What Is a CMV Immunostain and What Do Results Mean?

A CMV immunostain is a laboratory technique that uses antibodies to detect cytomegalovirus proteins directly in a tissue biopsy. When your pathology report mentions CMV immunohistochemistry (often abbreviated IHC), it means the lab applied specially designed antibodies to your tissue sample to see whether cells are actively infected with this virus. The test is widely considered the gold standard for confirming CMV tissue infection, particularly in the gut, and the results typically come back as either positive or negative, sometimes with additional detail about how many infected cells were found and where they were located.

Why Routine Staining Often Misses CMV

When a pathologist first examines a tissue biopsy under the microscope, they use a standard stain called hematoxylin and eosin, or H&E. CMV-infected cells sometimes show a distinctive appearance on H&E: an enlarged cell with a large dark inclusion in the nucleus, often described as an “owl’s eye” because of how it looks. These inclusions are highly specific, meaning that when a pathologist spots one, it almost certainly is CMV. The problem is that H&E staining has poor sensitivity. Many infected cells do not display that classic appearance, so the pathologist can look at a slide and see nothing unusual even when the virus is there.

In one study of lung transplant recipients, standard H&E staining picked up CMV inclusions in only about 8% of biopsy samples, while adding the immunostain boosted detection to 33%. That fourfold improvement is why clinicians order the immunostain rather than relying on routine microscopy alone.¹ The same pattern holds in the colon: H&E staining may show the classic viral inclusions in some cases of CMV colitis, but its sensitivity is low compared to immunohistochemistry, which is considered the reference standard for diagnosing CMV infection in colonic tissue.²

How the Stain Actually Works

The basic concept is straightforward. The lab takes your tissue biopsy, slices it extremely thin, and mounts it on a glass slide. Then, instead of just applying colored dyes the way H&E works, the lab applies antibodies that are designed to latch onto CMV-specific proteins. These antibodies are linked to a chemical marker, usually an enzyme that produces a brown or red color when activated. If CMV proteins are present in the cells, the antibodies bind to them and the color shows up under the microscope. If no CMV proteins are present, the antibodies wash away and no staining appears.

The staining can show up in the nucleus of the cell, the surrounding cytoplasm, or both, depending on what stage of viral replication the cell is in and which antibody the lab uses. Some antibodies target early viral proteins that appear before the virus has fully assembled, while others target late proteins. The pattern can vary, and pathologists evaluate the location and intensity of staining to confirm a true positive rather than an artifact.³

What a Positive Result Means

A positive CMV immunostain means the pathologist found cells in your biopsy that contain CMV proteins. This confirms that the virus is present and actively producing proteins in that tissue, which is the hallmark of tissue-invasive disease rather than just the virus circulating in the bloodstream. In practical terms, a positive result usually prompts your doctor to consider antiviral treatment, most commonly ganciclovir or valganciclovir.

The report may also mention how many positive cells were seen. Some pathologists describe this in approximate terms (“rare positive cells,” “scattered positive cells,” “numerous positive cells”), while others give a more precise count per area of tissue. The number of infected cells matters because it can influence how aggressively the infection is treated. In inflammatory bowel disease, for example, research suggests that patients who are IHC-positive are the ones most likely to benefit from antiviral therapy, supporting the idea that the immunostain identifies clinically meaningful infection rather than incidental viral presence.⁴

Context matters enormously, though. A positive CMV immunostain in a transplant recipient who is feeling unwell and has organ dysfunction is a very different clinical scenario from a single positive cell found incidentally in someone whose symptoms have another clear explanation. Your doctor interprets the stain alongside your symptoms, immune status, and other lab results.

What a Negative Result Means

A negative immunostain means no CMV-positive cells were identified in the tissue examined. In most situations, this is reassuring and suggests CMV is not causing the problem. But there are caveats worth knowing about.

The immunostain can only detect what is on the slide. If CMV-infected cells are sparse and scattered through a large area of tissue, the particular slice that was stained might not contain any of them. One study evaluating the reproducibility of CMV IHC in gastrointestinal biopsies found that when CMV-positive cells are sparse, the reliability of detection across serial tissue sections is uncertain.⁵ This is a sampling problem: the virus might be present in the tissue but absent from the particular section that was examined. For this reason, if clinical suspicion remains high despite a negative immunostain, your doctor may request additional biopsies or turn to molecular testing like PCR.

It also helps to know that a negative CMV immunostain does not mean you have never been exposed to CMV. Most adults have been infected at some point, and the virus remains dormant in the body for life. The immunostain only detects active viral protein production in that specific piece of tissue. A negative result simply means the virus was not actively replicating there at the time of the biopsy.

