Globulin in Cats: What High and Low Levels Mean

Normal globulin in cats is 2.5–5.0 g/dL. In cats, the most clinically important thing about globulin is its relationship to albumin — because FIP (feline infectious peritonitis) creates a uniquely distinctive pattern: very high globulin with low albumin, producing an albumin:globulin (A:G) ratio below 0.4. This pattern, especially in a cat with fluid accumulation, is one of the most reliable bloodwork clues for FIP before more specific testing. Low globulin with low albumin points in the opposite direction: gut protein loss from IBD or GI lymphoma.

The Two Globulin Patterns That Matter Most in Cats

Globulin Levels: What the Numbers Mean

Causes of High Globulin in Cats

FIP — The Most Important Cause

Feline infectious peritonitis (FIP) is caused by a mutant form of feline coronavirus (FCoV) that infects macrophages and triggers uncontrolled systemic inflammation. This process has two simultaneous effects on protein values:

• • Globulins rise dramatically — the liver massively overproduces acute-phase proteins (fibrinogen, haptoglobin, alpha-globulins), and the immune system produces immunoglobulins in response to the viral antigens.
• • Albumin falls — consumed by inflammation, lost into effusions, and production is outpaced by demand.

The result: globulin is very high, albumin is low, and the A:G ratio drops below 0.4. In a cat with ascites or pleural effusion and this bloodwork pattern, FIP should be the working diagnosis until proven otherwise. Rivalta test on effusion fluid, FCoV PCR, and FCoV antibody titers confirm the diagnosis.

Chronic Infections

Chronic bacterial infections (pyothorax, deep abscesses, osteomyelitis) and fungal infections (Cryptococcosis — common in cats, especially immunocompromised) produce sustained immune activation and polyclonal globulin elevation. Unlike FIP, albumin is usually preserved unless the infection is very severe. SPEP shows a broad polyclonal band.

Lymphoma and Multiple Myeloma

High-grade lymphoma can drive globulin elevation through immune activation. Multiple myeloma is rare in cats but produces a monoclonal globulin spike (M-spike on SPEP) and is associated with hypercalcemia and bone lesions. When myeloma is suspected, SPEP and urine Bence-Jones protein testing are indicated.

Causes of Low Globulin in Cats

Low globulin in cats is almost always accompanied by low albumin — the two fall together because the cause is non-selective protein leakage through a damaged gut wall. This is called protein-losing enteropathy (PLE).

• • IBD (inflammatory bowel disease) — chronic inflammation of the intestinal wall increases permeability and leaks both albumin and globulin into the gut lumen.
• • Small cell GI lymphoma — the most common GI tumor in cats, difficult to distinguish from IBD without intestinal biopsy. Causes the same pan-hypoproteinemia pattern.
• • Intestinal lymphangiectasia — lymphatic dilation causes protein-rich lymph to leak into the gut; less common in cats than dogs.

Key differentiator: In PLE, both albumin and globulin fall proportionally — the A:G ratio stays in the normal range. In FIP, only albumin falls while globulin rises — the A:G ratio inverts. This single ratio is the fastest way to distinguish PLE from FIP at the blood panel level.

The A:G Ratio in Practice

What Happens Next?

• • Calculate A:G ratio — the most important first step for any abnormal globulin in a cat
• • Check for fluid (effusion) — if present + A:G <0.4, start FIP workup immediately
• • Rivalta test on effusion fluid — simple, fast, high specificity for FIP
• • FCoV PCR and antibody titers — confirm FIP diagnosis
• • SPEP — if globulin >6 g/dL or myeloma features present
• • GI workup (ultrasound, biopsy) — if both albumin and globulin are low (PLE workup)
• • Fungal testing — Cryptococcal antigen test if respiratory signs or CNS signs present