading but does not directly interfere with measurement.⁶

Note

Certain methodologies that measure human albumin give falsely low values for canine albumin.

Drug Therapy That May Alter Protein Values

Hormonal changes generally have a slight effect on serum proteins, even though physical changes (e.g., body weight, muscle mass) may be marked. Hyperproteinemia may be caused by anabolic steroids, progesterone, insulin, and thyroid hormones in people, but a similar effect is not expected in dogs and cats. Prolonged, high-dose corticosteroid therapy can cause hyperproteinemia and hyperalbuminemia in normal dogs, but values return to normal within weeks after cessation of therapy.¹⁵ Hypoproteinemia may be due to estrogens; hypoalbuminemia may be due to anticonvulsants, acetaminophen, estrogens, and various antineoplastic agents in people. Anticonvulsants and antineoplastic drug administration in dogs and cats are not expected to cause similar changes by themselves without associated underlying causes (e.g., hepatic cirrhosis).

Causes of Alteration in Plasma and Serum Protein

Serum total protein concentration is a direct reflection of cumulative serum albumin and globulin values. Therefore the value only gives an overview of the general state of protein homeostasis.

Causes of Hyperalbuminemia

Only clinically relevant cause is dehydration.

The first consideration is typically to concurrently determine the serum globulin concentration and determine if it is also similarly decreased (nonselective), or only albumin is decreased (selective). If both are decreased (i.e., panhypoproteinemia), nonselective causes for hypoproteinemia such as hemorrhage, exudation from severe skin lesions, protein-losing enteropathy (PLE), and hemodilution are usually more likely (Box 12-1). Overt bleeding should be apparent, but gastrointestinal hemorrhage can be difficult to determine if it is relatively mild and chronic. GI hemorrhage can be due to many types of gastrointestinal diseases (see Chapter 9), including PLE, but other nonspecific disorders such as hypoadrenocorticism are also of consideration. Causes of PLE are numerous and are discussed in Chapter 9. Although both serum albumin and globulin are often decreased in PLE, globulin concentrations may be normal to increased in some cases (especially those with other concurrent diseases such as heartworm infection, ehrlichiosis, or chronic skin disorders). Hemodilution rarely causes hypoalbuminemia but can occur due to intravenous fluid overload or plasma expanders (e.g., hetastarch), diseases causing edema (e.g., congestive heart failure), and rarely from excess antidiuretic hormone (ADH) secretion. Hemodilution usually causes mild decreases (albumin 2.1 to 2.4 g/dl), whereas PLE can cause moderate (1.5 to 2.0 g/dl) to severe (<1.5 g/dl) hypoalbuminemia.

Box 12-1 Causes of Hypoalbuminemia in Dogs and Cats

Decreased Production

Chronic hepatic insufficiency *^,¹

Inadequate protein intake ^†^,^‡

Maldigestion ^†

Malabsorption ^†

Hypergammaglobulinemia

Sequestration

Body cavity effusion ¹

Vasculopathy

Increased Loss

Protein-losing nephropathy (PLN) because of glomerular disease *^,¹

Gastrointestinal: protein-losing enteropathy (PLE)*^,²

Exudates from cutaneous lesions ²

Hemorrhage ²

Hemodilution²

^* Most common and important causes of serum albumin ≤2.0 g/dl. Other causes rarely, if ever, cause serum albumin ≤2.0 g/dl.

¹ Important selective causes.

^† Doubtful importance as a sole cause of serum albumin ≤2.0 g/dl. Probably more important as a contributing factor when there is another problem that results in hypoalbuminemia.

^‡ Can be important in very young animals or animals fed diets that are extremely restricted in protein for prolonged periods.

² Important nonselective causes.

Decreased albumin plus normal to increased globulins can be referred to as selective hypoalbuminemia. The most common and clinically significant causes are decreased albumin production from chronic hepatic insufficiency, increased loss from protein-losing nephropathy (PLN), or sequestration in a body cavity due to major effusion (see Box 12-1). Chronic hepatic insufficiency can produce marked hypoalbuminemia (<1.0 g/dl) if the liver is severely affected. Causes for hepatic insufficiency are numerous and can be congenital (e.g., congenital portosystemic shunt) or acquired (e.g., cirrhosis, neoplasia) (see Chapter 9). Hypoalbuminemia from PLN can be substantial (e.g., <2.0 g/dl, even <1.5 g/dl). If renal protein loss is detected, urinalysis ± urine protein : creatinine ratio (see Chapter 7) are typically indicated to confirm presence and severity of albuminuria. Causes for PLN are discussed in Chapter 7.

Note

PLN of very severe magnitude can sometimes cause concomitant hypoglobulinemia.

Sequestration of albumin may occur in pleural or peritoneal cavities or subcutaneous (SC) tissues. Patients with effusion caused by hypoalbuminemia may further lower their serum albumin concentration via sequestration. Alternatively, sequestration can be secondary to increased hydrostatic pressure (e.g., portal hypertension, right-sided cardiac failure). Immune-mediated or infectious vasculopathies (e.g., endotoxemia and bacteremia, ehrlichiosis, Rocky Mountain spotted fever [RMSF]) also allow albumin loss from the vascular compartment. Hypoalbuminemia as the result of sequestration or vasculopathy is usually mild.

