Serum IgG Levels in Latent SSPE
In latent SSPE, serum IgG antibodies against measles virus are markedly elevated, typically with titers significantly higher than those seen in healthy individuals with prior measles exposure or vaccination. 1, 2
Specific Serum IgG Findings
Serum measles-specific IgG levels in SSPE patients are dramatically elevated compared to controls, with geometric mean titers of 6.58 ± 0.98 (log scale) versus normal post-vaccination or post-infection levels. 3 This represents a several-fold increase over what would be expected from natural immunity or vaccination alone.
Quantitative Characteristics
Using enzyme immunoassay (EIA), all confirmed SSPE cases demonstrate elevated serum measles IgG, with a positive correlation between serum antibody values measured by both IgG-EIA and traditional hemagglutination inhibition (HI) methods. 1
**The serum/CSF measles antibody titer ratio is characteristically <20 in most SSPE patients**, which is abnormally low and reflects the massive intrathecal antibody synthesis occurring in the CNS. 1 In normal individuals with measles immunity, this ratio would be much higher (typically >100).
Measles-specific IgG antibodies to nucleocapsid (NC) protein show the highest titers, while anti-matrix (M) protein antibodies are present but at lower levels (geometric mean 3.35 ± 0.53 for anti-M versus 6.58 ± 0.98 for anti-NC). 3
Additional Immunoglobulin Classes
Measles-specific IgD antibody is significantly elevated in SSPE serum compared to multiple sclerosis patients or normal controls with high measles antibody titers, consistent with chronic viral infection patterns. 4
The elevated IgD is not simply due to high IgG levels, as significant measles IgD activity persists even after eluting IgG from SSPE serum. 4
Clinical Diagnostic Context
The key diagnostic feature is not just elevated serum IgG, but the relationship between serum and CSF antibodies. 2 The CSF/serum measles antibody index (CSQrel) ≥1.5 confirms intrathecal synthesis, with typical values ranging from 2.3 to 36.9 (mean 12.9) in confirmed SSPE cases. 2
Important Diagnostic Pitfalls
Do not rely on serum antibody levels alone for diagnosis—the demonstration of intrathecal synthesis through simultaneous serum and CSF analysis is crucial. 2
The oligoclonal IgG band patterns in serum and CSF are nearly identical in SSPE patients, suggesting the same cell clones produce measles-specific IgG in both compartments. 5
Even in "latent" or early SSPE, these antibody abnormalities are typically already present, as the immune response is fundamental to the disease pathophysiology rather than a late phenomenon. 6