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Blood Test Tracker

Johnson Grass IgE

Measures IgE antibodies to Johnson grass pollen, a grass of warm climates. A higher level means sensitization, which doesn't always cause symptoms.

AI-verified against cited sources. Awaiting expert reviewAlso called g10LOINC 6152-3

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What it is

Johnson grass (Sorghum halepense) is a tall grass that grows mostly in warm climates. Its pollen can cause hay fever symptoms in people who are allergic to it. This test measures IgE antibodies in your blood that recognize Johnson grass allergens. Labs report the result in kU/L, often with a class from 0 to 6.

Why it matters

Grass pollen is a common cause of hay fever, with sneezing, a runny or stuffy nose, and itchy, watery eyes. A result above your lab's cutoff means your immune system has made IgE to Johnson grass. It can't tell you how strong your symptoms are. Johnson grass and Bermuda grass share similar allergens, so a positive result for one can go with a positive result for the other.

If it’s high

A higher level means sensitization to Johnson grass pollen. Many people with hay fever in warm regions have it, and some people have it with no symptoms at all. Labs describe higher classes as a higher chance of a reaction, but the level may not match how strong your symptoms are.

Common causes

  • Hay fever (allergic rhinitis) from grass pollen
  • Sensitization without any symptoms
  • Cross-reactivity with related grasses, especially Bermuda grass

What can help

  • Take steps to reduce pollen exposure, such as checking local pollen forecasts, keeping windows closed on high-pollen days and showering after time outdoors
  • Ask your clinician about ways to manage hay fever, including allergy shots (immunotherapy), which can lower sensitivity to pollen
  • Ask whether tests for single grass allergens could help tell related grasses apart

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: 0–0.09 kU/L. Source: Labcorp, ARUP Laboratories, Quest Diagnostics, Thermo Fisher Scientific (Phadia), Journal of Allergy and Clinical Immunology.

ImmunoCAP class scale as printed by ARUP (the manufacturer's revised scale, per UR Medicine Labs 2022)

  • Class 0: absent or undetectablebelow 0.1 kU/L
  • Class 0/1: clinical relevance undetermined0.1 to under 0.35 kU/L
  • Class 1: low level0.35 to under 0.71 kU/L
  • Class 2: moderate level0.71 to under 3.51 kU/L
  • Class 3: high level3.51 to under 17.51 kU/L
  • Class 4: very high level17.51 to under 50.01 kU/L
  • Class 5: very high level50.01 to under 100.01 kU/L
  • Class 6: very high level100.01 and above kU/L

Labcorp class scale (ImmunoCAP, as printed on Labcorp sample reports dated 2016 to 2019)

  • Class 0: negativebelow 0.1 kU/L
  • Class 0/I: equivocal or low0.1 to under 0.32 kU/L
  • Class I: low0.32 to under 0.56 kU/L
  • Class II: moderate0.56 to under 1.41 kU/L
  • Class III: high1.41 to under 3.91 kU/L
  • Class IV: very high3.91 to under 19.01 kU/L
  • Class V: very high19.01 to under 100.01 kU/L
  • Class VI: very high100.01 and above kU/L

What can affect your result

  • Where you live: Johnson grass pollen is mainly a concern in warm regions
  • Cross-reactivity with related grasses can make several grass tests positive at once
  • Lab and method: class cutoffs differ slightly between labs
  • Results from different labs and assay systems can differ, so compare results from the same lab.
  • Cross-reactivity with related grasses, especially Bermuda grass, can make several grass tests positive at once.

When to talk to a clinician

Call 911 or your local emergency number right away for signs of a severe allergic reaction, such as trouble breathing, swelling of the tongue, lips or throat, dizziness or fainting, a fast heartbeat, or cold, clammy skin that looks pale, gray or bluish. Talk with a clinician if hay fever keeps you from sleeping or doing daily activities, or if you have wheezing, chest tightness or a cough that doesn't go away.

