Your Cat's Blood Work During FIP Treatment: What the Numbers Are Really Telling You

The lab results are back. Your cat is eating again, showing more energy, acting more like themselves after days that felt impossible. You open the chemistry panel — and your stomach drops.

Globulins still sky-high. ALT trending up. White blood cells outside the reference range. You start Googling. The anxiety spikes.

Here's what most cat owners don't find out until later: many of those abnormal values during active Feline Infectious Peritonitis (FIP) treatment aren't just explainable — they're predictable. They are not proof that the treatment is failing. In many cases, they're evidence that your cat's immune system and liver are actively doing exactly what they're supposed to do while the virus gets suppressed.

FIP Cat Blood Work During Treatment: What to Expect

This guide cuts through the confusion. It explains — clearly and without unnecessary medical jargon — what happens to your cat's blood work during GS-441524 therapy, which values improve first, which are the last to normalize, and what a result actually looks like when something needs your vet's attention right away. Whether you're managing treatment at home or working alongside a veterinary practice, this is the reference you needed on day one.

Why Blood Work Is the Single Most Important Monitoring Tool in FIP Treatment

Feline Infectious Peritonitis (FIP) is caused by a mutated feline coronavirus (FCoV) that triggers a systemic inflammatory cascade throughout the body. GS-441524 — now legally prescribable by U.S. veterinarians since mid-2024 — works by blocking viral replication at the cellular level. But tissue recovery and lab value normalization take time, even as viral load is dropping.

In the United States, where many cat owners are managing treatment through their veterinarian or directly through CureFIP USA, blood panels give you three things you cannot get from observing your cat alone:

• Objective confirmation that the disease is reversing — inflammation markers declining, protein fractions rebalancing

• Early warning of potential side effects — particularly hepatic or renal stress that may not show any outward symptoms yet

• Dosing guidance — if your cat isn't responding as expected, the panel tells you and your vet whether a dosage adjustment is warranted

The standard monitoring schedule is: baseline panel before treatment starts, repeat panels at weeks 4 and 8, and a final comprehensive panel at day 84. After treatment ends, the 12-week observation period calls for additional blood work at weeks 4, 8, and 12 post-treatment.

The trend across multiple draws matters far more than any single value at any single point in time.

The Key Lab Values in FIP Treatment, Explained

Globulins

What they are: Globulins are immune proteins the body produces in response to infection. In cats with active Feline Infectious Peritonitis (FIP), globulins are almost universally elevated — often dramatically, frequently in the 50–90 g/L range — because the immune system has been in a state of sustained overdrive.

What to expect during treatment: Globulins are typically the last marker to normalize. It is completely normal for them to remain elevated at weeks 4 and 8, even in cats who are visibly thriving. The number that matters isn't the absolute value — it's whether the trend line is heading downward.

Red flag: Globulins that continue climbing or remain completely flat after week 8, alongside other clinical signs of deterioration, may indicate the virus isn't being adequately suppressed at the current dose.

Albumin-to-Globulin Ratio (A:G Ratio)

What it is: The A:G ratio compares albumin — a structural protein produced by the liver — to total globulins. In healthy cats, this ratio typically sits above 0.8. In cats with active Feline Infectious Peritonitis (FIP), it often falls below 0.5, sometimes to 0.3 or lower.

What to expect during treatment: A rising A:G ratio is one of the earliest and most reliable indicators of treatment response. As globulins fall and albumin recovers, this ratio climbs back toward the normal range. Seeing A:G improve at the week-4 draw — even when individual values are still outside normal limits — is a meaningful, concrete positive signal.

Why you need to read it alongside individual values: A cat whose globulins dropped from 72 g/L to 55 g/L while albumin climbed from 15 g/L to 21 g/L is responding well to therapy — even if neither number has crossed back into the normal reference range. The A:G ratio captures that progress where individual markers still don't show it.

ALT (Alanine Aminotransferase)

What it is: ALT is a liver enzyme that rises when hepatic cells are stressed or damaged. Elevated ALT in FIP cats can stem from the disease itself, but it is also a documented, generally transient side effect of GS-441524 — meaning the medication can cause moderate ALT elevation that is not indicative of liver failure.

What to expect during treatment: A moderate ALT rise during the first four to eight weeks of GS-441524 therapy is a well-documented finding. It does not mean your cat is in liver failure. It does not mean the treatment is failing.

When to act: An ALT that spikes to more than three to five times the upper limit of the normal range, or that keeps climbing after week 8, warrants a closer look. Concurrent jaundice — yellowing of the gums, whites of the eyes, or earflaps — alongside high ALT is a more urgent sign that requires same-day veterinary evaluation.

Do not stop treatment on ALT alone without speaking to your vet first. Discontinuing GS-441524 prematurely carries a significantly higher risk of relapse than a moderate, monitored enzyme elevation in the vast majority of cases.