How CMV Immunostains Compare to PCR Testing

PCR (polymerase chain reaction) testing detects viral DNA rather than viral proteins, and it can be performed on blood, tissue, or other body fluids. The two tests answer somewhat different questions, and understanding the distinction helps make sense of situations where they disagree.

Blood PCR measures how much CMV DNA is circulating in your bloodstream. This is useful for monitoring viral reactivation, especially in transplant patients, but it does not directly tell you whether the virus is invading a specific organ. When researchers compared blood PCR levels to immunostain results from colon biopsies in ulcerative colitis patients, the correlation was only moderate, with many cases where one test was positive and the other was not.⁶ A positive blood PCR with a negative tissue immunostain could mean the virus is reactivating in the blood but not actually invading the gut wall. A negative blood PCR with a positive immunostain could mean the virus is causing localized tissue damage without spilling much DNA into the bloodstream.

Tissue PCR, which is performed directly on the biopsy sample, is more closely related to what the immunostain detects. In a ten-year retrospective study, tissue PCR had perfect sensitivity compared to histopathology but lower specificity, around 72%.⁷ That means tissue PCR catches essentially every case that immunostaining catches, but it also flags some cases where viral DNA is present without the virus actively causing disease. Another study found that CMV DNA was detected by PCR in about 91% of IHC-positive tissues but also in roughly 15% of IHC-negative tissues, and nearly all of the IHC-positive patients with active colitis showed no CMV DNA in their adjacent normal-looking tissue.⁸ That finding reinforces the value of immunostaining: it tends to light up in the areas where CMV is causing actual damage, not just passively sitting in tissue.

In practice, some clinicians use PCR as a screening step. If tissue PCR is negative, the immunostain is likely negative too, which can save time and resources. If tissue PCR is positive, the immunostain helps determine whether the viral DNA represents clinically important infection or just low-level presence.⁴

The CMV Immunostain in Gut Disease

The gastrointestinal tract is one of the most common sites where CMV immunostaining is ordered. CMV can infect the esophagus, stomach, small bowel, or colon, and the clinical picture often overlaps with other conditions, making the immunostain essential for sorting things out.

A particularly tricky scenario arises in people with inflammatory bowel disease, especially ulcerative colitis. When a flare does not respond to standard treatment with steroids or other immunosuppressive drugs, the question comes up: is this a true IBD flare, or has CMV reactivated in the inflamed colon and taken over? The immunosuppressive drugs used to treat IBD can weaken the immune system’s control of CMV, letting the virus flare up in already-damaged tissue. An increasing body of evidence suggests that CMV reactivation in this context is not always an innocent bystander and may actively worsen the flare, making it less responsive to steroids.⁹ Current recommendations call for checking for CMV reactivation with quantitative tools in colonic biopsies when someone with ulcerative colitis has a steroid-refractory flare, and treating with ganciclovir when the viral load is high or the disease is severe.

That said, the overall yield of CMV testing in IBD flares may be low. One study of 99 IBD patients with suspected CMV found biopsy-proven CMV colitis in only 1%.¹⁰ That does not mean the testing is useless; finding the rare case where CMV is driving the flare can change the treatment plan entirely. But it does illustrate that most IBD flares are not caused by CMV, even when the clinical picture is suspicious enough to warrant checking.

Lung Transplant and Respiratory Applications

In lung transplant recipients, CMV pneumonitis is a feared complication. The immunostain is often performed on transbronchial biopsies or bronchoalveolar lavage specimens. One study found that positive CMV immunostaining on bronchoalveolar lavage had a sensitivity of about 89% and a specificity of nearly 99% for diagnosing CMV pneumonitis, with a negative predictive value above 99%.¹¹ That high specificity means a positive result in the right clinical context strongly supports the diagnosis. The high negative predictive value means a negative result can help rule it out. These numbers make the immunostain a particularly reliable tool in lung pathology.

CMV remains a significant cause of complications following solid organ transplantation generally, contributing to increased illness and healthcare costs. Beyond the lungs, transplant teams regularly use CMV immunostaining on biopsies from the liver, kidney, and heart to check whether rejection-like symptoms are actually being caused or worsened by CMV infection.

Congenital CMV and Placental Testing

CMV is the most common congenital infection worldwide, and when a baby is suspected of being affected, pathologists may examine the placenta for signs of viral infection. In a study of placentas from confirmed congenital CMV cases, immunohistochemistry detected CMV in about 44% of cases, while PCR on the same placentas detected it in about 70%.¹² The gap between IHC and PCR detection was substantial, highlighting that in placental tissue, the immunostain alone may miss a meaningful number of cases. Typical findings in infected placentas included chronic inflammation of the villi, plasma cell infiltration, viral inclusions, and iron deposits. When congenital CMV is suspected, many centers now use PCR alongside or instead of immunostaining on placental tissue to improve detection.

