IFNL4 Genotype

Most people who catch hepatitis C never clear it on their own. Somewhere between one in six and one in three do, and a large part of that difference comes down to a single inherited two-letter change in a gene called IFNL4 (interferon lambda 4). This test reads that change.

The result never changes, so you test once. What it tells you is whether your innate immune system builds a protein much of the world's population has lost the ability to make. Whether that protein helps or hurts you depends on which virus you are facing.

What the Test Actually Reads

The gene sits on chromosome 19, right next to IFNL3, which older papers call IL28B. The variant that matters is a two-letter insertion or deletion, written rs368234815 or ss469415590. Inherit the TT version from both parents and the gene's instructions shift out of register, so no working interferon lambda 4 protein gets built at all. Carry the ΔG version and you make a functioning protein.

That is the strange part. The genotype that breaks the gene is the one linked to better outcomes in hepatitis C. Losing a piece of your antiviral machinery makes you better at clearing this particular virus.

There is a second variant worth knowing about, rs117648444. It swaps one amino acid and produces a weakened version of the protein rather than none at all. Sequencing reads both. The older tests read neither directly.

Hepatitis C Clearance

In a Chinese Han cohort of 795 people, those carrying the non-functional TT/TT genotype cleared hepatitis C on their own about twice as often as carriers of the active allele. In the same population, carrying three or four unfavorable alleles across the interferon lambda region cut the odds of clearing the virus by around 90%.

The paradox resolves once you look at what the liver is doing. When you make working interferon lambda 4, your liver keeps its antiviral alarm switched on constantly during infection. The genes that fight viruses stay elevated all the time. That sounds protective. It isn't: running the system flat out leaves it unable to mount a fresh response when one is needed. People who make no interferon lambda 4 start from a quiet baseline, so they can respond sharply when the virus shows up, or when a doctor gives them interferon as a drug.

This is why the same genotype that predicts spontaneous clearance also predicted who responded to the old interferon-based regimens. Pooling studies across populations, the variant predicted who stayed virus-free after interferon treatment for hepatitis C genotype 1. In 207 people coinfected with hepatitis C and HIV, treatment on interferon and ribavirin failed in 77% of ΔG carriers against 48% of non-carriers.

Does It Still Matter Now That Modern Drugs Cure Almost Everyone?

Direct-acting antivirals cure the large majority of people regardless of genotype, which is a fair argument that this test no longer changes treatment. The argument is not quite complete. Unfavorable alleles have been linked to higher relapse after shortened courses of those drugs, and shortening courses is exactly what cost-conscious treatment programs are trying to do.

Genotype also tracks how fast the virus falls during treatment. On sofosbuvir and ribavirin, ΔG carriers cleared virus more slowly. And in hepatitis C genotype 1, people with the favorable genotype are more likely to be carrying a virus that already has the NS5A Y93H resistance mutation before treatment starts, which affects which drug class works.

Hepatitis B and Interferon Therapy

In 126 people with chronic hepatitis B who test negative for the e antigen and were treated with interferon, combining the two IFNL4 variants predicted clearance of hepatitis B surface antigen over 15 years of follow-up. Carrying the null version, or the version that makes a weakened protein, made clearance considerably more likely. It is one study and a small one. But interferon is still in use for chronic hepatitis B, and this is the setting where the genotype comes closest to changing what you would actually do.

Liver Scarring, Including in Fatty Liver Disease

The link runs beyond viral infection. Across 4,172 people with chronic liver disease from both viral and non-viral causes, the linked rs12979860 genotype predicted liver inflammation and scarring independently of what caused the disease. In a separate cohort of 946 people of European ancestry with fatty liver disease, the ΔG variant was associated with how severe the scarring was.

So the result carries information for someone with a fatty liver and no virus at all. The mechanism is disputed: one large analysis argued the neighboring IFN-λ3 protein, not IFN-λ4, drives the inflammation and scarring signal, since the two genes travel together on the same stretch of chromosome. Either way, the genotype marks the risk.

