Lymphogranuloma venereum

If a swab comes back positive for chlamydia, the next question is which kind. Most chlamydia stays on the surface of the genital lining, but a small group of strains gets through it and travels into the lymph glands. There it can cause swollen, painful glands and lasting tissue damage. This test looks for those strains, which cause an infection called LGV (lymphogranuloma venereum).

The answer changes how the infection is treated and how hard you should look for other infections that tend to come with it. In one study of women living with HIV (human immunodeficiency virus) in South Africa whose vaginal swabs were positive for chlamydia, about one in five (17 of 85) carried the invasive type, and most of those women had no symptoms.

What a Positive Result Means

LGV is caused by a small group of chlamydia strains. The classic three are labeled L1, L2, and L3, and genetic surveillance has since identified a newer L4 lineage. Most cases in current outbreaks are a variant of L2 called L2b. Ordinary genital chlamydia (strains D through K) mostly infects the surface cells lining the cervix, urethra, or rectum. The L strains get through that lining and into the lymph glands that drain the area, which is why the infection can spread well beyond the spot where it started.

The result is reported as detected or not detected. The lab checks whether genetic material from an L strain was found on your swab, so there's no level to compare and nothing to trend. A detected result means LGV bacteria were present in the sample, and a not-detected result means none were found at a level the test can pick up.

Most of what's known about LGV today comes from rectal infections in men who have sex with men, because that's where the large outbreaks in Europe and North America have been. Vaginal LGV has been studied far less, mostly in South Africa, so some of the evidence below comes from other body sites and populations, and it's labeled when it does.

The Second Step After a Positive Chlamydia Test

Standard chlamydia tests detect the bacterium's genetic material but treat every strain the same, so a positive result doesn't say whether you have the ordinary kind or the invasive kind. Telling them apart takes a second test that looks for genetic signatures found only in the L strains. A common target is a gene called pmpH. In L strains, a short stretch of that gene is missing, and tests aimed at it are built to spot the gap. Other labs use different targets.

That second step is mostly done in specialized reference labs, and routine screening panels usually skip it. So if you've had a positive chlamydia result and nobody mentioned the strain, it probably was never checked.

Genital Sores and Swollen Groin Glands

The first sign of LGV, when there is one, is a small bump, shallow sore, or raw patch where the bacteria entered. It can appear anywhere from about 3 to 30 days after exposure. These early sores are often painless and heal on their own, so they're easy to miss.

If the infection isn't treated, it can spread to the lymph glands in the groin, typically 2 to 6 weeks after the first sore. Those glands can swell into painful, fluid-filled lumps (called buboes) that break open through the skin in up to about a third of people, sometimes leaving draining channels that are slow to heal. In recent European data, this groin form has become uncommon compared with rectal infection, which is one reason it now catches people and clinicians off guard.

What this means for you: a painless genital sore followed by a swollen, tender groin is the pattern worth testing for directly, including a swab of the sore itself if one is still present.

Rectal Infection and Long-Term Damage

In current outbreaks, LGV shows up most often as an infection of the rectum, with pain, bleeding, discharge, and a constant urge to pass stool. On a scope and under the microscope it can look so much like inflammatory bowel disease (Crohn's disease or ulcerative colitis) that people get worked up for the wrong condition. In a small Polish case series, the delay before the right diagnosis ran past six months, and some people kept having symptoms even after the bacteria were cleared.

Left untreated, rectal and genital LGV can cause deep abscesses, narrowing of the rectum (strictures), abnormal tunnels between organs or out to the skin (fistulas), and scarring of the lymph channels.

What this means for you: a vaginal swab samples the vagina and nothing else. If you've had anal sex, a rectal swab is the test that would catch a rectal infection, and rectal symptoms deserve that test whatever your vaginal result says.

Infection Without Symptoms

LGV often causes no symptoms at all. Among the South African women described above, most of those with vaginal LGV felt fine. In rectal-screening studies of men who have sex with men, a meaningful share of confirmed cases had no symptoms or visible inflammation, and that share has grown in recent French and Canadian surveillance.

