What Is a CMV Immunostain Test for Cytomegalovirus?

A CMV immunostain is a lab stain a pathologist adds to a slide of biopsy tissue. It colors cells that contain cytomegalovirus (CMV) proteins, so the pathologist can see the virus inside the tissue itself. Unlike a blood test, it shows whether CMV is actually attacking the gut, liver or other organ that was sampled. It is mostly ordered for people with weakened immune systems or badly inflamed bowels, because CMV that sleeps harmlessly in most adults can flare in those settings. A positive stain is a strong clue rather than an automatic order for antiviral drugs, and doctors read it alongside how sick the person is.

How the CMV Immunostain Works on a Biopsy

Your biopsy is preserved, embedded in wax and cut into slices thin enough to see through on a glass slide. The slide is then covered with lab-made antibodies that stick only to proteins the virus makes early after it enters a cell.¹ Because of that, infected cells can be marked even before they develop the swollen look of advanced infection.² A chemical step turns the bound antibody brown or red, and a blue background stain shows the rest of the tissue.³ On the slide, infected cells appear as colored spots against blue tissue. Brown versus red depends on the chemical system the lab uses and says nothing about how severe the infection is.

This technique is different from the routine pink-and-purple stain every biopsy gets first to show tissue structure. On that routine stain, the pathologist looks for enlarged cells with dark centers, often called “owl’s eye” cells. The immunostain is an extra step added when CMV is suspected or those cells are unclear. It answers one narrow question, whether CMV proteins are present, rather than describing the whole tissue.

Labs run a known CMV-infected tissue and a known uninfected tissue alongside each case.⁴ Wrong timing, temperature or contaminated reagents can cause misleading color,⁵ so if the control samples don’t stain as expected, the result isn’t reported.

Why Your Doctor or Pathologist Ordered It

How often the stain turns positive depends heavily on who is tested. In one large hospital series of gut biopsies, fewer than 1 in 20 stained positive overall.⁶ But about 1 in 5 biopsies from transplant patients and nearly 1 in 4 from people with HIV or inherited immune deficiency were positive, while in inflammatory bowel disease the rate was under 1 in 25.⁷ Biopsies with severe inflammation were about twice as likely to be positive as mildly inflamed ones.⁸

Anti-rejection drugs deliberately dampen the immune cells that hold CMV in check, so the virus can wake up in the transplanted organ. Diarrhea, abdominal pain, fever or abnormal liver tests overlap with many other problems, so tissue is needed to tell them apart.

In ulcerative colitis or Crohn’s disease, steroids and other immune-suppressing drugs can let dormant CMV reactivate. CMV in the bowel can look like a flare that won’t respond to the usual treatment, so finding it can explain why a flare isn’t improving and change the plan.

For bowel biopsies from people with inflammatory bowel disease, pathologists may run the stain even if no CMV-looking cells are seen, especially with severe inflammation.⁹ Common triggers are ulcers that look deep or punched-out at endoscopy, a flare that fails steroids, or a transplant patient with diarrhea or stomach pain.

In transplants, the stain can catch infection early. In liver transplant recipients, it picked up CMV hepatitis an average of about 10 days earlier than the routine look in some patients.¹⁰ CMV proteins were also found in many kidney grafts lost to chronic rejection, even when the person never had obvious CMV illness.¹¹

What a Positive or Negative Result Means

The amount of staining carries meaning. Reports may describe “rare” positive cells, meaning two or fewer stained cells, versus clearly positive with more.¹² When more than two cells stained, over half of patients also had CMV in their blood within a week, while with only rare cells, under a quarter did.¹³

Wording on the report varies. “Positive” or “CMV immunostain positive” means virus proteins were found in the sample. “Negative” means none were seen in the slices examined, not that you never had CMV. “Rare” or “focal” means only a few cells stained. “Equivocal” means the pathologist couldn’t be sure.

Tiny infected cells, dying cells and background color can be hard to tell apart, especially in inflamed or damaged tissue.¹⁴ Two pathologists can occasionally read the same slide differently, so an “equivocal” or “suspicious” result is worth discussing, and it can be followed by a DNA test of the tissue.

A positive stain is not a diagnosis by itself. It shows CMV proteins in the tissue, but cannot say whether the virus is the cause of your symptoms or an incidental passenger. Doctors usually combine it with how many cells stained, your symptoms, how the bowel looked at endoscopy, and a blood CMV DNA test. A small number of positive cells in a person with a severe, steroid-resistant flare may be read differently than the same finding in someone with mild symptoms.

If your report says equivocal or suspicious, ask whether a second pathologist has reviewed the slide, a common way to settle a hard call. Treatment is usually based on the whole picture, so an unclear stain by itself rarely forces an immediate decision.

