At a Glance
IgM P23 Ab: Presentmeans the p23/OspC band reacted.- P23 is one of three scored IgM bands; at least two must react for a positive IgM immunoblot.
- Positive IgM results should be disregarded after more than 30 days of illness.
- The immunoblot is a second-tier result and should be read with the first-tier test.
If you remember only one thing
A reactive p23 band is one piece of an IgM immunoblot. It becomes interpretable only alongside p39, p41, the first-tier result, and how long you have had symptoms.
What the p23 band detects
Lyme immunoblots separate Borrelia burgdorferi proteins by molecular weight and show which ones your antibodies bind to. Each band is named for that weight in kilodaltons (kD or kDa). Labcorp currently reports this component as part of its Lyme disease line blot.
The p23 band corresponds to OspC (outer surface protein C), a protein the bacterium produces early in infection, particularly as it moves from the tick into a human host. Because OspC appears early, antibodies to it also appear early — which is why it is one of the three bands scored on the IgM blot.
One practical wrinkle: OspC does not always land at exactly 23 kD. CDC notes that depending on the assay, OspC may be indicated by a band at 21, 22, 23, 24, or 25 kD. If your report shows a 24 kD IgM band instead, it is the same protein, scored the same way.
Gate 1: the other two IgM bands
A single band is a sub-result. The immunoblot result is the pattern across all three scored bands.
Current CDC and APHL criteria: the IgM immunoblot is positive when at least 2 of 3 scored proteins are reactive, and negative when fewer than two are. The three scored IgM proteins are p23, p39, and p41.
So a reactive p23 band does not by itself make the immunoblot positive. What it means depends entirely on p39 and p41 — and if your report shows only the p23 line, you do not yet have enough to know the overall result. Find the other two before drawing a conclusion.
This is not a technicality. The multi-band requirement exists because no individual band is specific enough alone. The criteria hold specificity high across the whole test, and applying them to one band removes the thing that makes the test work.
Gate 2: how long you have had symptoms
IgM antibodies typically appear early in infection. They do not always fade on schedule, though — IgM reactivity can persist well after an infection has resolved, and that persistence is one reason the following rule exists.
CDC states it operationally: positive IgM results should be disregarded if the patient has been ill for more than 30 days.
So if you have been unwell for more than a month, even a genuinely positive IgM immunoblot is set aside, because false-positive and prolonged IgM reactivity both become more likely with time. In that situation the IgG immunoblot is the result that informs interpretation.
Gate 3: the first-tier test
The immunoblot is the second step of a two-step process. It is meant to be run only on samples that were positive or equivocal on a first-tier immunoassay (EIA or IFA).
Labcorp states this directly for its line blot: the sensitivity of a line blot by itself is not sufficient to assess Lyme infection adequately, and both the EIA and the line blot should be performed. Interpreting blot results without knowing the first-tier result is strongly discouraged, because it raises the probability of a false positive.
If you do not know whether a first-tier test was done, that is worth establishing before reading anything into the bands.
"IgM P23 Ab" means the same thing
Laboratories use different notation for identical findings, which causes a lot of unnecessary searching.
| What your report says | What it means |
|---|---|
| 23 kD (IgM) Band: Reactive | Antibodies to OspC detected |
| IgM P23 Ab.: Present | Antibodies to OspC detected |
| IgM P23 Ab.: Present (Abnormal) | Antibodies to OspC detected — "Abnormal" flags a non-default value, not severity |
| 23 kD (IgM) Band: Non-reactive | Antibodies to OspC not detected |
| IgM P23 Ab.: Absent | Antibodies to OspC not detected |
The "P" stands for protein and the number is the same molecular weight, so "P23" and "23 kD" are interchangeable. On a Labcorp line blot report the line reads IgM P23 Ab. and sits alongside IgM P39 Ab., IgM P41 Ab., and an overall Lyme IgM Line Blot Interp. — that last line is the one carrying the actual immunoblot result.
Reading the three IgM bands together
Once you have all three results, the interpretation is straightforward.
| p23 | p39 | p41 | Complete IgM immunoblot |
|---|---|---|---|
| Reactive | Non-reactive | Non-reactive | Negative — 1 of 3 |
| Non-reactive | Reactive | Non-reactive | Negative — 1 of 3 |
| Non-reactive | Non-reactive | Reactive | Negative — 1 of 3 |
| Reactive | Reactive | Non-reactive | Positive — 2 of 3 |
| Reactive | Non-reactive | Reactive | Positive — 2 of 3 |
| Non-reactive | Reactive | Reactive | Positive — 2 of 3 |
| Reactive | Reactive | Reactive | Positive — 3 of 3 |
The criteria do not weight the bands differently — any two of the three satisfy the rule equally. And a positive here is still subject to Gates 2 and 3: it is disregarded beyond 30 days of illness, and it depends on a positive or equivocal first-tier result.