That's why testing only people with symptoms misses cases. When clinics in Alberta, Melbourne, and the Netherlands moved toward checking every rectal chlamydia sample for LGV, they found more infections in people who had no complaints, and a pilot across several European countries concluded that LGV is substantially underdiagnosed.

What the Vaginal Evidence Shows

Vaginal LGV is documented, though it turns up far less often in surveillance records than rectal LGV. In South Africa, the invasive L2 strain was found in seven women being evaluated for vaginal discharge, alongside the larger study of women with HIV described at the top of this page. Most of those women had no symptoms that would have pointed toward LGV on their own.

For chlamydia in general, the risk for women reaches the upper reproductive tract. In a Dutch cohort of 5,704 women previously tested in a screening trial, those who had tested positive for chlamydia later had more pelvic inflammatory disease (infection spreading to the uterus and fallopian tubes) and more infertility from tubal damage, while ectopic pregnancy rates were similar. That study looked at chlamydia overall, not the L strains, so it doesn't tell you whether LGV carries more or less of that risk.

HIV and the Infections That Travel With It

LGV clusters with other sexually transmitted infections. It's strongly associated with HIV, and it often turns up alongside gonorrhea and syphilis. Among French men using PrEP (a daily HIV-prevention pill) whose rectal swabs were positive for chlamydia, nearly one in five (18.7%) turned out to have LGV.

What this means for you: a detected result is a reason to check for HIV, syphilis, gonorrhea, trichomonas, and herpes at the same time, since one exposure can pass along several of them.

How Accurate the Testing Is

Accuracy has been measured at each step: finding chlamydia at all, telling LGV strains apart, and the older blood-based alternative.

Sources: Aaron et al.; Touati et al.; de Vries et al.

What this means for you: a vaginal swab is a good sample for finding chlamydia in the first place. The LGV-specific step has been validated mostly on rectal samples, and direct validation on vaginal swabs is limited. A blood antibody test gives too many wrong answers in both directions to stand in for a swab, and guidelines advise against using it routinely for LGV.

When Results Can Be Misleading

Because the result is detected or not detected, the main ways it can mislead you are a missed infection or a result that answers a narrower question than you think.

• Too few bacteria on the swab: early or light infections can sit below what the test can detect, so a negative result is possible even when you're infected.
• Infection at another site: a vaginal swab can't detect infection in the rectum, throat, or groin glands, so a negative result covers the vagina only.
• Strain typing that never happened: if a lab ran only a general chlamydia test, a positive result says nothing about LGV either way, and labs differ in which genetic targets they use, so results from different labs aren't always interchangeable.
• A blood antibody test mistaken for this one: ordinary chlamydia can push antibody levels high enough to look like LGV, which makes blood-based LGV results prone to false alarms.

Lab factors, including the type of collection tube, can occasionally affect how much genetic material is recovered from the swab.

Testing Again After Treatment and New Exposures

There's no number to trend with an infection test like this one. What you track over time is whether an infection has cleared and whether you've picked up a new one, and partners who weren't tested and treated are a common source of a second infection.

A sensible plan: retest after you finish treatment to confirm the infection is gone, test again whenever you have a new partner or learn that a partner tested positive, and if you have new partners, get a full sexually transmitted infection screen at least once a year. More often makes sense if you take PrEP, live with HIV, or have several partners. A single negative swab describes one site on one day.

What to Do With a Detected or Unexpected Result

The next step depends on which pattern you're looking at.

• Chlamydia positive, strain unknown: ask whether LGV typing can be run on the same sample, or order this test, since the answer changes the treatment plan.
• LGV detected: you'll need a prescription antibiotic course, so this is the point to bring in a clinician; get the companion infection tests above at the same visit, and let partners from the last six months know so they can be tested and treated.
• Vaginal swab negative but symptoms continue: a genital sore, a swollen groin gland, or rectal pain or bleeding calls for testing the affected site, and a sexual health or infectious disease specialist if the picture stays unclear.
• Rectal symptoms being treated as bowel disease: if inflammatory bowel disease is on the table and you've had anal sex, a rectal chlamydia and LGV test belongs in the workup before that diagnosis is settled.