Why a Negative RT-PCR Result Does Not Always Mean No Infection

A negative RT-PCR result means the test did not find the germ in that sample on that day. It does not always mean the animal is free of infection, because the sample may have been taken too early, too late or from the wrong place. A registered veterinarian reads the result together with the animal's signs.

What does a negative result really tell you?

RT-PCR looks for the genetic material of a germ in the sample that reaches the lab. It can only find what is in that tube. So a negative result is an answer about one swab or one blood sample, not a promise about the whole animal.

That does not make a negative useless. It can make some causes less likely and help your vet decide what to look at next. But if the animal is still sick, the signs still matter.

Can the timing of the sample change the answer?

Yes. The amount of a germ in the body rises and falls during an illness. Very early on there may be too little to find. Later, the body may have cleared the germ from the nose or blood while the animal is still unwell.

Dogs with distemper show how this can go. The Canine Distemper RT-PCR looks for the virus, but in some dogs the signs that bring them to the clinic, such as twitching, appear after the virus has left the places a swab or blood sample can reach. The vet may then rely on the examination and other tests.

In cows and buffaloes, the BVD Virus RT-PCR looks for the bovine viral diarrhoea (BVD) virus. In a short, passing infection the virus may be in the blood only briefly. Some animals are infected before birth and carry the virus for a long time while looking quite normal. That is a different situation, and the vet reads it separately.

Why can a germ be missed when it is really there?

There are several common reasons.

• Low numbers. The sample holds too few germs for the test to catch.
• Shedding that comes and goes. Some cats pass coronavirus in their dung only now and then, so one negative result on the FCoV / FIP RT-PCR speaks only for that day.
• A quiet infection. Horse herpesvirus, tested with the Equine Herpesvirus (EHV-1 & EHV-4) RT-PCR, can stay quiet in a horse's body after infection and become active again at times, for example when the horse is stressed. A swab taken in a quiet phase can be negative.
• A poor sample. The swab may have missed the right spot, held too little material, or got hot in transport so the genetic material broke down.
• Interference. Substances in some samples, such as dung, can get in the way of the test reaction.

What is a carrier and why does it matter?

A carrier is an animal that holds a germ without obvious illness and may pass it on at times. A carrier can test negative on a day it is not shedding. In a stable, a cattle shed, a kennel or a home with several cats, one negative result therefore cannot speak for every animal or every day.

The opposite also happens. A healthy carrier can test positive, which is why a positive result is read with the signs too.

What happens after a negative result?

Your vet may choose to:

• repeat the test on a fresh sample taken at a different stage
• take a different kind of sample
• add other tests, such as a blood count or an antibody test
• examine the animal again as the signs change

A sick animal needs prompt care whatever the test says, so please never delay seeing a vet because a report came back negative. Ask the vet whether the animal should be kept apart from others while the cause is unclear.

You can help the next result mean more. Give the vet a clear history of when signs began, which animals are affected and any recent vaccine, and keep every sample cool and well labelled.

Read next

If you keep cows or buffaloes, Cattle Viral RT-PCR Panel: FMD, LSD, BVD, Bluetongue and IBR shows how one panel looks at several viruses with overlapping signs, which helps when a single result is not enough. For a closer look at one disease where several tests are used side by side, see Canine Distemper: RT-PCR Versus the Rapid Test.

Testing in Bhopal

If a result leaves you unsure, ask your vet about a repeat sample rather than a guess. To book, call or WhatsApp +91 88175 01118 and the lab at C-200, C Sector, Shahpura, Bhopal will help. Collection is free anywhere in Bhopal, and other districts can use courier, bus parcel or a local vet. The lab confirms the method, sample needs and reporting time when you book. For horses, cows and buffaloes, the vet usually takes the sample.