What Can Cause a False-Positive AFP Test?

Alpha-fetoprotein (AFP) is a protein produced in large quantities during fetal development, and its levels normally drop to very low concentrations after birth. When a blood test shows unexpectedly high AFP, the assumption is often cancer or a serious fetal abnormality, but a long list of benign and technical factors can push the number up without either being present. Whether you are a pregnant person receiving prenatal screening results or someone being monitored for liver cancer, understanding these non-alarming causes matters for avoiding unnecessary panic and invasive follow-up procedures.

Why AFP Is Tested in the First Place

AFP serves as a screening marker in two very different clinical settings. In pregnancy, maternal serum AFP is measured during the second trimester to flag possible neural tube defects or chromosomal abnormalities. Outside of pregnancy, AFP is tracked primarily in people at risk for liver cancer, particularly those with chronic hepatitis B or C, cirrhosis, or certain testicular cancers. In both scenarios, the test is a screening tool, not a diagnostic one. That distinction is critical: a screening test is designed to cast a wide net, which means it will inevitably catch some people who do not actually have the condition. Those are the false positives.

In a healthy adult, serum AFP concentrations sit roughly between 0.5 and 15 micrograms per liter.¹ Anything above the expected range for a person’s age, sex, and clinical context triggers further investigation. But “above the expected range” does not automatically mean cancer or birth defect. Here is where things get interesting.

Gestational Dating Errors in Pregnancy Screening

The single most common reason for a falsely elevated maternal serum AFP is getting the gestational age wrong. AFP levels in pregnancy rise steeply week by week during the second trimester. When the lab calculates your expected AFP, it uses your estimated due date. If that date is off by even a week or two, the math breaks down: the lab compares your blood level to the wrong reference range, and what looks like an abnormally high result is actually perfectly normal for the true gestational age.

One study found that incorrect menstrual dating accounted for roughly a fifth of all elevated maternal serum AFP results.² This is why an ultrasound to confirm dates is considered essential before acting on a high AFP result. Many false alarms evaporate the moment a sonographer measures the fetus and recalculates the due date.³

Vanishing Twin and Multiple Pregnancies

A “vanishing twin” occurs when a pregnancy starts as twins but one embryo stops developing and is gradually reabsorbed. The vanished twin’s tissue can continue to release AFP into the mother’s bloodstream well after the second embryo is no longer viable. In a multicenter study, pregnancies with a vanishing twin had median AFP levels roughly a third higher than normal singleton pregnancies, and the rate of abnormally increased AFP was also significantly higher in the vanishing-twin group.⁴ A separate analysis found AFP increased by about 10 percent in vanishing-twin pregnancies compared to controls.⁵

This is a sneaky source of false positives because many vanishing twins occur so early that the mother never knew she was carrying twins. If the extra tissue is not detected on early ultrasound, the elevated AFP looks unexplained. Clinicians are increasingly aware of this, but it remains an underappreciated factor, particularly when early imaging was not performed.

Other Obstetric Factors That Raise AFP Without a Fetal Defect

Beyond dating errors and vanishing twins, several pregnancy-related conditions can elevate maternal AFP even when the fetus is completely healthy. The common thread is anything that disrupts the normal barrier between fetal and maternal blood or stresses the placenta. Conditions linked to elevated maternal AFP in the absence of fetal abnormality include:

Placenta previa: When the placenta implants low in the uterus, covering or approaching the cervix, more fetal AFP can leak into maternal circulation.
Premature rupture of membranes: Damage to the membranes surrounding the fetus allows fetal proteins to enter the mother’s bloodstream.
Multiple gestation: Carrying more than one viable fetus means more total AFP production; screening protocols adjust for twins, but the correction is imperfect.
Low birth weight and placental dysfunction: Ischemic placental disease, where the placenta does not get adequate blood flow, is associated with elevated AFP as well.

A retrospective cohort study found that placenta previa, hepatitis B carrier status, premature rupture of membranes, and advanced maternal age were all independent risk factors for elevated AFP alongside adverse pregnancy outcomes.⁶ None of these causes indicate a fetal neural tube defect, yet they can trigger the same screening alarm.

