Understanding Cat Bloodwork During FIP Treatment: What Every Result Means

Your cat has been diagnosed with Feline Infectious Peritonitis (FIP). Treatment has started. Then comes the first blood test — and the results look nothing like what you expected.

Globulin is elevated. ALT is climbing. White cell counts look abnormal. You read the numbers and feel more confused, not less.

Understanding Cat Bloodwork During FIP Treatment

This is one of the most common and least discussed challenges in Feline Infectious Peritonitis (FIP) treatment: understanding what blood test results actually mean during active therapy, and why many values will behave in ways that seem alarming but are, in fact, completely expected.

This guide explains, in clear terms, what each key marker does during Feline Infectious Peritonitis (FIP) treatment, when abnormal values are a sign of progress, and when they warrant immediate veterinary attention. It is designed for cat owners managing treatment at home and for veterinarians who want a structured reference for client communication.

Why Blood Tests Are Essential During FIP Treatment

Feline Infectious Peritonitis (FIP) is caused by a mutated form of the feline coronavirus that triggers a systemic immune and inflammatory response. The antiviral drug GS-441524 works by blocking viral replication — but suppression takes time, and the body's recovery from weeks or months of viral damage does not happen overnight.

Blood tests serve three functions during Feline Infectious Peritonitis (FIP) treatment:

• Confirm the diagnosis is being reversed — viral load and inflammation are declining

• Detect side effects early, particularly kidney and liver stress

• Guide dosage adjustments if the cat is not responding as expected

The standard protocol recommends testing at baseline (before or at treatment start), at weeks 4 and 8 during treatment, and at the end of the 84-day course. A post-treatment panel is then recommended at weeks 4, 8, and 12 of the 12-week observation period.

Understanding the trend across these tests matters far more than any single result in isolation.

The Key Blood Markers in FIP — And What They Tell You

Globulin

What it is: Globulins are a family of proteins produced by the immune system. In cats with active Feline Infectious Peritonitis (FIP), total globulin is almost always elevated — often dramatically so — because the immune system is in a state of sustained activation.

What to expect during treatment: Globulin is typically one of the last markers to normalize. It is not unusual for globulin to remain elevated through weeks 4 and 8 of treatment, even in cats who are visibly improving. A gradual, consistent decline is the goal — not a sudden drop.

Red flag: Globulin that rises or remains static after week 8, alongside other signs of deterioration, may indicate treatment is not achieving adequate viral suppression.

Albumin-to-Globulin (A:G) Ratio

What it is: The ratio of albumin (a structural protein made by the liver) to total globulin. In healthy cats, this ratio is typically above 0.8. In cats with active Feline Infectious Peritonitis (FIP), it is often below 0.5 — sometimes below 0.3.

What to expect during treatment: A rising A:G ratio is one of the most reliable early markers of treatment response. As globulin decreases and albumin recovers, the ratio climbs back toward the reference range. Seeing the A:G ratio improve at the week-4 panel — even if absolute globulin is still elevated — is a positive signal.

Why this matters: Some owners focus on individual numbers and miss the trend. A cat whose globulin dropped from 65 g/L to 52 g/L and whose albumin rose from 16 g/L to 22 g/L is responding well, even if neither value is yet within the healthy reference range.

ALT (Alanine Aminotransferase)

What it is: ALT is an enzyme released when liver cells are stressed or damaged. Mildly elevated ALT is common in cats with Feline Infectious Peritonitis (FIP) due to the inflammatory process and is also reported as a known side effect of GS-441524 therapy.

What to expect during treatment: ALT may rise during the first four to eight weeks of GS-441524 treatment. This elevation is usually mild to moderate and does not indicate liver failure. Research has documented this as a recognized, generally transient response to the antiviral compound.

When to act: ALT that rises sharply — more than three to five times the upper limit of normal — or that continues to climb after week 8 should be investigated. Concurrent jaundice (yellowing of the skin, eyes, or gums) alongside high ALT is a more serious finding requiring immediate veterinary assessment.

Do not stop treatment based on ALT alone without veterinary consultation. Premature discontinuation carries a higher risk than mild, monitored enzyme elevation in most cases.

ALP (Alkaline Phosphatase)

What it is: ALP is another liver-associated enzyme. Elevations during Feline Infectious Peritonitis (FIP) treatment are documented, often occurring alongside ALT rises.

What to expect: Mild ALP elevation during treatment is common and typically resolves during or after the treatment course. Persistently elevated ALP post-treatment warrants further hepatic investigation.