ALP (Alkaline Phosphatase)

What it is: ALP is another liver-associated enzyme. Elevations during Feline Infectious Peritonitis (FIP) treatment are documented and commonly accompany ALT changes.

What to expect: Mildly elevated ALP during treatment is common and typically self-resolves during or after the treatment course. Persistently elevated ALP after confirmed recovery warrants specific hepatic follow-up.

SDMA and Creatinine (Kidney Function)

What they are: SDMA (symmetric dimethylarginine) and creatinine are markers of kidney filtration function. SDMA is a more sensitive early indicator than creatinine — it can detect a reduction in renal function before creatinine rises.

What to expect during treatment: Transient SDMA elevations have been reported in some cats during GS-441524 therapy. This does not automatically mean kidney damage — GS-441524 is renally excreted, and some functional shifts can occur without structural injury.

When to act: Rising SDMA alongside elevated creatinine, decreased urine specific gravity, or increased water intake requires veterinary evaluation. Cats who started treatment with pre-existing chronic kidney disease need closer, more frequent renal monitoring throughout.

Complete Blood Count (CBC): White Blood Cells and Differential

What it is: Total white blood cell count plus the breakdown of cell types: neutrophils, lymphocytes, eosinophils, monocytes.

What to expect during treatment:

• Lymphocytosis (elevated lymphocytes) has been reported during GS-441524 therapy and is generally not clinically significant when isolated

• Eosinophilia (elevated eosinophils) is also documented as a transient finding and does not by itself point to parasites or an allergic reaction unless accompanied by other clinical signs

• Neutrophilia (elevated neutrophils) early in treatment reflects the active systemic inflammation from Feline Infectious Peritonitis (FIP) itself and should gradually decrease as treatment takes effect

A CBC trending toward the normal reference range between weeks 4 and 8 is a good sign. Persistent or worsening neutrophilia may point to a secondary bacterial infection or insufficient viral control at the current dose.

Hematocrit (PCV) and Red Blood Cells

What it is: The percentage of red blood cells in circulating blood. Many FIP cats are anemic at diagnosis due to immune-mediated red cell destruction and bone marrow suppression.

What to expect: Anemia improves gradually as viral load decreases and the immune system stabilizes. Do not expect hematocrit to normalize in the first two to four weeks. Cats who remain severely anemic beyond week 8 may need a supportive care conversation with their veterinarian.

Total Protein and Fibrinogen

What they are: Total protein is albumin plus globulins combined. Fibrinogen is an acute-phase inflammatory protein that rises significantly with active systemic inflammation.

What to expect: Total protein normalizes as albumin climbs and globulins fall. Very high baseline values — above 90–100 g/L in some presentations — are consistent with active Feline Infectious Peritonitis (FIP). A sustained downward trend during treatment is the key indicator to watch.

Reading Your Cat's Blood Work Week by Week

Before Treatment Starts (Baseline)

The pre-treatment panel is your reference point for everything that follows. Expect elevated globulins, low albumin, a depressed A:G ratio, possible anemia, and disrupted inflammatory markers. Every subsequent result gets measured against this baseline — not against the healthy reference range for a normal cat.

Week 4 (Day 28)

In cats responding to treatment, the week-4 draw typically shows:

• A:G ratio improving, even if absolute values remain outside reference limits

• Globulins lower than baseline (may still be elevated in absolute terms)

• CBC trending toward normal

• Partial anemia improvement

• ALT possibly elevated but stable, not climbing

If globulins haven't moved and clinical signs haven't improved, verify that the current dose is appropriate for your cat's actual body weight using the CureFIP Dosage Calculator before drawing conclusions about treatment failure.

Week 8 (Day 56)

By week 8, a well-responding cat should show values moving clearly toward the normal reference range. Globulins should be measurably lower than at week 4. ALT and ALP, if elevated, should be stable or declining. Albumin should show a visible recovery trend.

If values are still moving in the wrong direction at week 8, discuss whether a dose escalation is appropriate with your veterinarian. Do not make dosing changes unilaterally.

Day 84 (End of Treatment)

The goal at the end of the 84-day course is not perfect lab results — it's clinical remission supported by a clear trend toward normal across most parameters. Some markers, particularly globulins, may still be slightly outside the reference range. The criteria for ending treatment are:

• Clinical remission: normal appetite, stable weight, normal activity level, normal body temperature

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

• No active effusions

• No other signs of ongoing active disease

For a full breakdown of the clinical milestones expected at each stage of the treatment course, see the GS-441524 FIP Treatment Timeline Guide.

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

The 12 weeks following treatment require continued blood monitoring. At each of the three post-treatment check-ins, values should continue normalizing. Any marker that moves in the wrong direction after treatment ends — particularly globulins rising again or albumin falling — should be reported to your veterinarian immediately. This pattern can indicate relapse, and early intervention makes a meaningful difference in outcomes.

The Most Common Misreads During FIP Treatment

"ALT is elevated — I need to pull my cat off the medication." In most cases, no. Moderate ALT elevation during GS-441524 therapy is a documented, typically reversible finding. Pulling treatment prematurely is almost always riskier than monitored enzyme elevation, because it opens the door to relapse. Have the conversation with your vet before making any changes.