False Positives and Technical Pitfalls

No lab test is perfect, and the CMV immunostain has a few known pitfalls. The most well-documented involves endogenous biotin, a naturally occurring molecule in tissue that can interfere with certain staining methods. When labs use a detection system called the streptavidin-biotin complex method, endogenous biotin in the tissue can produce a false-positive signal, making it look like CMV is present when it is not.¹³ Biotin shows up in both the nucleus and cytoplasm of certain cell types, which can mimic the staining pattern of actual CMV infection. The main defense against this is careful use of negative control slides and awareness on the pathologist’s part. Many modern labs have switched to polymer-based detection systems that avoid the biotin problem entirely.

Other potential sources of confusion include background staining from tissue processing, cross-reactivity with other herpes family viruses (though this is rare with well-validated antibodies), and the challenge of interpreting very faint or atypical staining patterns. Experienced pathologists look at the overall pattern: is the staining in the right cellular location? Does it have the expected intensity? Are there accompanying tissue changes like inflammation or the characteristic enlarged cells? A single ambiguous cell in an otherwise normal biopsy typically warrants caution and possibly repeat testing rather than an immediate diagnosis.

Immunocompetent Versus Immunocompromised Patients

Most people associate CMV disease with weakened immune systems, and for good reason: transplant recipients, people on chemotherapy, and those with HIV make up the majority of CMV tissue disease cases. But CMV can also cause tissue-invasive disease in people with apparently normal immune systems, and the clinical picture differs in some important ways.

In a study comparing the two groups among patients with confirmed tissue-invasive gastrointestinal CMV, those with healthy immune systems tended to be older, were more likely to present with gastrointestinal bleeding, and had a shorter time between symptom onset and diagnosis. The immunocompromised patients, by contrast, were more likely to have diffuse involvement across multiple parts of the GI tract and to have the virus show up in the esophagus. Spread to organs outside the gut was seen only in the immunocompromised group. Small bowel involvement was actually more common in the immunocompetent group.

For the CMV immunostain, this means the test has clinical value regardless of immune status. If a doctor suspects CMV-related gut disease in an older adult with no known immune problems, an immunostain on a biopsy specimen can confirm the diagnosis just as effectively. The difference lies more in who gets tested and how the result is managed afterward. In transplant patients, CMV monitoring is routine and results feed into ongoing antiviral prophylaxis decisions. In immunocompetent patients, CMV is often not suspected until other explanations have been ruled out, so the immunostain may be ordered later in the diagnostic workup.

When Your Report Says “Equivocal”

Occasionally, the pathology report will describe the CMV immunostain as equivocal or indeterminate rather than clearly positive or negative. This typically means the pathologist saw some staining that could represent CMV but was not confident enough to call it positive. There might be very faint staining, staining in an unexpected location, or only a single questionable cell.

An equivocal result is genuinely uncertain, not a soft positive. In one comparison study, PCR testing of equivocal IHC cases detected CMV DNA in only about 20% of them, compared to 91% of clearly positive cases.⁸ That means most equivocal immunostains do not represent true CMV infection, but a meaningful minority do. When your report comes back equivocal, your doctor will typically weigh the clinical suspicion: if everything else points to CMV, they may treat empirically or order additional testing such as tissue PCR or repeat biopsies. If the clinical picture does not fit, the equivocal stain is more likely to be a technical artifact.

How Results Feed into Treatment Decisions

A positive CMV immunostain does not automatically mean you will receive antiviral medication. Treatment decisions depend on several factors: how many positive cells are seen, what symptoms you have, what your immune status is, and whether the CMV appears to be causing the tissue damage or is just along for the ride.

In inflammatory bowel disease, the recommendation is to treat with ganciclovir when the viral load in colonic tissue is high or the disease is severe, particularly if the flare is not responding to immunosuppressive therapy.⁹ A handful of scattered positive cells in an otherwise mild flare might be managed differently from widespread CMV positivity in someone whose colon is severely inflamed and not improving.

In transplant medicine, antiviral treatment thresholds are often more aggressive. Protocols vary by center, but many transplant programs will initiate treatment at the first sign of tissue-invasive CMV, particularly in high-risk recipients. Some centers have experimented with different threshold cutoffs for starting treatment based on quantitative measures of CMV detection, and the optimal threshold remains an area of active study.

For immunocompetent patients, the approach is more case-by-case. Many cases of CMV disease in people with healthy immune systems resolve without antiviral treatment as the immune system brings the virus back under control. Antivirals carry their own side effects, including bone marrow suppression, so the decision to treat involves weighing the severity of the CMV disease against the risks of medication. A positive immunostain in an immunocompetent patient provides diagnostic clarity but does not always dictate a course of antivirals the way it might in a transplant recipient.