Liver Disease and Hepatocyte Regeneration

Outside of pregnancy, the liver is the biggest non-cancer source of AFP elevations. The connection is straightforward: AFP is primarily made by liver cells, and when those cells are damaged and then regenerate, they can temporarily reactivate AFP production. This reactivation is a normal part of the healing process and does not necessarily signal that anything has gone cancerous.

In acute viral hepatitis, AFP levels rise in the vast majority of patients. A classic study tracking 32 patients with acute viral hepatitis found increased AFP in all but one. In several patients, the AFP spike came as their liver enzyme levels were falling back to normal, suggesting the elevation reflected liver cells rebuilding rather than being destroyed.⁷

Chronic liver conditions cause similar problems. Cirrhosis from any cause can produce fluctuating AFP levels. In one reported case, a patient with known liver cirrhosis had a remarkably elevated AFP, and imaging could not determine whether liver cancer was present. Ultimately the cause turned out to be portal vein thrombosis (a blood clot in the main vein feeding the liver) and pseudo-infarction of the liver tissue, not cancer at all.⁸

Non-alcoholic fatty liver disease, the most common liver condition in Western countries, adds another layer. As fatty liver progresses to its more inflammatory form, the repair mechanisms activated in liver cells can nudge AFP levels upward. Several molecular pathways involved in liver cell damage and regeneration have been implicated, and mildly elevated AFP may actually serve as an early signal that fatty liver is worsening.⁹ For clinicians screening for liver cancer, these non-malignant elevations are a constant nuisance because they produce exactly the kind of borderline results that are hardest to interpret.

Benign Liver Tumors

Not all liver masses are cancerous, and a few of the benign ones can raise AFP just enough to cause confusion. Focal nodular hyperplasia is a common benign liver growth that typically does not produce AFP. However, a Chinese prospective study found increased AFP in up to about 3 percent of patients with focal nodular hyperplasia.¹⁰ That is rare, but when it happens, it creates a diagnostic headache. Finding a liver mass on imaging and a rising AFP in the blood makes liver cancer look very likely, and distinguishing a benign growth from an early hepatocellular carcinoma can require a biopsy.

Liver adenomas, another class of benign tumors, are even more rarely associated with AFP elevation, and hemangiomas (the most common benign liver tumors) appear not to raise AFP at all.¹⁰ Still, the possibility of AFP coming from a benign tumor is something clinicians need to hold in mind when working up an unexpectedly high result.

Non-Liver Cancers That Produce AFP

This is a category where “false positive” depends on your frame of reference. If AFP is being tracked to monitor for liver cancer, a high reading caused by a completely different cancer is, in practical terms, a false positive for the condition being surveilled. AFP is not exclusive to liver cells; certain cancers outside the liver can produce it.

Testicular germ cell tumors, particularly the non-seminomatous subtypes, are well known to produce AFP. Along with two other markers, AFP plays a central role in the diagnosis, staging, and surveillance of these cancers.¹¹ If someone is being screened for liver cancer and happens to have an undiagnosed testicular tumor, the AFP rise can look like hepatocellular carcinoma until further workup sorts things out.

A rarer and trickier scenario involves AFP-producing gastric cancer. These tumors can metastasize to the liver and produce high AFP levels, mimicking primary liver cancer so closely that even tissue samples can be ambiguous. In reported cases, pathologists needed a full panel of immunohistochemical stains to confirm that the liver mass was a metastasis from a stomach cancer rather than liver cancer itself.¹² AFP-producing gastric carcinoma displays a wide spectrum of tissue patterns that can mimic hepatocellular carcinoma, making identification especially difficult.¹³

Genetic Conditions That Keep AFP Elevated for Life

Some people have persistently elevated AFP due to inherited conditions unrelated to cancer or liver disease. The most straightforward is hereditary persistence of alpha-fetoprotein, caused by mutations in the AFP gene’s promoter region. People with this condition never fully turn down AFP production after birth and carry mildly elevated levels their entire lives.¹⁴ This is entirely benign. The difficulty is that it can only be confirmed through genetic testing, and until it is, every elevated AFP reading triggers a cancer workup.