Two people with the same p23 result
The same reactive p23 band can sit inside two completely different outcomes.
| Result context | Interpretation |
|---|---|
| P23 reactive; p39 and p41 non-reactive | Complete IgM immunoblot is negative: 1 of 3 bands |
| P23 and p39 reactive; symptoms began 10 days ago; first-tier test positive | IgM immunoblot is positive: 2 of 3 bands, interpreted with symptoms and exposure |
Read together with
- IgM p39
- IgM p41
- Lyme IgM line-blot interpretation
- First-tier Lyme immunoassay
- IgG result when illness has lasted more than 30 days
What can cause a reactive band that is not Lyme disease
Not every band is equally specific. The p41 band detects flagellin, a protein structurally similar across many bacteria, which makes it frequently cross-reactive — one reason the criteria never rely on a single band.
CDC notes that false-positive cross-reactions may occur in patients with other conditions, including relapsing fever, syphilis, rheumatoid arthritis, and Epstein-Barr virus infection.
A related consideration runs the other way: serologic assays may be falsely negative during the first four to six weeks after infection, and patients treated with antibiotics early in disease may be less likely to seroconvert at all.
What this result cannot tell you
- It cannot diagnose Lyme disease. Diagnosis requires compatible symptoms and exposure history alongside laboratory support. Even a complete positive immunoblot is not a diagnosis on its own.
- It cannot establish that an infection is active. Once elevated, antibody titers remain so for months to years.
- It cannot be used to check whether treatment worked. Lyme serology should not be used to monitor treatment response or establish cure.
- It cannot distinguish recent from past exposure on its own.
- It cannot be read in isolation from the first-tier test. See Gate 3.
Common mistakes when reading this band
- Treating one reactive band as a partial positive. One band is a sub-result; the immunoblot result is the pattern across all three.
- Concluding the overall result from a single visible line. If your report shows only
IgM P23 Ab., you do not yet know the immunoblot result — find p39 and p41. - Assuming "Abnormal" means severe. It flags a non-default value.
- Reading a positive IgM beyond 30 days of illness. Current guidance is to disregard it.
- Comparing band counts across laboratories using non-standard criteria. Some laboratories offer tests or interpretive criteria that are not FDA-cleared or recommended; those band counts are not comparable to the criteria described here.
Questions your doctor may ask
- When did your symptoms begin?
- What did the first-tier immunoassay show?
- Were p39 or p41 also reactive?
- What symptoms and possible tick exposure prompted testing?
- Did you receive antibiotics early in the illness?
Clinical pearls
Present (Abnormal)describes the individual band, not the overall Lyme result.- IgM and IgG bands are scored separately; they cannot be added together.
- A result without bands may come from a modified two-tier algorithm using two immunoassays.
Clinical Takeaway
IgM P23 Ab: Present means the p23/OspC band reacted. It does not, by itself, establish a positive Lyme test or diagnose Lyme disease.
What to do next
Find the rest of the result. The three things that make your p23 band interpretable are the p39 and p41 lines, the overall interpretation line, and the first-tier immunoassay result — plus how long you have had symptoms. Bring the complete report to your clinician rather than the single band.
If recent infection is clinically suspected and the complete two-tier result is negative or not confirmed, your clinician may recommend testing a new sample in 7 to 14 days to look for seroconversion.
If you have a spreading rash after a possible tick bite, that is a situation where treatment may be appropriate without waiting on serology at all — contact a clinician promptly.
You can upload your lab report to keep the full band panel together in one place.
In one sentence
P23 reactivity is one component of the Lyme IgM immunoblot, not a positive Lyme test by itself.
Bottom line
Find p39, p41, the overall interpretation, the first-tier result, and your symptom duration before interpreting p23.
23 KD (IGM) Band
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Frequently Asked Questions
What does a reactive 23 kD IgM band mean?
Is a reactive 23 kD IgM band on its own a positive Lyme test?
What does "IgM P23 Ab: Present" or "Present (Abnormal)" mean?
Which bands are scored on the Lyme IgM immunoblot?
Why does it matter how long I have had symptoms?
How do I combine this band with my other results?
Can something other than Lyme disease cause a reactive band?
Can this band tell me whether my infection is active or cured?
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