"The blood work looks almost normal at week 4 — we can probably stop early." Early normalization is encouraging, but it does not mean the virus has been cleared. The 84-day protocol is grounded in the biology of feline coronavirus persistence inside macrophages. Normalized labs are not a virological cure. This is one of the most common reasons for preventable relapse in the US FIP community.

"The numbers didn't improve at week 4 — the treatment isn't working." Some cats are slower responders. Clinical improvement — better appetite, reduced fever, returning energy — often precedes laboratory normalization by two to four weeks. Evaluate the clinical picture and the blood work together before drawing conclusions.

"Globulins are still high at week 8 — that's a bad sign." Globulins are the last parameter to normalize in almost every successfully treated FIP case. Many cats still show elevated globulins at week 8 while responding excellently to treatment. The direction of the trend line is what matters — a consistent downward trajectory is reassuring even when the absolute number remains elevated.

FAQ: Cat Blood Work During FIP Treatment

What blood tests are needed to monitor FIP treatment?

The standard panel includes a full chemistry profile — total protein, albumin, globulins, ALT, ALP, creatinine, SDMA — plus a complete blood count (CBC) with differential. The A:G ratio is calculated from albumin and globulins. Recommended minimum testing points: baseline before treatment, week 4, week 8, and day 84 at the end of the treatment course.

What do high globulins in a cat mean during FIP treatment?

Elevated globulins during Feline Infectious Peritonitis (FIP) treatment reflect sustained immune system activation in response to the feline coronavirus infection. This is an expected finding during the early and middle phases of treatment. A consistently declining trend — even if values remain above the reference range — is a positive indicator of GS-441524 response.

Is it normal for ALT to increase during GS-441524 treatment?

Yes. Moderate ALT elevation is a documented side effect of GS-441524 therapy and does not automatically indicate liver damage or treatment failure. Regular monitoring is essential; a significant or progressive rise should be evaluated by a veterinarian before any treatment modifications are made.

What is the A:G ratio and why does it matter in FIP?

The albumin-to-globulin ratio (A:G ratio) compares two protein fractions in the blood. In healthy cats, it typically sits above 0.8. In cats with active Feline Infectious Peritonitis (FIP), it often falls below 0.5. A rising A:G ratio during treatment is one of the earliest and most reliable indicators that GS-441524 is working.

How often should blood tests be done during FIP treatment?

Minimum recommended schedule: baseline before starting, week 4, week 8, and day 84 at end of treatment. After treatment, draws at weeks 4, 8, and 12 of the observation period confirm sustained remission.

Can blood work look normal even if my cat relapses?

Yes. Lab values can normalize before the virus is fully eliminated. This is one of the core reasons both the complete 84-day treatment course and the 12-week post-treatment observation period are non-negotiable, regardless of how clean the blood work looks.

My cat's blood work is getting worse at week 4. What do I do?

Contact your veterinarian immediately. Deteriorating values at week 4 may indicate subtherapeutic dosing, a product quality issue, or a form of FIP requiring a higher-dose protocol. Verify the correct weight-based dose with the CureFIP Dosage Calculator and do not make changes without professional guidance.

What happens to SDMA during FIP treatment?

Transient SDMA elevation has been documented in some cats during GS-441524 therapy and does not necessarily indicate permanent renal injury. Cats with pre-existing chronic kidney disease require more frequent renal monitoring throughout the course. Persistently elevated or rising SDMA after treatment ends warrants further veterinary investigation.

My vet hasn't seen many FIP cases. What should I tell them about blood work monitoring?

This is common in the US, where GS-441524 only became legally prescribable in mid-2024. The most useful thing you can provide is a complete chronological history of every blood panel from baseline forward, along with the recommended monitoring schedule: weeks 4, 8, and 84 during treatment, and weeks 4, 8, and 12 post-treatment. UC Davis research and the CureFIP clinical team are both available as resources for veterinarians who want clinical support.

Tracking Blood Work: The Practical Side

Keep every result. Take a photo of each panel the day it comes in and add it to a running document or spreadsheet. When you go back to your vet, bring the full timeline — not just the most recent draw. Trends are the story. Individual values are just data points.

Many American cat owners managing FIP treatment for the first time find the blood work to be the most stressful part of the process — not because the news is bad, but because they don't have a framework for interpreting it. Now you do.

If your cat shows warning signs after treatment ends — returning fever, appetite loss, weight drop, abdominal swelling — don't wait for the next scheduled appointment. The CureFIP USA Relapse Guarantee is built exactly for this situation and allows your cat to restart treatment with the correct protocol without delay.

If you have questions about a blood result during active treatment and aren't sure what you're looking at, the CureFIP USA support team is available at info@curefip.com. No cat owner should have to figure out their cat's lab values alone in one of the hardest moments of pet ownership.