Certain neurological conditions also feature chronically raised AFP. Ataxia-telangiectasia, a rare genetic disorder affecting coordination and immune function, is commonly associated with high AFP. All patients in one study showed increased serum AFP during the course of the disease.¹⁵ Milder AFP elevation has also been documented in related conditions such as ataxia with oculomotor apraxia types 2 and 4.¹⁶ For people living with these conditions, chronically elevated AFP is an expected finding, but it can still cause confusion if a new doctor unfamiliar with the patient’s history orders the test and interprets the result in isolation.

Laboratory Artifacts and Assay Interference

Sometimes the number on the report is simply wrong because something in the blood sample interfered with the test itself. AFP is typically measured using immunoassays, which rely on antibodies binding to the protein. Several substances in the blood can disrupt that binding and produce a result that does not reflect the true AFP concentration.

Heterophilic antibodies are perhaps the most well-known culprit. These are antibodies circulating in the patient’s blood that can bridge the test’s capture and detection antibodies, creating a signal where there should not be one. All major AFP assay platforms are susceptible to this type of interference.¹⁷ In at least one documented case involving a patient with chronic hepatitis B, heterophilic antibodies were considered the most likely explanation for persistently elevated AFP values despite no evidence of cancer.¹⁸

Rheumatoid factor, an antibody found in many people with rheumatoid arthritis and some other autoimmune conditions, is another source of trouble. In laboratory experiments, adding rheumatoid factor to blood samples caused significant measurement error in AFP results. The interference kicked in at rheumatoid factor concentrations above about 260 IU/mL.¹⁹ Interestingly, the direction of the error is not always upward. On one widely used testing platform, rheumatoid factor caused AFP results to be falsely low rather than falsely high.²⁰ That means rheumatoid factor can produce either false positives or false negatives depending on the assay technology the lab happens to use. Clinicians may not even know which direction the error goes without checking which specific platform ran the test.

A third laboratory pitfall is the “hook effect.” When AFP concentrations are extremely high, as in some advanced cancers, one-step immunoassays can paradoxically report a falsely low or even normal result because the excess protein overwhelms the test’s antibodies.²¹ This is technically a false negative rather than a false positive, but it is worth knowing because it represents the mirror-image problem: the test failing to flag a genuinely dangerous AFP level. Labs can catch this by re-running the sample at a diluted concentration.

Chemotherapy-Related AFP Surges

Patients being treated for AFP-producing cancers sometimes see their AFP levels spike shortly after starting chemotherapy, which can look alarmingly like the cancer is getting worse. In testicular cancer, transient elevations of AFP after the first cycles of treatment have been recognized as a known phenomenon.²² The spike is thought to reflect dying tumor cells releasing stored AFP into the bloodstream all at once, rather than new tumor growth.

Certain chemotherapy regimens can also elevate AFP through direct liver toxicity. One regimen used for testicular cancer caused temporary AFP elevations that corresponded with rising liver enzymes and bilirubin during the early days of each treatment cycle. The elevations resolved before the next cycle began, and the researchers concluded the spikes were caused by reversible liver stress from the drugs, most likely driven by a heavy dose of one particular platinum-based agent, and should not be interpreted as treatment failure.²³ Misreading these surges as disease progression could lead to premature changes in treatment strategy, which is why oncologists typically expect and account for early AFP bumps in the treatment timeline.

Infants and Newborns Have Naturally Sky-High AFP

AFP levels in a fetus can reach concentrations millions of times higher than a normal adult value. After birth, that number falls rapidly but does not reach the adult range until around age two.¹ Some degree of AFP production continues after birth, with the rate of synthesis tapering gradually rather than shutting off instantly.²⁴

This matters clinically because AFP is sometimes measured in infants being evaluated for liver tumors or metabolic diseases. Interpreting those results requires age-specific reference ranges that are dramatically different from adult values. A level that would be terrifying in an adult is entirely normal in a three-month-old. Labs and clinicians who do not use the correct pediatric reference charts can easily generate false alarms, and parents who encounter AFP numbers without proper context can understandably panic.