Other Infections and Outcomes

A focused analysis of UK Biobank participants found the ΔG allele protective against hepatitis A infection while raising risk for hepatitis C and giant cell arteritis. In 480 Rwandan children with upper respiratory infections, genotypes common in people of African ancestry predicted slower clearance of respiratory RNA viruses. In men who have sex with men, the null genotype was associated with lower odds of acquiring HIV-1. A small case-control study in Egypt put IFNL4 among the gene differences linked to COVID-19 severity, which should be read as preliminary and unreplicated.

A different use entirely: across 3,242 unrelated-donor stem cell transplants for acute leukemia, the donor's IFNL4 genotype predicted death from causes other than relapse, with donors carrying the null genotype linked to better recipient survival. That is donor selection, not your own risk. And the direction is not universal. In 116 liver transplants for hepatitis C, a donor with the null genotype was linked to more early scarring in the recipient, the reverse of what the genotype does before transplant.

The gene is not a universal immune switch. In 2,310 men with HIV-1, genotype showed no association with Kaposi sarcoma, cytomegalovirus, herpes simplex, or opportunistic infections and cancers generally.

Ancestry Changes What the Result Means

The active ΔG allele is far more common in people of African ancestry, which is a large part of why hepatitis C outcomes have historically differed by ancestry. It also creates a testing problem. Most clinics have used rs12979860 as a stand-in for the real variant, and in people of European ancestry the two track each other almost perfectly. In people of African ancestry they do not. The stand-in can call you favorable when the functional variant says otherwise.

That mismatch is the main reason to sequence the functional variant directly rather than accept a surrogate result.

Where Sequencing Can Get It Wrong

Insertions and deletions like this one are among the harder variants for sequencing to call correctly. Local sequence complexity around this gene can leave too few reads to make a confident call, which is why some pipelines quietly fall back on a linked marker with better coverage instead of reading the real thing. Misalignment and primer problems can both produce false negatives. Across genetic testing generally, an analysis of 450,000 patients found roughly one in seven clinically important variants is hard for standard sequencing to detect.

Two practical consequences. First, check whether your lab called rs368234815 directly or inferred it from a linked marker. Second, a rapid cheek-swab test exists, but it reads the linked marker rather than the functional variant. In validation it caught 90% of non-CC results and correctly cleared 98% of CC results. Good, not perfect, and sequencing is the more thorough read.

Nothing about your day distorts this result. You cannot eat, exercise, or medicate your way into a different genotype. Saliva, buccal swab, and blood all read the same inherited DNA.

A One-Time Result You Keep Using

There is no trend to follow. You sequence this once and the answer holds for life. The value is in what you do with it afterward, repeatedly, over years.

If you have hepatitis B or C, the genotype belongs in the conversation about which regimen and what duration, especially if a shortened course is on the table. If you have fatty liver disease, an unfavorable result is a reason to take scarring seriously earlier: liver enzymes, a blood fibrosis score, and a liver stiffness scan on a regular schedule rather than an occasional one. If you have neither, the result sits in your file until a hepatitis diagnosis or a transplant decision makes it relevant.

What an Unexpected Result Should Make You Do

If the result is unfavorable and you have any liver history, run the companion tests that show what your liver is doing now: ALT, AST, GGT, platelet count, and albumin. The first three are enzymes that rise when liver cells are injured. A falling platelet count alongside rising enzymes is the combination that should push you toward a hepatologist rather than watchful waiting.

If you have active hepatitis C, the genotype is one input among several, and the virus's own genotype and resistance profile matter more for drug selection. Ask for baseline resistance testing if a shortened course is being considered. If you have chronic hepatitis B and interferon is on the table, this result deserves real weight in that decision.

If the sequencing report flags low coverage or an uncertain call at this position, treat it as unresolved rather than negative, and ask for confirmation by a second method.