What do positive Mycoplasma (Infectious Agent) IgM (Immunoglobulin M) and IgG (Immunoglobulin G) test results indicate?

Interpretation of Mycoplasma IgG and IgM Test Results

Positive Mycoplasma IgM indicates a current or recent infection, while positive IgG with negative IgM typically indicates past infection or immunity; however, IgM can remain elevated for months after infection, making interpretation challenging without clinical correlation.

Understanding Mycoplasma Antibody Testing

IgM Antibodies

• IgM antibodies appear first in the immune response to Mycoplasma pneumoniae infection
• Typically become detectable early in infection (within 1-2 weeks of symptom onset)
• Important limitation: IgM can remain elevated for months or even years after infection 1
• False positive IgM results can occur due to:
  • Cross-reactivity with other infections
  • Rheumatoid factor
  • Blood product transfusions or IVIg infusions 2

IgG Antibodies

• IgG antibodies develop later in the immune response
• Persist long-term after infection
• Indicate either current infection (when rising titers are demonstrated) or past infection/immunity
• May remain elevated for many weeks and are less useful for diagnosing acute infection 3

Interpretation of Results

Positive IgM, Negative IgG

• Suggests very early/acute Mycoplasma infection
• Most commonly seen in primary infections, especially in children and teenagers 4
• Requires clinical correlation as false positives can occur

Positive IgM, Positive IgG

• Indicates recent or ongoing Mycoplasma infection
• Most reliable pattern for confirming active infection
• Combined measurement of specific IgM and IgG has shown 97.8% sensitivity and 99.7% specificity 4

Negative IgM, Positive IgG

• Usually indicates past infection with immunity
• In adults over 40 years, this pattern may represent reinfection (56% of cases show only IgG response) 4
• May also indicate very early infection before IgM develops or late infection after IgM has declined

Negative IgM, Negative IgG

• No evidence of current or past Mycoplasma infection
• May be seen very early in infection before antibodies develop

Age-Related Differences in Antibody Response

• Children and teenagers: Predominantly respond with IgM antibodies 4
• Adults (especially >40 years): Often show only IgG response (56% of cases), likely due to reinfection 4

Clinical Pitfalls in Interpretation

1. False positive IgM results:

   • A study found 32% of children with pneumonia and 38.3% of healthy children had elevated IgM titers 1
   • IgM can persist for months after infection, making it unreliable for diagnosing acute infection

2. Reinfection patterns:

   • Adults with reinfection often lack IgM response but show IgG elevation 4
   • This can lead to missed diagnosis if only IgM is tested

3. Test quality varies:

   • Commercial assays show significant differences in specificity (25-100%) and sensitivity 5
   • EIA-based tests have shown better performance than some other methods

Recommendations for Optimal Diagnosis

1. Test both IgM and IgA when available (not just IgM) to maximize detection of current infection, including reinfections 3

2. For adults with suspected Mycoplasma infection:

   • Consider paired sera (acute and convalescent samples 2-4 weeks apart)
   • Look for a 4-fold rise in antibody titers
   • PCR testing of respiratory specimens can provide additional diagnostic information 5

3. For children with suspected Mycoplasma infection:

   • IgM testing is more reliable than in adults but still requires clinical correlation
   • PCR testing may help confirm diagnosis but can also detect colonization without active infection 1

4. In cases of suspected false positive results, repeat testing after 5-10 days (half-life of IgM is approximately 5 days) 2