When the Result Is Discrepant Across Platforms

One practical clue that an AFP result might be falsely elevated is a discrepancy between assay platforms. When clinicians suspect interference, a standard strategy is to re-measure AFP using a different manufacturer’s test. Because heterophilic antibodies and rheumatoid factor interfere with specific antibody pairings rather than with AFP itself, switching platforms often produces a dramatically different result. In one reported case, five different AFP assays were evaluated and found to be susceptible to heterophilic antibody interference, but the degree of interference varied across platforms.¹⁷ If two different labs return meaningfully different numbers from the same blood draw, that inconsistency itself is diagnostic: it points toward a testing artifact rather than a biological cause.

Serial dilution is another tool. True AFP elevations scale proportionally when the sample is diluted. If diluting a sample by half does not cut the measured AFP roughly in half, something other than AFP is generating the signal. These are standard troubleshooting steps in laboratory medicine, but they only happen if someone thinks to question the initial result rather than acting on it immediately.

1
PubMed. Alpha-fetoprotein, a fascinating protein and biomarker in neurology
2
Acta Obstetricia et Gynecologica Scandinavica. Inaccurate Gestational Dating and Maternal Serum Alpha‐Fetoprotein Screening
3
PubMed. Elevated Maternal Serum AFP (MSAFP) During the Second Trimester
4
PubMed Central. The Effect of Vanishing Twin on First- and Second-Trimester Maternal Serum Markers and Nuchal Translucency: A Multicenter Prospective Cohort Study
5
PubMed. First and second trimester maternal serum markers in pregnancies with a vanishing twin
6
Clinical and Experimental Hypertension. Correlation between elevated maternal serum alpha-fetoprotein and ischemic placental disease: a retrospective cohort study
7
PubMed Central. Serum alpha-fetoprotein levels in acute viral hepatitis
8
Exploration of Digestive Diseases. Remarkable alpha-fetoprotein elevation and pseudo-infarction of cirrhotic liver: case report
9
Gene Expression. Elevated Serum Alpha-fetoprotein Levels in Non-alcoholic Steatohepatitis: Possible Molecular Mechanisms and Potential Clinical Significance
10
PubMed Central. Alpha‐fetoprotein and focal nodular hyperplasia: An unconventional couple
11
PubMed Central. Role of biochemical markers in testicular cancer: diagnosis, staging, and surveillance
12
PubMed Central. An isolate alpha-fetoprotein producing gastric cancer liver metastasis emerged in a patient previously affected by radiation induced liver disease
13
American Journal of Clinical Pathology. Unveiling the Unexpected: A Metastatic AFP-Producing Carcinoma of Gastric Origin Masquerading as Hepatocellular Carcinoma on Liver Biopsy
14
Clinical Chemistry / Oxford Academic. Rapid determination of alpha-fetoprotein gene promoter mutations in hereditary persistence of alpha-fetoprotein
15
PubMed. Serum alpha fetoprotein in Ataxia Telangiectasia: New lessons about an old biomarker
16
PubMed Central. Progressive Ataxia with Elevated Alpha-Fetoprotein: Diagnostic Issues and Review of the Literature
17
PubMed Central. A challenging case of alpha-fetoprotein-result discrepancies in a patient with chronic hepatitis B
18
American Journal of Gastroenterology. 2250 An Unusual Case of Elevated AFP in a Patient With Chronic Hepatitis B
19
PubMed. Rheumatoid factor interference of α-fetoprotein evaluations in human serum by ELISA
20
PubMed. Negative interference by rheumatoid factor in alpha-fetoprotein chemiluminescent microparticle immunoassay
21
Clinical Laboratory. Alpha-1-Fetoprotein Measurements and the High-Dose Hook Effect
22
PubMed Central. Serum alpha-fetoprotein surge after the initiation of chemotherapy for non-seminomatous testicular cancer has an adverse prognostic significance
23
PubMed. VAB-6 chemotherapy causes spurious elevation of alpha-fetoprotein associated with liver dysfunctions
24
PubMed. Serum alpha fetoprotein (AFP) levels in normal infants