Infectious Disease Testing: Choosing the Right Test for the Sign
Quick Answer — Most infectious disease testing errors are not analytical; they are categorical. A clinician orders the wrong kind of test (antibody where antigen is needed), at the wrong time (outside the shedding window), on the wrong sample (a nasal swab where a conjunctival one is needed). This page is organised around the decision, not around the pathogen.
Quick Answer
Antigen for the sick animal today. Antibody for the immune question tomorrow. Everything else is timing and sample quality.
The categorical distinction
| Antigen | Antibody | |
|---|---|---|
| Question | Is the pathogen present now? | Has this animal met it? |
| Use in acute disease | Primary | Supplementary (unvaccinated animals) |
| Effect of vaccination | None | Confounds interpretation completely |
| Window | During shedding — often narrow | Weeks after exposure; persists |
| Typical false result | Negative (early, late, poor sample, hook effect) | Positive (vaccination) |
In practice, this table prevents the two dominant errors: diagnosing infection from a vaccinated animal’s antibody, and excluding disease on a single antigen test taken at the wrong moment.
By clinical sign
Haemorrhagic diarrhoea, puppy. Parvovirus antigen first. Add coronavirus antigen where the animal is severe or co-infection is suspected. Consider that a positive coronavirus result in an unwell puppy often accompanies parvovirus rather than replacing it.
Haemorrhagic diarrhoea, kitten. Feline panleukopenia antigen. Note that parvovirus assays may cross-detect FPV and CPV, which is useful but must be interpreted with the species in mind.
Upper respiratory signs, cat. Calicivirus and herpesvirus antigen from oropharyngeal/conjunctival swabs. Herpesvirus is a latency virus — a negative between episodes is expected, and recurrent ocular disease should raise suspicion regardless of the swab result.
Neurological signs, dog. Distemper antigen, with the caveat that antigen may be undetectable at the neurological stage. Antibody is not diagnostic in a vaccinated dog.
Effusion or fever of unknown origin, young cat. Feline coronavirus — but on effusion, not on serum antibody. An antibody titre does not diagnose FIP.
Weight loss, anaemia, recurrent infection, cat. Feline leukaemia virus antigen and feline immunodeficiency virus antibody, as a pair. Two retroviruses, one sample.
Weight loss, skin lesions, endemic region. Leishmania antibody, interpreted with species, endemicity and clinical signs.
Building a co-infection panel
A panel is justified when the result changes management beyond what a single test would. Four patterns cover most of the value:
| Setting | Panel | Why |
|---|---|---|
| Shelter intake, diarrhoea | Parvovirus + coronavirus | Co-infection alters prognosis and isolation |
| Cattery, respiratory outbreak | Calicivirus + herpesvirus | Management differs (antiviral vs supportive) |
| Sick kitten | Panleukopenia + FeLV + FIV | Retrovirus status changes long-term plan |
| Breeding group | Progesterone timing + (later) relaxin | Two different questions, sequenced |
Panels that only confirm an already-made diagnosis add cost without changing action.
Why a negative result is not the end
The four mechanisms behind a false negative deserve equal weight in training:
- Timing — before shedding or after it has declined.
- Sample — wrong site, poor technique, insufficient material.
- Antigen excess — very high concentrations read low; dilution confirms.
- Interference — haemolysis, lipaemia, or a heavily contaminated sample.
The practical rule: in a strongly suspected case, a negative result is a prompt to repeat with a better sample, not a conclusion.
Where this menu does not reach
Data pending. This section reports measured values from internal validation studies. They are published once the supporting study is complete; the current report is available from technical support on request.
Stating a limitation is more useful than omitting it: a clinician who knows the boundary orders the test that exists, rather than assuming a panel covers everything.
The Author’s Take
Position. An index page organised by clinical sign will outperform one organised by pathogen name, because clinicians arrive with a sign and a patient, not with a taxonomy.
Reasoning. Pathogen-organised menus are convenient for manufacturers and useless at the point of care: nobody wakes up wanting to test for a virus, they want to know why the puppy has bloody diarrhoea. A sign-first index also does something a pathogen list cannot — it makes the trade-offs visible, including when not to test, which is where most of the cost and most of the harm sits. For the vendor, the practical benefit is that such a page becomes the page clinicians bookmark, and a bookmarked page is worth more than ten pages that rank once and are never returned to.
Disclosure. This page is clinical decision guidance, not a performance claim. Figures marked indicate where menu coverage must be stated accurately.
Related disease pages
- Canine parvovirus · Canine distemper · Canine coronavirus · Canine adenovirus · Canine leishmaniasis · Canine toxoplasmosis
- Feline panleukopenia · Feline calicivirus · Feline herpesvirus · Feline coronavirus / FIP · Feline leukaemia · Feline immunodeficiency
Sources
- Reference to be added.
- Reference to be added.
- CLSI EP17 / EP06 — detection capability and measuring interval (basis for the false-negative discussion).
Frequently asked questions
- Should I test for antigen or antibody?
- Antigen answers 'is the pathogen here now' and is the test for acute disease. Antibody answers 'has this animal met the pathogen or the vaccine' and is the test for immune status. In a vaccinated population a positive antibody result usually means vaccination, not infection, which is why antigen testing is the diagnostic anchor for acute disease.
- When should a co-infection panel be run instead of a single test?
- When the clinical picture is severe or the setting is group housing. A puppy with haemorrhagic diarrhoea should have parvovirus and coronavirus assessed together, because co-infection changes prognosis. In a cattery, upper respiratory signs warrant both calicivirus and herpesvirus, since both circulate and management differs for herpes.
- Why do some infected animals test negative?
- Four common reasons: sampling too early or too late in the shedding window, poor sample site or technique, antigen excess in very high-titre samples (the hook effect), and interference from a haemolysed or otherwise compromised sample. A negative result in a strongly suspected case is a reason to repeat with a better sample, not to close the case.