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Diphtheria antitoxoid IgG

Measures antibodies against the diphtheria toxin to show whether vaccination or past exposure still gives you protection.

AI-verified against cited sources. Awaiting expert reviewAlso called Diphtheria IgG, Anti-DT IgGLOINC 13227-4

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What it is

Diphtheria is a serious infection caused by a bacterium that makes a toxin. The vaccine, which is usually given as part of a combined shot, teaches your body to make antibodies against that toxin. This test measures the amount of those IgG antibodies in your blood, in international units per milliliter (IU/mL). Labs such as Quest treat 0.10 IU/mL or higher as a protective level.

Why it matters

A clinician may order this test to see how well your immune system responded to a vaccine, often with a blood sample before the vaccine and another about a month after. It can also help when vaccination records are missing. A level below 0.10 IU/mL does not mean you are sick, and it does not predict that you will get diphtheria. It means your clinician may want to look at your vaccination history and decide whether a booster makes sense.

If it’s low

A level below 0.10 IU/mL is below the cutoff Quest uses for protection. It often just means that antibodies have faded since your last dose. Your clinician reads it along with your vaccination history and health.

Common causes

  • Many years since your last diphtheria-containing vaccine dose
  • A vaccine series that was never completed, or no vaccination
  • A weaker immune response to the vaccine
  • A condition or treatment that weakens the immune system, or an antibody deficiency

What can help

  • Find your vaccination records and share them with your clinician
  • Ask whether a booster dose is recommended for you
  • Ask whether a follow-up level about a month after any booster would be useful

If it’s high

A higher level means more antibody protection, and labs do not set an upper limit. A high result usually means a recent vaccine dose.

Common causes

  • A recent dose of a vaccine that contains diphtheria toxoid (such as Td, Tdap or DTaP)
  • Recent treatment with immune globulin, which carries donor antibodies

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 0.1 and above IU/mL. Source: Quest Diagnostics, ARUP Laboratories.

Quest Diagnostics interpretive criteria

  • Nonprotective antibody levelbelow 0.1 IU/mL
  • Protective antibody level0.1 and above IU/mL

What can affect your result

  • Time since your last diphtheria-containing vaccine dose
  • Whether you finished the vaccine series
  • Treatment with immune globulin or other blood products
  • The immune system's health
  • Which lab and method were used
  • Immune globulin (IVIG or similar) or other blood products given before the draw carry donor antibodies and can raise the result without reflecting your own immunity.
  • Results from different labs and kits can differ, so compare trends only within the same lab and method when you can.

Levels rise within weeks of a vaccine dose (ARUP asks for the follow-up sample about 30 days after the dose) and tend to fall slowly over the years.

When to talk to a clinician

A low level on its own is not urgent. Call a clinician the same day if you have been in close contact with someone with diphtheria, or if you have a severe sore throat with trouble swallowing or breathing. For trouble breathing, call 911 or your local emergency number.

Questions you could ask

  • Are my vaccination records complete and up to date?
  • Why was this test ordered, and how will the result change what we do?
  • Is a booster dose recommended for me?
  • Could anything about my health or treatment explain a low level?
  • Should we recheck my level about a month after a booster?

Questions people ask

What does 0.10 IU/mL mean?

It is the level that Quest and ARUP treat as the protective threshold for diphtheria antibodies. Quest reports 0.10 IU/mL or higher as protective. ARUP says above 0.1 IU/mL is usually considered protective.

My level is between 0.01 and 0.10 IU/mL. Am I unprotected?

Below the lab's protective cutoff, but not necessarily without any protection. Some studies call 0.01 to under 0.1 IU/mL basic protection. Your clinician can look at your vaccination history and decide if a booster is worth discussing.

Why do some labs ask for a before and after sample?

Comparing the level before a vaccine dose with the level about 30 days after shows how strongly your immune system responded. ARUP asks for both samples to be sent together for this reason.

Show the science and sources

Units

  • 1 IU/mL = 1 IU/mL
  • 1 IU/mL = 1000 mIU/mL

Open the Diphtheria IgG unit converter

For clinicians and researchers

Antitoxin IgG is measured against diphtheria toxoid by immunoassay (Quest) or by a quantitative multiplex bead assay (ARUP, laboratory-developed test), reported in IU/mL. Quest uses 0.10 IU/mL or higher as protective and notes that after a primary series of three properly spaced doses in adults, or four in infants, more than 95% of immunized persons reach at least 0.10 IU/mL. ARUP states that a concentration above 0.1 IU/mL is usually considered protective. Serosurveys use graded cut-offs: the Guyana serosurvey (2026) classed below 0.01 IU/mL as no protection, 0.01 to 0.09 as minimal protection, 0.1 to 0.99 as full protection and 1.0 or higher as long-term protection, while the Greek adult survey (Papagiannis 2024) used below 0.01 as seronegative, 0.01 to 0.099 as basic protection and above 0.1 as full protection. These are study conventions, not a single named guideline. ARUP interprets paired samples drawn before and about one month after vaccination: a post-vaccination level below 1.0 IU/mL is a nonresponse. At 1.0 IU/mL or higher, a post/pre ratio under 1.5 is a nonresponse, 1.5 to under 3.0 is a weak response and 3.0 or higher is a good response. If the pre-vaccination level is already above 1.0 IU/mL, a post level above 2.5 IU/mL is usually adequate.

LOINC 13227-4 (primary), 58787-3, 5115-1, 49189-4. Specimen: serum.

Sources

  1. Diphtheria Antitoxoid (test code 4865), Quest Diagnostics
  2. Diphtheria Antibody, IgG (test 0050210), ARUP Laboratories
  3. Diphtheria and Tetanus Immunity Status among Greek Adults: Results from a Nationwide Seroprevalence Study, Vaccines (Basel) (2024)
  4. Integrated serosurveillance of multiple communicable diseases in the hinterland of Guyana, Revista Panamericana de Salud Pública (2026)
  5. Chapter 7: Diphtheria, US Centers for Disease Control and Prevention (Pink Book)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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