FIP Diagnosis UK: A Cat Owner's Fast-Track Guide

If your vet has mentioned Feline Infectious Peritonitis (FIP), or you have been searching symptoms late at night and the same three letters keep appearing, you are not alone. Thousands of UK cat owners face this moment every year, and what happens next, specifically how fast a diagnosis is reached and how quickly GS-441524 treatment begins, often makes the difference between recovery and loss.

FIP Diagnosis UK: A Cat Owner's Fast-Track Guide

CureFIP™ has supported caregivers across the UK and Europe as part of a network of 100,000+ cats treated since 2019. This guide walks you through how FIP is diagnosed in the UK, what tests are worth pushing for, which results actually matter, and how to avoid the slow or invasive workups that cost precious days.

Why FIP Diagnosis in the UK Is Often Delayed

FIP is caused by a mutation of the feline coronavirus (FCoV). It progresses quickly, and untreated cases are almost always fatal within weeks. Yet many UK clinics still default to lengthy, stepwise workups, sometimes including referral to a second-opinion internal medicine specialist, before confirming the diagnosis.

The reasons are understandable. FIP can mimic other diseases (lymphoma, cholangiohepatitis, toxoplasmosis, cardiomyopathy), and no single test gives a 100% definitive answer in every case. But waiting for absolute diagnostic certainty is the most common mistake UK owners encounter. By the time biopsy results return, the cat may have lost critical ground.

The modern approach, used by FIP-experienced vets, is to combine pattern recognition (signalment, symptoms, bloodwork) with targeted confirmatory tests, and to start the 84-day antiviral protocol as soon as the clinical picture is convincing.

The Typical FIP Patient Profile

Before any test, your vet will consider the patient. Classic FIP risk factors include:

1. Age under 2 years (though older cats can develop it).

2. Recent stressor: rehoming, neutering, vaccination, boarding, or new household member.

3. Multi-cat origin: shelters, rescues, breeders, ferals.

4. Pedigree breeds, particularly Birmans, Ragdolls, Bengals, British Shorthairs and Maine Coons.

5. Persistent fever unresponsive to antibiotics.

6. Failure to thrive in a young cat.

If your cat matches several of these, you should ask your vet directly: "Could this be FIP, and what is the fastest way to find out?"

The Four Types of FIP and What Diagnosis Looks Like for Each

FIP does not present the same way in every cat. UK vets typically classify it into four types, and the diagnostic pathway differs slightly for each.

Wet (Effusive) FIP

The most recognisable form. Fluid accumulates in the abdomen (most common), chest, or pericardium. Cats often have a distended belly, laboured breathing, lethargy and weight loss despite a swollen appearance.

Diagnosis is usually the fastest here, because the effusion itself can be sampled and tested.

Dry (Non-Effusive) FIP

No significant fluid. Instead, granulomas form in organs such as the kidneys, liver, intestines or lymph nodes. Symptoms are vaguer: chronic fever, weight loss, jaundice, palpable masses. Diagnosis is harder and relies more on bloodwork, imaging and sometimes fine-needle aspiration of affected tissue.

Ocular FIP

The eye is affected, often as the first visible sign. Look for colour change in the iris, cloudiness, uveitis, keratic precipitates (small deposits on the inside of the cornea), or sudden blindness. An ophthalmic exam combined with systemic bloodwork is usually diagnostic.

Neurological FIP

The most dangerous form to delay. Signs include wobbliness (ataxia), seizures, head tilt, behavioural change, incontinence, or hind-limb weakness. Diagnosis may involve MRI and cerebrospinal fluid (CSF) analysis if available, but in practice many UK vets diagnose neurological FIP on clinical signs plus supportive bloodwork, because waiting for imaging can be fatal.

Step 1: The Bloodwork That Actually Points to FIP

If you take nothing else from this guide, take this: a standard haematology and biochemistry panel, run in-house or at a UK reference lab, gives most of what is needed to suspect FIP within 24 hours.

Ask your vet to look specifically at:

Globulins and the A/G Ratio

FIP cats typically have elevated globulins (often above 51 g/L) and low albumin, producing a low albumin-to-globulin ratio. An A/G ratio below 0.6 is highly suggestive of FIP. Above 0.8, FIP becomes less likely (though not impossible).

