Leptospirosis IgM / IgG Testing

Overview

Available Serological Tests

IgM vs IgG: Timing and Interpretation

IgM

• Appears: ~5-7 days after onset of symptoms (day 6-12 post infection)
• Significance: Indicates current / recent active infection
• Peaks: ~2-4 weeks into illness
• Wanes: Gradually over months
• A positive IgM rapid test places a patient in the "Probable Case (laboratory definition)" category (WHO/Goldman-Cecil criteria)
• IgM ELISA sensitivity is high in the first 1-3 weeks; far superior to MAT during acute phase - Goldman-Cecil Medicine, 2-Vol Set

IgG

• Appears: 2-3 weeks after onset
• Significance: Indicates past exposure or late/convalescent infection
• Persists: For months to years after recovery
• A positive IgG alone does NOT confirm active disease
• IgG ELISA alone has very low sensitivity (~17-18%) for acute infection per recent research

MAT

• Can detect both IgM and IgG agglutinating antibodies but cannot distinguish between them
• A single titer ≥1:400 is suggestive; a 4-fold rise between acute and convalescent samples confirms diagnosis
• MAT ≥400 in a single sample = Confirmed Case (WHO definition) - Goldman-Cecil Medicine
• Low sensitivity in the first week (misses many early cases)

WHO Case Definitions (from Goldman-Cecil Medicine)

• Suspect case (fever ≥38.5°C + myalgia + exposure history) with a positive rapid IgM test

• Isolation of leptospires from clinical specimen
• Positive PCR
• Seroconversion (negative → positive) or 4-fold rise in MAT titer
• MAT titer ≥400 in a single sample

Rapid IgM/IgG Lateral Flow Tests (e.g., SD Bioline Leptospira IgG/IgM)

⚠️ These tests provide a preliminary result only - confirmation with MAT, ELISA, or PCR is required per WHO. Do not use as the sole diagnostic criterion.

Limitations to Know

1. Serology is negative in the first 5-7 days - test early, then repeat in convalescent phase (10-14 days later) if negative initially
2. IgM can persist for months; a single positive IgM does not always mean current active disease - clinical context is essential
3. Cross-reactivity occurs with other spirochetal infections (Borrelia/Lyme, syphilis, relapsing fever) causing false positives
4. MAT cannot differentiate current infection vs. past infection vs. vaccination (in animals)
5. PCR is better in the first week before seroconversion; serology is better from day 7 onward - Goldman-Cecil Medicine

Practical Algorithm

Suspected leptospirosis
        |
Day 1-7: PCR (serum) preferred
        |
Day 7+: IgM ELISA / Rapid IgM test
        |
IgM positive → Probable case → treat empirically
        |
Confirm with MAT (paired sera if possible)
or single MAT titer ≥1:400

Counseling the Attender of a 16-Year-Old with Grade 3 Tonsillitis, Fever Spikes & CRP 56

First - Acknowledge Their Concern

"I completely understand why you're worried - seeing your child with a high fever and chills is very distressing. Let me explain exactly what is happening in his body and why this is expected."

Explain What Grade 3 Tonsillitis Means (in simple language)

"His tonsils are significantly enlarged and infected - Grade 3 means they are quite large and inflamed. This is a serious throat infection caused by bacteria. His body is working hard to fight this infection."

Explain Why Fever and Chills Are EXPECTED (and actually GOOD signs)

"Fever is NOT the disease itself - it is your child's immune system fighting the bacteria. When the body detects infection, it intentionally raises its temperature to kill the bacteria. The chills happen just before the temperature rises - the body is shivering to generate more heat."

"This is actually a sign that his immune system is responding and fighting back. If there was no fever, it would sometimes mean the body was not fighting properly."

Explain Why Fever Doesn't Drop Immediately After Starting Antibiotics

"Antibiotics have been started to kill the bacteria. But here is what most people don't know: antibiotics take 48 to 72 hours (2-3 days) to start showing their effect. During this time, the bacteria are being killed, but there is still inflammation in the tonsils. This inflammation is what keeps producing the fever."

"Even after the bacteria are gone, the inflamed tissue takes a few more days to settle down. So fever coming down slowly over 2-3 days is completely normal and expected - it does NOT mean the medicine is not working."

Explain the CRP Value

"We checked a blood test called CRP (C-Reactive Protein). His value is 56. This is an inflammation marker - it rises when there is infection and inflammation in the body. A value of 56 confirms there is a significant infection, which is consistent with what we are seeing. As he responds to antibiotics, this number will come down over several days."

"This also tells us that antibiotics are definitely the right treatment for him."

What the Attender Should Watch For (Warning Signs)

• Fever goes above 40°C and doesn't respond to paracetamol at all
• Child is unable to swallow even water or saliva (drooling, neck stiffness - may suggest peritonsillar abscess)
• Breathing difficulty or muffled/hot-potato voice
• Fever persisting beyond 4-5 days despite antibiotics
• Rash developing on the body
• Child becomes very drowsy, confused, or too weak to respond

"These warning signs are rare, but if you notice any of them, please call us or bring him in immediately."

Practical Instructions to Give

Timeline of Expected Recovery (Set Realistic Expectations)

"By day 3-4, you should start to see some improvement. If there is absolutely no change by day 4, please come back so we can reassess."

One Reassuring Closing Statement

"Your child is in the right place, on the right treatment, and being monitored. The fever and chills you are seeing are part of his recovery process. Give the antibiotics time to work. We are watching him closely."

Clinical Notes for You (Doctor's Perspective)

• CRP of 56 with Grade 3 tonsillitis strongly suggests bacterial pharyngotonsillitis - most likely Group A beta-hemolytic Streptococcus (GABHS)
• First line: Amoxicillin/Penicillin for 10 days; if penicillin failure suspected, consider amoxicillin-clavulanate or cephalosporin - Cummings Otolaryngology, Head & Neck Surgery
• Add NSAIDs (ibuprofen) or paracetamol for symptomatic relief - aspirin is contraindicated in adolescents (Reye's syndrome risk)
• Consider throat swab/ASLO if not already done to confirm streptococcal etiology
• Watch for peritonsillar abscess (Grade 3 tonsils with asymmetric swelling, uvular deviation, trismus, muffled voice)
• If no improvement by 48-72 hrs on antibiotics, reassess and consider changing antibiotic coverage or further workup (mono spot test for EBV if lymphocytosis, abscess formation)