The essentials

"Reactive" means the test found what it looks for, usually antibodies or an antigen above the lab's cut-off; "nonreactive" means it did not. A reactive screen is not a diagnosis: it is often checked with a second, more specific test, and it may simply show a past infection or a vaccine. "Indeterminate" or "equivocal" means "not sure yet".

How do I read a reactive or nonreactive result on my report?

Antibody and antigen tests (serology) often report words, sometimes with a number: the index or signal-to-cut-off ratio (S/CO), which compares your sample's signal with the threshold set by the test maker[4]. Here is a fictitious US report with several serology lines.

Fictitious example of serology report lines (United States format)
TestResultReference rangeFlag
Hepatitis C antibody (anti-HCV), S/CO 1.4ReactiveNonreactive (S/CO < 1.00)A
HCV RNA, reflexNot detectedNot detected-
Hepatitis B surface antibody (anti-HBs)Reactive, 85 mIU/mLImmune if > 10 mIU/mL-
Toxoplasma IgGReactiveNonreactive-
Toxoplasma IgM, index 0.92EquivocalNegative < 0.80; equivocal 0.80-0.99; positive ≥ 1.00A
Syphilis screen (RPR)NonreactiveNonreactive-
  1. Reactive with an S/CO: the signal is 1.4 times the cut-off. Results just above the cut-off are more often false positives than strongly reactive ones, so labs use the ratio to decide on further testing[4].
  2. Reflex test: after a reactive hepatitis C antibody, labs should automatically look for the virus itself. RNA not detected means no current infection: a cleared past infection or a false-positive antibody[5].
  3. A reactive result can be good news: anti-HBs above 10 mIU/mL after the vaccine series means you are protected against hepatitis B[7].
  4. IgG reactive, IgM equivocal: IgG usually points to a past contact; the IgM index sits in the gray zone. These cut-offs are examples: each test maker sets its own, and your laboratory's report prevails[4].

Reactive, equivocal, an index you cannot place? BloodSense explains each line of your report and helps you prepare questions for your doctor.

Get my results explained

What is a reactive test used for?

Most serology runs in two steps: when a screening test is reactive, a more specific supplemental or confirmatory test checks the result[2][4]. In the US laboratory algorithm for HIV, for example, a reactive screen with negative follow-up tests is classed as a false positive on the first test[2].

Likewise, a positive syphilis screen leads to a second, different test[13], and a reactive hepatitis C antibody leads to a test for the virus[5]. Details are in our guides to the anti-HCV test, the RPR syphilis test and HIV test results. For results with a normal range, see how to read blood test results.

What do reactive, nonreactive, indeterminate and titer mean?

Wording differs between labs; your laboratory's report prevails.

Common result words on serology reports
On the reportWhat it usually meansSource
Nonreactive, negativeThe test did not find what it looks forMedlinePlus[1]; CDC[2]
Reactive, positiveThe test found it; for a screen, a confirmation step often followsCDC[2]
Repeatedly reactiveThe sample was retested and stayed reactive before being reportedCDC[2][4]
Indeterminate, equivocal, borderline, inconclusiveNeither clearly positive nor negative; repeat or different testMedlinePlus[1]; CDC[2]
Index or S/COSignal compared with the cut-off; 1.0 is often the threshold, but each test sets its ownCDC[4]
Titer (for example 1:16)The highest dilution that still reacts; a fourfold change between two results with the same test is significantCDC[6]
Not detectedUsed for tests of the germ's genetic material (RNA, DNA) rather than antibodiesCDC[5]

IgG or IgM? IgM is the early antibody; IgG comes later and usually lasts. IgM suggests a recent infection, but can be falsely positive, and for some germs, such as toxoplasma, it persists for months or years after infection[8]. When both are reactive, a high IgG avidity rules out an infection in the last 4 months or so[8]. See our pages on IgG antibodies, IgM antibodies and the TORCH panel in pregnancy.

Some antibodies stay for life: treponemal syphilis antibodies typically remain reactive after treatment[6].

Why can a test be reactive if I am not sick?

From the most frequent explanations to the rarest:

  • Common Past infection or vaccination: a reactive anti-HBs after vaccination means immunity[7]; a reactive hepatitis C antibody with no virus found can be a cleared infection[5].
  • Common A current or recent infection, which the confirmation step and your doctor must establish.
  • Possible A biological false positive: for syphilis screens, other infections, recent vaccinations, autoimmune disorders and injection drug use are known causes[6].
  • Possible A low chance of infection before the test: the rarer an infection among the people tested, the larger the share of false positives[4]. With older hepatitis C antibody tests, about 35% (15-60%) of screening-positive results were false in populations where fewer than 10% were infected[4]. Illustration: a 99% specific test used where 1 person in 1,000 is infected gives about 10 false reactive results for each true one[12].
  • Possible A mother's antibodies in a baby: for this reason, the hepatitis C antibody test is not done before 18 months of age in exposed children[9].
  • Rare Interference in the test itself: heterophilic antibodies can bind the test's antibodies and give false results[10]; high doses of biotin supplements can push some immunoassays falsely up or down[11].