SDMA (Symmetric Dimethylarginine) and Creatinine

What it is: SDMA and creatinine are markers of kidney filtration function. SDMA is an earlier and more sensitive indicator of reduced kidney function than creatinine.

What to expect during treatment: Studies have reported transient increases in SDMA during GS-441524 therapy in some cats. This does not necessarily indicate kidney damage — GS-441524 is excreted renally, and some functional shift may occur without structural injury.

When to act: Rising SDMA alongside elevated creatinine, reduced urine specific gravity, or increased water intake should prompt veterinary evaluation. Cats who entered treatment with pre-existing kidney compromise require closer renal monitoring throughout.

White Blood Cell Count (WBC) and Differential

What it is: The total white blood cell count and the proportions of each cell type (neutrophils, lymphocytes, eosinophils, monocytes).

What to expect during treatment:

• Lymphocytosis (high lymphocytes) has been reported during GS-441524 treatment and is generally not clinically significant in isolation

• Eosinophilia (elevated eosinophils) is also documented as a transient finding and does not typically indicate a parasitic or allergic complication unless accompanied by other signs

• Neutrophilia (high neutrophils) early in treatment often reflects active systemic inflammation from Feline Infectious Peritonitis (FIP) itself. As treatment progresses, this should normalize

A WBC count trending toward the reference range from weeks 4 to 8 is a positive sign. Persistent or worsening neutrophilia may indicate a secondary infection or inadequate viral control.

Hematocrit (PCV) and Red Blood Cell Count

What it is: The proportion of red blood cells in the blood. Many cats with Feline Infectious Peritonitis (FIP) are anemic at diagnosis due to immune-mediated destruction of red cells and bone marrow suppression.

What to expect during treatment: Anemia often improves gradually once viral load declines and the immune system stabilizes. Do not expect hematocrit to normalize within the first two to four weeks. Cats that remain severely anemic beyond week 8 may need supportive care discussion with their veterinarian.

Total Protein and Fibrinogen

What it is: Total protein is the combined measurement of albumin and globulin. Fibrinogen is an acute-phase protein that rises significantly with systemic inflammation.

What to expect: Total protein normalizes as albumin recovers and globulin declines. Very high total protein at baseline — above 90–100 g/L in some cases — is consistent with active Feline Infectious Peritonitis (FIP). A declining trend over the treatment course is the key indicator.

How to Read Your Cat's Blood Tests Week by Week

Baseline (Before or at Day 1)

The baseline panel establishes the severity of disease. Expect globulin elevation, reduced albumin, a low A:G ratio, possible anemia, and potentially elevated inflammatory markers. This is the starting point — every subsequent result should be measured against it, not against the healthy reference range.

Week 4 (Day 28)

By week 4, most cats who are responding to treatment will show:

• Improved or improving A:G ratio

• Declining globulin (may still be elevated but trending down)

• Stabilizing or improving white cell counts

• Partial anemia correction

• ALT possibly elevated but not worsening

If globulin has not moved or has increased, and clinical signs have not improved, dosage adjustment should be discussed with a veterinary professional. Use the CureFIP Dosage Calculator to verify your cat is receiving the correct weight-based dose before assuming treatment failure.

Week 8 (Day 56)

By week 8, a cat responding well to treatment should show values trending clearly toward the normal range across most markers. Globulin should be measurably lower than at week 4. ALT and ALP elevations, if present, should be stable or declining. Albumin should be visibly recovering.

Day 84 (End of Treatment)

The goal at the end of the 84-day course is not necessarily complete normalization of all values — some parameters, particularly globulin, may still be slightly outside the reference range. The key markers for a decision to stop treatment are:

• Clinical remission (normal appetite, weight, activity, temperature)

• A:G ratio trending toward or within the reference range

• No signs of active effusion

• No other indicators of ongoing viral disease

For a full picture of what clinical milestones to expect at each stage, see the GS-441524 Treatment Timeline Guide.

Post-Treatment Observation (Weeks 4, 8, 12 After Day 84)

The 12-week observation period requires continued blood monitoring. At each of these three checkpoints, values should continue to normalize. Any marker that worsens after treatment ends — particularly globulin rising again, or albumin declining — should be communicated to a veterinary professional immediately, as this may indicate relapse.

Common Misconceptions About Bloodwork During FIP Treatment

"If ALT is high, I should stop the medication." Not necessarily. Mild-to-moderate ALT elevation during GS-441524 therapy is a documented, typically transient finding. Stopping treatment prematurely risks relapse and can be more harmful than the enzyme elevation itself. Always consult your veterinarian before making changes.