How Often the Test Misses CMV

A negative stain can be wrong, because infection is patchy and sampling matters. Cases tested with several tissue pieces turned positive more than twice as often as single-piece cases.¹⁵ Testing 3 to 4 pieces nearly tripled detection, but 5 or more added nothing further.¹⁶ In nearly all patients with a positive stain in an inflamed area, normal-looking areas biopsied at the same time had no CMV.¹⁷

CMV tends to sit in clusters, so one forceps bite can land on healthy tissue next to an infected spot. Each extra piece samples a different spot, but the gain levels off, so the goal is a few well-chosen pieces, not dozens.

During the scope, the doctor can aim at ulcers, particularly the edges and base, because infected cells tend to be in damaged tissue. Biopsies from healthy-looking lining are less likely to be useful and are no substitute. If CMV is suspected beforehand, telling the pathologist lets them order the stain on every piece, rather than only when owl’s eye cells are seen.

Smaller amounts of virus can also slip past the stain. About 1 in 7 stain-negative gut biopsies still had CMV DNA on a more sensitive test.¹⁸ A negative stain lowers the odds of CMV disease but doesn’t rule it out when symptoms persist, and asking whether a DNA test on the tissue is possible is reasonable if you’re not improving.

If your stain was negative but you’re still very unwell, ask how many biopsies were taken and from which areas. A repeat scope may be suggested if your symptoms don’t fit the result.

Immunostain Versus PCR and the Routine Stain

The routine stain alone misses some infections. Compared against the immunostain, it caught about three-quarters to five-sixths of CMV-positive gut biopsies in two large studies.¹⁹²⁰ When it did show CMV, the immunostain almost always confirmed it.²¹ In one hospital mostly testing people with weak immune systems, the routine stain caught nearly all cases.²² The immunostain therefore matters most when the routine look is negative or unclear.

PCR is a DNA test: it detects the virus’s genetic material rather than its proteins. It copies tiny amounts of that material until they can be measured, so it can detect far less virus than a stain can see. It can be run on tissue or blood, and results are usually reported as a number showing how much DNA is present. It does not show which cells hold the virus, since the tissue is ground up for the test.

In stem cell transplant patients, a DNA test on fresh gut tissue performed as well as the immunostain.²³ But in inflammatory bowel disease, only about 1 in 5 patients with CMV DNA in the colon had a positive stain.²⁴ European bowel disease guidelines therefore recommend reading the stain together with DNA tests.

Finding CMV DNA doesn’t always mean CMV is causing illness. A DNA test can pick up tiny amounts of virus that may be bystanders, while the stain shows virus actively making proteins in cells. Doctors therefore look at the amount of DNA, not just its presence, and both tests are often reported together.

When the two disagree, a DNA-positive, stain-negative result usually means a small amount of virus, which may or may not be causing disease. A stain-positive, DNA-negative result is less common but can happen if the sample tested did not contain the infected area. Neither is a lab error; the tests measure different things and cross-check each other.

Does a Positive Stain Mean You Need Antiviral Treatment

Treatment depends on the whole picture. People with stain-positive ulcerative colitis and mild disease may recover without antiviral drugs.²⁵ Antiviral treatment is favored for moderate to severe flares that don’t respond to steroids and show a high viral load.²⁶ A negative blood DNA test, very low tissue viral DNA and only a single stained cell can support skipping antivirals,²⁷ which matters because the main antiviral, ganciclovir, carries real risks.²⁸

Viral load is the amount of CMV DNA measured in blood or tissue, and higher numbers suggest more active infection. Doctors look at it alongside the stain because a stain only shows presence, while a DNA number shows quantity. Labs use different methods, so numbers are best compared with the same lab over time rather than across labs.

Ganciclovir is often started through a vein and may switch to tablets (valganciclovir). It can lower blood counts, so regular blood tests are needed. Courses typically last two to three weeks, and your IBD or transplant drugs may be adjusted at the same time.

Holding off doesn’t mean ignoring the result. It usually means treating the underlying flare and checking in at set intervals. Your doctor may repeat a blood CMV test or review symptoms after a short period, and if you worsen, treatment can still be started.

Immunostains Beyond the Gut

When a baby is born with CMV infection, the placenta may be stained to look for the virus.²⁹ In one series, the stain was positive in most placentas from congenitally infected babies.³⁰ In another, it found CMV in under half of cases while a DNA test found it in about 7 in 10.³¹ A negative placenta stain therefore doesn’t rule out infection in the baby.

Newborn CMV is confirmed by testing the baby, usually with a DNA test on urine or saliva collected in the first weeks of life. The placenta stain is supporting evidence that can help explain what happened but does not decide whether the baby is infected.

CMV infects many tissue types: connective tissue, lining cells, blood vessel walls and muscle cells, and less often white blood cells.³² In severe cases with very weak immunity, it can be stained in brain and eye tissue.³³

On a transplant biopsy, the stain can explain a graft that isn’t working well, and early detection allows antiviral treatment before more damage. Transplant teams also monitor blood CMV levels routinely, so the stain adds tissue-level detail. In a kidney graft, it helps separate CMV damage from rejection, which can look similar but is treated differently. In a liver graft, it helps explain abnormal liver tests by showing whether the virus is in the liver cells.