Total Protein

Often elevated due to the globulin spike, despite the cat being clinically wasted.

Lymphocytes

Lymphopenia (low lymphocyte count) is very common in FIP.

Bilirubin

Elevated bilirubin without obvious haemolysis or biliary obstruction is a red flag, especially in a young cat with fever.

ALT and Liver Enzymes

Mild to moderate elevations are common. ALT is also one of the markers tracked week by week during the 84-day antiviral protocol.

SDMA and Kidney Markers

SDMA, creatinine and urea help establish renal baseline before treatment, which matters because dosing and monitoring depend on kidney function.

Haematocrit

Mild non-regenerative anaemia is typical.

A young cat with fever, weight loss, low A/G ratio, elevated bilirubin and lymphopenia is, until proven otherwise, an FIP suspect. That single pattern can justify starting treatment in many UK clinics that are experienced with FIP.

Step 2: Imaging

Ultrasound

For suspected wet or dry FIP, abdominal ultrasound is the most useful single imaging test. It detects:

1. Free abdominal fluid (even small volumes invisible on X-ray).

2. Enlarged mesenteric lymph nodes.

3. Kidney capsule changes ("granulomatous nephritis").

4. Intestinal wall thickening.

5. Liver or splenic nodules.

Thoracic X-ray or Ultrasound

If breathing is laboured, this rules in or out pleural effusion or pericardial fluid.

MRI

Reserved for neurological cases at referral centres. Useful but rarely essential. Most UK owners cannot wait the days or weeks for a referral slot, and most FIP-experienced vets will start treatment based on bloodwork plus neurological exam.

Step 3: Effusion Analysis, The Fastest Confirmation

If there is any abdominal or chest fluid, sampling it is the single most powerful diagnostic step. A small needle tap (often done conscious or with light sedation) gives fluid that can be tested immediately.

Appearance

FIP effusion is classically straw-yellow, viscous, and may froth slightly when shaken (due to high protein content).

Protein Content

FIP effusions are exudates with very high protein (typically above 35 g/L).

Cell Count

Low to moderate, predominantly macrophages and neutrophils. A low cell count with high protein is a hallmark.

The Rivalta Test

A simple, low-cost test that many UK clinics can perform in minutes. A drop of effusion is added to a tube of distilled water with one drop of acetic acid. If the drop holds its shape and slowly sinks, the test is positive (suggestive of FIP). If it dissolves, it is negative.

A positive Rivalta test in a young cat with classic bloodwork is, for most FIP-experienced clinicians, sufficient to begin treatment.

PCR on Effusion

Reverse transcription PCR (RT-PCR) for feline coronavirus on effusion fluid is highly specific. A positive PCR on abdominal or chest fluid is essentially confirmatory in a clinically suspect cat. UK reference labs such as Langford Vets and IDEXX offer this test, with turnaround typically 2 to 5 working days.

Step 4: When Biopsy Is (and Is Not) Necessary

Histopathology with immunohistochemistry on affected tissue remains the gold standard. But here is the honest reality for UK owners: biopsy is invasive, expensive, requires general anaesthesia in a cat already weakened by disease, and the results take a week or more.

In modern FIP practice, biopsy is reserved for:

1. Cats with no effusion and no clear ultrasound abnormality.

2. Cases where bloodwork is genuinely ambiguous (lymphoma is the main differential).

3. Suspected dry FIP where a palpable mass can be safely aspirated.

Fine-needle aspiration of an enlarged lymph node or organ lesion, sent for cytology with PCR, is a much faster and less invasive alternative and is increasingly used in the UK.

Putting It Together: A Realistic UK Diagnostic Timeline

A well-coordinated FIP workup looks like this:

Day 1: Clinical exam, full bloodwork (haematology, biochemistry, A/G ratio, SDMA, bilirubin), urinalysis, abdominal ultrasound. If effusion present, sample it. Run Rivalta in clinic. Send fluid for PCR and protein analysis.

Day 2 to 3: Bloodwork interpretation. If pattern fits FIP, many experienced vets will start the 84-day antiviral protocol immediately rather than wait for PCR.

Day 4 to 7: PCR result returns. Treatment continues or is adjusted based on response.