Severe malnutrition, malabsorption, or maldigestion leading to poor protein intake can be associated with hypoalbuminemia, but this generally causes very mild hypoalbuminemia. Likewise, hyperglobulinemia from inflammation can cause mild hypoalbuminemia due to down-regulation of albumin production to offset the increased globulin levels or because albumin is a negative acute phase protein that decreases during inflammation. Significant hypoalbuminemia (i.e., albumin < 2.1 g/dl) should not be attributed solely to decreased nutrition or down-regulation of albumin production until hepatic insufficiency and protein-losing disorders have been eliminated by definitive tests (not just history and physical examination), because starvation and down-regulation rarely cause serum albumin concentrations less than 2.1 g/dl, except perhaps in very young animals.

Basic diagnostic approach to hypoalbuminemic patients is outlined in Figure 12-1. Clinical pathology testing should include a complete blood count (CBC), clinical chemistry, and urinalysis on animals with physical exam findings suggestive of hypoalbuminemia. Serum bile acids and blood ammonia are indicated if hepatic insufficiency is suspected (see Chapter 9). If proteinuria is found without pyuria or hematuria and hypoalbuminemia is present, a urine protein : creatinine ratio (see Chapter 7) should be performed. Pyuria and hematuria can cause proteinuria, making it impossible to determine if there is glomerular loss; therefore follow-up urinalysis following resolution of pyuria or hematuria is indicated. An attempt should be made to categorize the degree of hypoalbuminemia (2.1 to 2.4 g/dl, mild; 1.5 to 2.0 g/dl, moderate; <1.5 g/dl, marked) in order to establish initial differential diagnoses. However, definitive exclusion of potential causes by this categorization should not occur until additional testing has been performed, because mild hypoalbuminemia can be observed in some cases of PLE and PLN and early hepatic insufficiency.

FIGURE 12-1 Diagnostic evaluation of hypoalbuminemia in dogs and cats when the serum albumin is less than or equal to 2.0 g/dl. BUN, Blood urea nitrogen; CBC, complete blood count; PLE, protein-losing enteropathy; PLN, protein-losing nephropathy.

Initial approach in hypoalbuminemic patients begins with clinical evaluation. Severe cutaneous exudative lesions may be diagnosed by physical examination, but the possibility of renal, hepatic, and alimentary disease should still be investigated. Subcutaneous edema and body cavity effusions associated with hypoalbuminemia are usually transudates. Hypoalbuminemia associated with PLN or PLE, chronic hepatic insufficiency, and immune-mediated or infectious vasculitis may cause body cavity effusion, primarily transudates. However, one should always evaluate fluid accumulations to be sure that they are in fact transudates as opposed to unexpected modified transudates or exudates (which would strongly suggest that more than hypoalbuminemia is causing the effusion).

Next, recognizing certain patterns on diagnostic samples can be suggestive of different disease processes. Hypercholesterolemia plus hypoalbuminemia suggests PLN. Significant proteinuria without pyuria and hematuria indicates a diagnostic workup for PLN (see Chapter 7). Hypocholesterolemia plus hypoalbuminemia is suggestive of hepatic insufficiency or PLE. Hypoalbuminemia associated with hepatomegaly; microhepatia; neurologic signs; icterus; decreased blood urea nitrogen (BUN) with or without increased alanine aminotransferase (ALT), serum alkaline phosphatase (SAP), or both; or abnormal hepatic function test results (e.g., serum bile acids) indicates a diagnostic workup for hepatic insufficiency (see Chapter 9).

Note

ALT and SAP may be normal in many patients with severe hepatic disease, and decreased mean corpuscular volume (MCV) (see Chapter 2) is sometimes present in dogs with congenital portosystemic shunts.

A congenital portosystemic shunt is more likely in young animals; however, congenital shunts can be diagnosed in animals more than 10 years old. Acquired hepatic disease is more common in adults and requires hepatic biopsy for diagnosis; however, some dogs less than 1 year old have severe, acquired hepatic disease with acquired shunting. Hypoalbuminemia with normal hepatic function tests and absence of proteinuria or cutaneous lesions allows one to diagnose PLE by exclusion (see Chapter 9), even if feces are normal. If the patient has renal or hepatic disease and PLE is still a concern, then measurement of fecal alpha₁-protease inhibitor concentrations (Chapter 9) may allow diagnosis of PLE by inclusion. Intestinal biopsy may then provide a definitive diagnosis of which intestinal disease is causing PLE. Endoscopic biopsies are safer than surgical biopsy, but it is critical that excellent-quality tissue samples be obtained; many endoscopically obtained samples are poor quality and nondiagnostic. If exploratory laparotomy is performed, hepatic biopsy should generally be performed along with intestinal biopsies. It is important to obtain biopsy specimens at several sites along the small intestine, even when no apparent gross lesions are found.