The reverse also happens: a nonreactive result can be falsely reassuring if done too early (the window period). For HIV, a lab antigen/antibody test usually detects infection 18 to 45 days after exposure, and an antibody-only test 23 to 90 days[3]. Autoantibody tests such as the ANA test are read with their own titers and patterns; see also abnormal results with no symptoms.

What can affect a reactive or nonreactive result?

Most antibody tests need no fasting; a syphilis blood test, for example, needs no special preparation[13]. Tell your doctor or the lab:

  • The date of a possible exposure: testing during the window period can give a nonreactive result despite infection[3].
  • Any vaccine received recently[6][7].
  • A known autoimmune disease or a recent infection[6].
  • Biotin (vitamin B7) supplements, especially at high doses[11].
  • Treatments made from mouse antibodies, which can trigger interfering antibodies[10].
  • For a baby: the mother's results, since her antibodies can show up in the child's blood[9].

See also what to expect at a blood draw.

When should I see a doctor about a reactive result?

When to see a doctor

  • Emergency: call 911 for high fever with a stiff neck or confusion, trouble breathing or fainting.
  • Within 24 to 48 hours if a reactive result comes with fever, a rash or yellow eyes, or if you are pregnant; after a recent possible exposure, ask when to repeat a nonreactive test[3].
  • At your next appointment if you feel well and the result is a reactive IgG, an equivocal index or a screen awaiting confirmation.

Questions to ask your doctor

  1. Was this a screening test, and has the confirmation test been done?
  2. What was my index or titer, and how far is it from the cut-off?
  3. Is it IgG, IgM or both, and does it mean a past or a recent infection?
  4. Do I need a repeat test, and when?

Frequently asked questions

Is "reactive" the same as "positive"?

In practice, yes: both mean the test found its target. Labs often keep the word "reactive" for screening results that still need confirmation[2].

My test was equivocal. Will the next one be clear?

Often. Antibodies may rise above the cut-off a few weeks later, or the lab may use a different test. In the US HIV algorithm, some indeterminate results are repeated 2 to 4 weeks later[2].

Does a pregnancy blood test say reactive too?

In some countries, labs report a qualitative hCG test as reactive (pregnant) or nonreactive. The quantitative test gives a number instead; see our page on hCG blood test results.

  1. MedlinePlus. How to Understand Your Lab Results. US National Library of Medicine. medlineplus.gov (accessed November 3, 2026)
  2. Centers for Disease Control and Prevention. 2018 Quick Reference Guide: Recommended Laboratory HIV Testing Algorithm for Serum or Plasma Specimens. stacks.cdc.gov (accessed November 3, 2026)
  3. Centers for Disease Control and Prevention. HIV Testing. cdc.gov (accessed November 3, 2026)
  4. Alter MJ, Kuhnert WL, Finelli L. Guidelines for laboratory testing and result reporting of antibody to hepatitis C virus. MMWR Recomm Rep. 2003;52(RR-3):1-13. cdc.gov (accessed November 3, 2026)
  5. Centers for Disease Control and Prevention. Clinical Testing and Diagnosis for Hepatitis C. cdc.gov (accessed November 3, 2026)
  6. Papp JR, Park IU, Fakile Y, Pereira L, Pillay A, Bolan GA. CDC Laboratory Recommendations for Syphilis Testing, United States, 2024. MMWR Recomm Rep. 2024;73(1):1-32. doi:10.15585/mmwr.rr7301a1 (accessed November 3, 2026)
  7. Centers for Disease Control and Prevention. Clinical Testing and Diagnosis for Hepatitis B. cdc.gov (accessed November 3, 2026)
  8. Teimouri A, Mohtasebi S, Kazemirad E, Keshavarz H. Role of Toxoplasma gondii IgG Avidity Testing in Discriminating between Acute and Chronic Toxoplasmosis in Pregnancy. J Clin Microbiol. 2020;58(9):e00505-20. doi:10.1128/JCM.00505-20 (accessed November 3, 2026)
  9. Panagiotakopoulos L, Sandul AL, Conners EE, Foster MA, Nelson NP, Wester C. CDC Recommendations for Hepatitis C Testing Among Perinatally Exposed Infants and Children, United States, 2023. MMWR Recomm Rep. 2023;72(4):1-19. doi:10.15585/mmwr.rr7204a1 (accessed November 3, 2026)
  10. Bolstad N, Warren DJ, Nustad K. Heterophilic antibody interference in immunometric assays. Best Pract Res Clin Endocrinol Metab. 2013;27(5):647-661. doi:10.1016/j.beem.2013.05.011 (accessed November 3, 2026)
  11. Balzer AHA, Whitehurst CB. An Analysis of the Biotin-(Strept)avidin System in Immunoassays: Interference and Mitigation Strategies. Curr Issues Mol Biol. 2023;45(11):8733-8754. doi:10.3390/cimb45110549 (accessed November 3, 2026)
  12. Trevethan R. Sensitivity, Specificity, and Predictive Values: Foundations, Pliabilities, and Pitfalls in Research and Practice. Front Public Health. 2017;5:307. doi:10.3389/fpubh.2017.00307 (accessed November 3, 2026)
  13. MedlinePlus. Syphilis Tests. US National Library of Medicine. medlineplus.gov (accessed November 3, 2026)

This page is general information. It does not replace the advice of a doctor, who interprets your results with your symptoms, history and examination.