If your clinic is recommending a longer pathway with multiple referrals before any treatment, and your cat is visibly deteriorating, it is reasonable to seek a second opinion from a vet experienced in FIP management.

What Happens After Diagnosis: Starting GS-441524 Treatment

Once FIP is confirmed or strongly suspected, the priority shifts to starting GS-441524, the antiviral that revolutionised FIP outcomes after the work of Dr. Niels Pedersen at UC Davis. The Pedersen 2019 study reported a 92% success rate with GS-441524 injectable monotherapy.

CureFIP™ offers three injectable strengths so dosing can be matched to your cat's weight and FIP type:

1. CureFIP™ GS-441524 Injectable 20mg/ml at €79.00, available in 8ml and 10ml.

2. CureFIP™ GS-441524 Injectable 30mg/ml at €89.00, available in 8ml and 10ml.

3. Cure FIP Antiviral 40mg/ml at €119.00, available in 8ml and 10ml.

All three follow the same dosing framework: 6 mg/kg for wet FIP, 8 mg/kg for dry FIP, 10 mg/kg for ocular FIP, and 10 mg/kg for neurological FIP. Schedule: one subcutaneous injection per day, every day, for 12 weeks (84 days). Reference: Pedersen et al., UC Davis (PMC6435921).

For owners who prefer an oral route, the CURE FIP™ Dual Antiviral Oral Capsules at €179.00 combine GS-441524 with EIDD-1931. Dosing by weight: under 2.5 kg uses GS-441524 25 mg plus EIDD-1931 5 mg; 2.5 to 5 kg uses GS-441524 35 mg plus EIDD-1931 8 mg; over 5 kg uses GS-441524 50 mg plus EIDD-1931 12 mg. One capsule daily for the recommended 12 weeks. The dual oral protocol has shown a 78.3% remission rate (Li and Cheah 2025). Note: the oral dual route is positioned for wet and dry FIP and is not recommended once ocular or neurological signs are present, or when the cat cannot eat or defecate.

Your vet will help you choose the right product and strength based on weight, FIP type, and clinical condition.

Monitoring During Treatment

Diagnosis is only the start. Throughout the 84-day protocol, your vet should repeat bloodwork roughly every 3 to 4 weeks to track:

1. Globulins and A/G ratio (should normalise).

2. ALT and bilirubin (should fall).

3. Haematocrit and lymphocytes (should recover).

4. SDMA and creatinine (should remain stable).

Week by week improvement on these markers is one of the strongest predictors of remission.

FAQ

How is FIP diagnosed in the UK?

FIP is diagnosed through a combination of clinical signs, bloodwork (low A/G ratio, elevated globulins and bilirubin, lymphopenia), imaging (ultrasound for effusion or organ lesions), effusion analysis (Rivalta test, protein, cytology), and RT-PCR for feline coronavirus on fluid or tissue. No single test is required, and most experienced vets diagnose based on a convincing pattern rather than waiting for biopsy.

What is the most reliable FIP test for cats?

RT-PCR for feline coronavirus on effusion fluid or affected tissue is the most specific test. Combined with classic bloodwork (A/G ratio under 0.6, high globulins, low albumin, hyperbilirubinaemia in a young febrile cat), it provides high diagnostic confidence within days.

Can FIP be diagnosed without a biopsy?

Yes, and in most UK cases it is. Biopsy is invasive and slow. Bloodwork, ultrasound, effusion analysis with Rivalta, and PCR usually provide enough certainty to begin the 84-day GS-441524 protocol. Biopsy is reserved for genuinely ambiguous cases.

How quickly should treatment start after FIP is suspected?

As early as possible, ideally within the first 1 to 2 weeks of clinical signs. FIP progresses fast, and early treatment with GS-441524 is consistently linked to better survival and recovery. Delaying treatment for diagnostic certainty is the most common cause of poor outcomes.

Will my UK vet support GS-441524 treatment?

Many UK vets now actively support GS-441524 treatment and will monitor bloodwork, hydration and weight gain throughout the 84-day protocol. If your current vet is not experienced with FIP, CureFIP™ can help you connect with clinicians who are, and our team has supported a network of 100,000+ cats treated since 2019.

Vet-reviewed guide from CureFIP™.