Causes of Altered Globulins

Changes in globulin levels are most often attributed to alterations in immunoglobulin values. Nonselective causes for hypoglobulinemia are similar to nonselective causes for hypoalbuminemia (see previous discussion of Causes of Hypoalbuminemia). True selective hypoglobulinemia (i.e., normal or increased albumin) occasionally occurs in dogs and cats from congenital or acquired immunodeficiencies. However, neonatal immunodeficiency patients are likely to succumb to this disorder early in life, and definitive diagnosis is often not established. Acquired immunodeficiencies are often secondary to chemotherapy or radiation therapy or directly from neoplastic transformation of lymphocytes (i.e., lymphoproliferative disorder) where antibody production is impaired or deficient. Hyperglobulinemia can be either nonselective (i.e., albumin elevated concurrently) from dehydration or selective due to three main processes, (1) acute phase protein increase (usually only induces mild elevation in globulins), (2) increased immunoglobulins from generalized antigenic stimulation with chronic inflammation, or (3) paraproteinemia (abnormal immunoglobulin production in blood) from a lymphoproliferative disorder (see following discussions).

Acute Phase Proteins

Occasionally Indicated

Acute phase protein analysis is performed in certain clinical situations where more specific information regarding inflammation or coagulation is needed. Collectively, acute phase proteins are part of the α and β globulins measured in protein electrophoresis (see Protein Electrophoresis later) and, in conjunction with gamma globulins, compose the globulin fraction of the total protein analysis. Acute phase proteins typically include fibrinogen, haptoglobin, C-reactive protein (see Chapter 9), complement (C3a), serum amyloid A, α1-acid glycoprotein, α₁-antiprotease, transferrin, α₂-macroglobulin, and ceruloplasmin.

Advantages

Some acute phase proteins (e.g., fibrinogen) can be measured rapidly and provide specific information regarding coagulation and inflammatory status of the animal.

Disadvantages

Additional testing is required for these proteins, and some require specialized testing procedures. Results of testing may not provide any more useful information as to the health status of the animal compared to total protein, albumin, and globulins.

Analysis

Most acute phase protein assays can be performed with serum samples with the exception of fibrinogen, which requires plasma. Methodologies of the various assays include heat precipitation (fibrinogen), immunoassays, enzyme-linked immunosorbent assay (ELISA), and spectrophotometry.

Normal Values

Normal reference intervals for fibrinogen in dogs and cats are generally accepted as approximately 100 to 300 mg/dl but are slightly variable depending on the methodology used to measure fibrinogen. Normal values on other acute phase proteins are variable depending on published data, so it is important to use reference intervals established at each laboratory performing the analysis.³

Danger Values

Alterations in values reflect only a change in health status (i.e., values increase with inflammation and dehydration or decrease with consumption or loss of protein); therefore there are no danger values.

Artifacts

Hemolysis and lipemia can interfere with certain testing methodologies. Heat precipitation methods for fibrinogen are not as accurate as instrument-based measurements. Dehydration will cause relatively increased values of some acute phase proteins such as fibrinogen. Laboratory or sampling error can cause spurious results.

Causes of Increased Acute Phase Proteins

Some acute phase proteins increase with associated inflammation and are appropriately termed positive acute phase proteins. Hyperfibrinogenemia is one of the best indicators of acute inflammation in large animal species, but has traditionally not been utilized in small animal medicine. This concept may change because of more accurate methods for fibrinogen analysis being employed by reference laboratories. Regardless, elevated levels of fibrinogen are frequently seen in infectious disease (e.g., bacterial, viral, protozoal, fungal), trauma, neoplasia, and necrosis. C-reactive protein is increased in pregnant dogs, glucocorticoid therapy increases haptoglobin in dogs, and phenobarbital treatment in dogs can cause elevated α1-acid glycoprotein.8–10,¹⁵

Note

Care must be utilized in excluding inflammation in animals with normal or low values of acute phase proteins because these proteins can decrease with hepatic insufficiency (similar to albumin), may be lost in generalized protein-losing disorders (PLE), or may be consumed in cases of coagulopathies.

Causes of Decreased Acute Phase Proteins

Acute phase proteins can decrease in patients as well and are often referred to as negative acute phase proteins when the decrease is associated with inflammation. Albumin (see previous discussion on Causes of Hypoalbuminemia) and transferrin are considered the classic negative acute phase proteins. However, other mechanisms such as lack of production or consumption can be associated with decreases in acute phase proteins that are not a result of the protein representing a negative acute phase protein. Hepatic insufficiency, if severe enough, can lead to decreased production levels of most acute phase proteins, similar to albumin. Fibrinogen, while most frequently considered a positive acute phase protein related to inflammation (see previous discussion on Causes of Increased Acute Phase Proteins), is equally clinically relevant in patients with decreased values. Primary consideration with hypofibrinogenemia is consumptive coagulopathy, such as disseminated intravascular coagulation (DIC). While not present in every case (in some instances fibrinogen is normal or increased), hypofibrinogenemia in conjunction with significant thrombocytopenia, decreased AT III, schistocytes, high D-dimer, and prolongation of activated partial thromboplastin time (aPTT) or prothrombin time (PT) is highly suggestive of DIC (see Chapter 5). Rare reports of inherited or congenital hypofibrinogenemia in dogs are documented.¹⁵