Nitroblue Tetrazolium Test
The NBT test evaluates neutrophil oxidative burst, helping diagnose chronic granulomatous disease and other phagocytic defects. It is a simple colour change assay used where advanced flow cytometry is unavailable.
Definition and diagnosis
The NBT test measures the ability of neutrophils to reduce the colourless dye nitroblue tetrazolium to a blue formazan precipitate during a respiratory burst. A negative or weak reaction suggests a phagocytic defect.
- Colourless NBT → blue formazan in normal neutrophils
Severity and staging
Severity is graded by the proportion of neutrophils that reduce NBT: >80% normal, 50–80% mild defect, <50% severe defect. Clinical severity correlates with infection frequency.
Causes, most likely first
The most common cause in low‑resource settings is chronic granulomatous disease (CGD). Other causes include severe combined immunodeficiency, neutropenia, and exposure to herbal toxins that inhibit NADPH oxidase.
- CGD (X‑linked or autosomal recessive)
- Severe neutropenia
- Herbal or environmental toxins
How it presents
Patients present with recurrent bacterial and fungal infections, especially with catalase‑positive organisms. Skin abscesses, lymphadenitis, and granulomatous lesions are common.
- Recurrent Staph aureus, Serratia marcescens, Aspergillus
Investigations
The NBT test is the first line. If unavailable, a dihydrorhodamine (DHR) flow cytometry assay is preferred. Blood cultures, chest X‑ray, and fungal imaging are adjuncts.
Red flags and complications
Severe infections, rapidly progressive abscesses, or failure to respond to antibiotics should prompt urgent evaluation for phagocytic defects.
- Rapidly enlarging abscess
- Unusual fungal infection
- Persistent fever despite antibiotics
Management
Treat the underlying infection aggressively with appropriate antibiotics or antifungals. Provide prophylaxis with trimethoprim‑sulfamethoxazole and itraconazole. Consider bone marrow transplant in confirmed CGD.
- High‑dose antibiotics for bacterial infections
- Itraconazole for fungal prophylaxis
- Bone marrow transplant for definitive cure
Follow-up and prevention
Regular monitoring for infections, immunizations (including pneumococcal and influenza), and early treatment of any new infection are essential.
- Annual review of infection history
- Vaccination schedule adherence
- Prompt treatment of any infection
Often confused with
Other causes of neutropenia such as severe combined immunodeficiency or drug‑induced neutropenia may mimic CGD clinically but have different NBT results.
- SCID – normal NBT but low neutrophil count
- Drug‑induced neutropenia – normal NBT
Where results can mislead
A false‑negative NBT can occur if the test is performed after antibiotic therapy or if neutrophils are immature. A false‑positive may arise from technical errors or contamination.
- Late sample processing
Questions an examiner may ask
- A 3‑year‑old boy presents with recurrent skin abscesses. What is the first test you would order? — NBT test to assess neutrophil oxidative burst.
- You find a negative NBT result. Which organism is most likely responsible for his current infection? — Staphylococcus aureus or Serratia marcescens.
- A child with suspected CGD is started on prophylactic itraconazole. Why is this drug chosen? — It prevents Aspergillus infection, a common cause of mortality in CGD.
- You have a patient with neutropenia and a negative NBT. What is the next step? — Perform DHR flow cytometry to confirm CGD or consider other causes of neutropenia.
- A child with CGD develops a lung cavity. What imaging study is most useful? — Chest X‑ray followed by CT if available.
- You are in a low‑resource setting and cannot perform DHR. What alternative confirms CGD? — Repeat NBT with careful technique or use a nitroblue tetrazolium reduction assay with a different dye.
When to seek help
- Severe or rapidly progressing infection
- Unusual fungal infection
- Failure to respond to standard antibiotics
Quick questions
What organisms are most common in CGD?
Staphylococcus aureus, Serratia marcescens, Aspergillus species, and other catalase‑positive bacteria.
Can the NBT test be done in a rural clinic?
Yes, it requires only a microscope and a small amount of blood; it is inexpensive and can be performed with minimal equipment.
Is the NBT test definitive for CGD?
It is a screening tool; a negative or weak result should be confirmed with DHR flow cytometry or genetic testing.