Why is the NS1 (Non-Structural 1) antigen test negative in a patient with suspected dengue fever, given the positive IgM (Immunoglobulin M) result?

Why is NS1 Negative with Positive IgM in Suspected Dengue?

The NS1 antigen test is negative despite positive IgM because the patient is likely in the later phase of dengue infection (day 5 of illness), when NS1 antigen levels typically decrease while IgM antibodies become detectable. 1, 2

Dengue Diagnostic Timeline and Test Interpretation

Dengue diagnostic testing involves both molecular and serologic methods, with timing being crucial for proper interpretation:

• Early phase (0-5 days):

  • NS1 antigen is highly detectable
  • Viral RNA can be detected by PCR
  • IgM antibodies begin to appear around day 3-5

• Later phase (>5 days):

  • NS1 antigen levels decrease
  • IgM antibodies become more prominent
  • IgG antibodies begin to appear (especially in secondary infections)

In this 9-year-old patient presenting with 5 days of intermittent fever, the negative NS1 with positive IgM is consistent with testing performed during the transition from acute to convalescent phase of dengue infection 1, 2.

Factors Affecting NS1 Detection

Several factors can influence NS1 test sensitivity:

1. Time since symptom onset: NS1 sensitivity decreases significantly after 3 days of illness 3, 4

   • Day 1-3: High sensitivity (>90%)
   • Day 4-5: Moderate sensitivity (70-80%)
   • Day >5: Lower sensitivity (<60%)

2. Primary vs. Secondary infection: NS1 tests are more sensitive in primary than secondary dengue infections 2, 4

   • In secondary infections, pre-existing antibodies may form immune complexes with NS1, reducing its detection

3. Viral serotype: NS1 detection varies by dengue serotype, with lower sensitivity for DENV-2 and DENV-4 5

4. Presence of antibodies: The presence of DENV-reactive IgG and IgM in the sample significantly lowers NS1 detection rates 3

5. Viral load: NS1 positivity correlates with viraemia levels - NS1-positive samples typically have higher viraemia than NS1-negative samples 3, 4

Clinical Implications and Management

For this patient with positive IgM but negative NS1:

• Diagnostic interpretation: The results strongly suggest dengue infection in the later phase (around day 5)
• Clinical correlation: The patient's symptoms (fever, headache, weakness, abdominal pain) are consistent with dengue
• Management focus: Monitor for warning signs of severe dengue, including:
  • Abdominal pain or tenderness
  • Persistent vomiting
  • Clinical fluid accumulation
  • Mucosal bleeding
  • Lethargy or restlessness
  • Liver enlargement >2 cm
  • Laboratory: increasing hematocrit with rapid decrease in platelet count 6

Improving Diagnostic Accuracy

To confirm the diagnosis:

1. Combined testing approach: Using NS1 along with IgM/IgG testing significantly improves diagnostic sensitivity 5

   • NS1 alone: ~62% sensitivity
   • NS1 + IgM: ~76% sensitivity
   • NS1 + IgM + IgG: ~84% sensitivity

2. Consider convalescent serology: A second IgG test in 2-3 weeks can confirm seroconversion

3. Clinical monitoring: Despite negative NS1, the patient should be managed as dengue based on clinical presentation and positive IgM 6

Common Pitfalls to Avoid

• Ruling out dengue based solely on negative NS1: NS1 sensitivity decreases after day 3-5 of illness
• Overlooking warning signs: Despite laboratory confirmation, clinical monitoring remains essential
• Unnecessary hospital admission based solely on positive tests: Admission criteria should be based on clinical warning signs rather than positive NS1 or IgM tests alone 2
• Ignoring cross-reactivity: IgM antibodies can cross-react with other flaviviruses, though this is less likely given the clinical presentation and epidemiological context 1