Isothermal amplification vs lateral-flow vs laboratory PCR: choosing the right test for infectious disease in dogs and cats

What each method actually detects

A lateral-flow assay (LFA) detects protein. Antibodies fixed on a nitrocellulose strip capture viral or parasite antigen from the sample, or the animal's own antibodies against a pathogen, and a coloured conjugate makes the capture visible as a line. The read-out is binary and by eye.

Laboratory PCR and chairside isothermal amplification both detect nucleic acid. Primers bind a sequence unique to the pathogen, an enzyme copies it, and a fluorescent signal accumulates as the copies multiply. Real-time PCR does this by cycling temperature; isothermal methods (LAMP, RPA, RHAM and related chemistries) do it at one temperature, which is what allows the instrument to shrink to the size of a hand.

The consequence is fundamental: protein is present only when enough intact pathogen is being shed at the sampling site, whereas nucleic acid can be found earlier in infection, at lower loads, and in the tail of shedding after the antigen signal has faded. This single difference explains most of the sensitivity gap discussed below.

Analytical and clinical sensitivity

Analytical sensitivity is how little the test can find in a clean sample. Lateral-flow cassettes for canine parvovirus need on the order of hundreds of thousands to millions of viral particles per gram of faeces before a line appears; nucleic-acid tests find a few hundred to a few thousand copies per reaction. Clinical sensitivity is what that means in real patients, and published comparisons put faecal antigen lateral-flow sensitivity for canine parvovirus somewhere between about 50 and 80 percent against PCR, depending on the day of illness, with specificity above 95 percent. In plain terms, the cassette rarely lies when positive but misses a meaningful share of true infections.

For the chairside isothermal platform, the validation studies hosted on our clinical trials page report 94.4 to 97.0 percent sensitivity against laboratory qPCR across the assays evaluated, with specificity at or near 100 percent. That places it within a few points of the laboratory, on the consulting table, in the same visit.

The detection window

Antigen appears when shedding peaks and disappears when it falls, so a cassette read on day one of clinical signs, or on day seven, can be negative in an animal that was strongly positive on day three. Nucleic acid is detectable from late incubation through convalescence. For a kitten with sneezing that started yesterday, or a dog whose diarrhoea started a week ago, the molecular test is simply looking through a wider window.

Antibody cassettes have the opposite profile: they turn positive one to three weeks after exposure and stay positive for months to years. They answer 'has this animal met the pathogen or the vaccine', which is useful for immunity checks and for FIV, but not for 'is this animal infectious today'.

Multiplexing and the syndromic case

Feline upper respiratory disease is the textbook example. Calicivirus, herpesvirus, Mycoplasma felis, Chlamydia felis and Bordetella bronchiseptica produce the same sneezing, discharge and conjunctivitis, and co-infection is common. A single-target cassette cannot resolve this; a laboratory panel can, but two days later; a six-target isothermal card resolves it during the consultation and lets the clinician choose between an antiviral, doxycycline or supportive care before the owner leaves.

Canine infectious respiratory disease is the same story with distemper, parainfluenza, adenovirus type 2, influenza, Bordetella and Mycoplasma. Gastrointestinal panels pair parvovirus and coronavirus with Giardia and Tritrichomonas. The value of the panel is not one extra pathogen; it is that the clinician stops guessing which single test to buy.

Turnaround, workflow and cost per answer

The cassette is cheapest per test and fastest, and it is the right tool when pre-test probability is high, the answer is binary and a negative result would be confirmed anyway. The laboratory panel is the most expensive per test once shipping and a second consultation are counted, and it is the right tool when quantitation, an unusual target or sequencing is needed. The isothermal card sits between the two in price and beats both on cost per answer whenever the alternative is a cassette followed by a send-out, or a treated-blind patient followed by a return visit.

Workflow is where clinics feel the difference. The card needs a swab, a buffer tube and a pipette; there is no extraction, no reagent preparation and no amplicon handling because the card is sealed. A nurse can run it. The dock reports to the practice-management system over Wi-Fi or USB.

Where isothermal does not replace the laboratory

Quantitation (viral load monitoring, treatment response), sequencing and resistance markers, targets not on any card, and forensic or regulatory confirmation remain laboratory work. Specimen quality still limits every platform: a dry swab from the wrong site is negative everywhere. And the platform's own validation applies to the targets it was validated for, not to every assay by extension; ask for the study on the pathogen you care about, and we will send it or say that it does not exist.

A practical decision rule

High pre-test probability, single pathogen, binary decision: lateral-flow. Syndromic presentation, treatment decision today, owner unlikely to return: isothermal card. Need a number, a rare target, a sequence or a legally defensible result: laboratory PCR. Most clinics end up running all three, and the distributor who can supply all three from one catalogue, with the validation data behind each, is the one who keeps the account.

Written by the Medicare Vet product team from manufacturer data, WOAH guidance and published validation. Not a substitute for veterinary or laboratory advice in your market.