"My cat's results are back in the normal range after four weeks — treatment must be done." Rapid apparent normalization is encouraging, but it does not mean the virus has been fully suppressed. The full 84-day protocol is based on the biology of feline coronavirus persistence in macrophages. Early normalization of blood markers does not equal virological cure.

"Bloodwork didn't improve at week 4, so the treatment isn't working." Some cats are slow responders. Clinical improvement — improved appetite, reduced fever, returning energy — often precedes laboratory normalization by two to four weeks. Assess both the clinical picture and the blood results together before drawing conclusions.

"My cat's globulin is still high at week 8. This is a bad sign." Globulin is the slowest marker to recover. Many cats still have elevated globulin at week 8 while otherwise responding well. What matters is the direction — a declining trend is reassuring even when the absolute value remains outside the reference range.

FAQ: Cat Bloodwork During Feline Infectious Peritonitis (FIP) Treatment

What blood tests are needed to monitor FIP treatment? A full biochemistry panel — including total protein, albumin, globulin, ALT, ALP, creatinine, SDMA, and glucose — and a complete blood count (CBC) are the core tests. Serial panels at baseline, week 4, week 8, and day 84 track treatment response. An albumin-to-globulin ratio calculation and coronavirus antibody titer are often used at diagnosis.

What does high globulin in a cat mean during FIP treatment? Elevated globulin during Feline Infectious Peritonitis (FIP) treatment reflects ongoing immune activation from the feline coronavirus infection. It is expected to remain elevated in early treatment and should decline progressively. A consistently declining globulin trend, alongside improving clinical signs, is a positive indicator of treatment response.

Is it normal for ALT to go up during GS-441524 treatment? Yes. Mild to moderate ALT elevation is a documented side effect of GS-441524 therapy and does not automatically indicate liver damage or treatment failure. Monitoring is recommended, and any sharp or sustained rise should be evaluated by a veterinarian before adjusting treatment.

What is the albumin-to-globulin ratio and why does it matter for FIP? The albumin-to-globulin ratio (A:G ratio) compares two protein fractions in the blood. In healthy cats it is typically above 0.8. In cats with active Feline Infectious Peritonitis (FIP) it is often below 0.5. A rising A:G ratio during treatment is one of the clearest early indicators that GS-441524 is working.

How often should blood tests be done during FIP treatment? The recommended minimum schedule is: baseline before treatment starts, week 4, week 8, and day 84 at end of treatment. After treatment, blood tests at weeks 4, 8, and 12 of the observation period confirm continued remission.

Can bloodwork look normal but the cat still relapse? Yes. Blood markers may normalize before the virus is fully cleared. This is one of the primary reasons the full 84-day treatment course and the 12-week post-treatment observation period are both necessary, regardless of how well the bloodwork appears.

My cat's bloodwork is worsening at week 4. What should I do? Contact your veterinary professional immediately. Deteriorating values at week 4 may indicate subtherapeutic dosing, product quality issues, or an atypical disease form requiring a higher-dose protocol. Review the correct weight-based dose using the CureFIP Dosage Calculator and do not adjust without professional guidance.

What happens to SDMA during FIP treatment? Transient SDMA elevation has been reported in some cats during GS-441524 therapy. This is generally not indicative of permanent kidney damage. However, cats with pre-existing kidney disease require more frequent renal monitoring. A persistently elevated or rising SDMA after treatment ends should be investigated further with your veterinarian.

Supporting Your Cat Through the Monitoring Phase

Regular blood testing throughout Feline Infectious Peritonitis (FIP) treatment is not just about watching for problems — it is how you confirm the treatment is working. For many cat owners, the moment the A:G ratio starts to climb, or globulin drops measurably from one panel to the next, is the first real evidence that their cat is going to recover.

Keep a record of every result from every test. Track the trends over time. When speaking to your veterinarian, bring the full history of results — not just the most recent one.

If your cat does show signs of relapse after treatment ends — returning fever, weight loss, reduced appetite, or swelling — do not wait. CureFIP's Relapse Program is designed specifically for these situations and ensures cats can re-enter treatment with the right protocol without delay.

If you are unsure how to interpret a blood result during treatment, or if a trend concerns you, contact the CureFIP support team directly at info@curefip.com. Our goal is to make sure no cat stops treatment unnecessarily — and no owner goes through this process without informed support.