Questions you could ask

  • Should we recheck this, and how often?
  • Do my symptoms follow the grass pollen season where I live?
  • Would tests for single grass allergens help tell related grasses apart?
  • What are my options for managing symptoms, including allergy shots?

Questions people ask

Why are my Johnson grass and Bermuda grass results both positive?

The two grasses have similar allergens, so IgE made against one can react with the other. A small study found that IgE from people allergic to Bermuda grass also reacted with Johnson grass pollen proteins. Two positive results may reflect one allergy or two. An allergist can sort this out using your symptoms and, if needed, tests for single allergens.

Does a positive result mean Johnson grass causes my symptoms?

Not by itself. The test shows that your body has made IgE to the grass, which is called sensitization. Some people with this IgE have no symptoms. Your clinician looks at the result together with when and where your symptoms happen.

What do the classes 0 to 6 on my report mean?

Labs sort the number into classes from 0 (negative) to 6 (very high). A higher class means more IgE was found. It does not tell you how strong a reaction would be. The cutoffs differ a little between labs. For example, Quest and ARUP start class 1 at 0.35 kU/L, while Labcorp starts it at 0.32 kU/L.

Show the science and sources

Units

  • 1 kU/L = 1 kU/L
  • 1 kU/L = 1 UA/mL

For clinicians and researchers

Johnson grass (Sorghum halepense) is a grass of warm climates. The test is ImmunoCAP g10, measured by fluoroenzyme immunoassay at Labcorp (602503) and ARUP (0055047) and reported in kU/L (LOINC 6152-3). A Clinical and Experimental Allergy review reports that subtropical grasses such as Bermuda grass and Bahia grass lack the group 5 allergens found in temperate Pooideae grasses like timothy, and that group 1 cross-reactivity is lower between timothy and those grasses (that review does not cover Johnson grass). In a small Thai study of nine patients, a 29-30 kDa beta-expansin was a major cross-reactive allergen among Bermuda, Johnson and Para grass pollens, and IgE above 0.35 kUA/L to Bermuda and Johnson grass was found in 9 and 8 patients. Extract-based tests can't separate true co-sensitization from cross-reactivity; tests for single allergens, such as Cyn d 1 for Bermuda grass, are used in specialist care. Class cutoffs differ between labs.

LOINC 6152-3 (primary), 102339-9. Specimen: serum.

Sources

  1. Johnson Grass (test 602503), Labcorp
  2. Sample report for test 602503 (G010-IgE Johnson Grass), showing the class table, Labcorp
  3. Allergen, Grass, Johnson Grass (test 0055047), ARUP Laboratories
  4. Beta-Expansin of Bermuda, Johnson, and Para grass pollens, is a major cross-reactive allergen for Allergic Rhinitis patients in subtropical climate, Asian Pacific Journal of Allergy and Immunology (2019)
  5. Multiple grass mixes as opposed to single grasses for allergen immunotherapy in allergic rhinitis, Clinical and Experimental Allergy (2013)
  6. Allergy Blood Test, MedlinePlus (National Library of Medicine)
  7. Allergic rhinitis (Medical Encyclopedia), MedlinePlus (National Library of Medicine)
  8. Food Allergy Testing, MedlinePlus (National Library of Medicine)
  9. VALIDA project: Validation of allergy in vitro diagnostics assays (Tools and recommendations for the assessment of in vitro tests in the diagnosis of allergy), Advances in Laboratory Medicine (2020)
  10. ImmunoCAP Specific IgE Blood Test: Allergy Test List (interpretation table and test codes), Quest Diagnostics (2017)
  11. ImmunoCAP Specific IgE blood test results: Interpretation (respiratory results interpretation guide), Thermo Fisher Scientific (Phadia) (2023)
  12. ImmunoCAP allergy: Change to Reference Interval Interpretation, UR Medicine Labs, University of Rochester Medical Center (2022)
  13. Prevalence of allergic sensitization in the United States: results from the National Health and Nutrition Examination Survey (NHANES) 2005-2006, Journal of Allergy and Clinical Immunology (2014)

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

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