Nitro-blue tetrazolium test is done for?
**Core Concept**
The nitro-blue tetrazolium (NBT) test is a diagnostic tool used to assess the function of the neutrophil NADPH oxidase complex, which is essential for the respiratory burst and subsequent killing of pathogens by neutrophils. This complex is comprised of several subunits, including the flavoprotein cytochrome b558 and the small GTP-binding protein Rac2.
**Why the Correct Answer is Right**
The NBT test is based on the principle that the NADPH oxidase complex reduces nitroblue tetrazolium to formazan, which is a blue-colored compound that can be visualized microscopically. This reaction is dependent on the presence and function of the NADPH oxidase complex. In cases of chronic granulomatous disease (CGD), where the NADPH oxidase complex is defective, the NBT test will be negative, indicating the absence or impaired function of the complex.
**Why Each Wrong Option is Incorrect**
* **Option A:** The NBT test is not used to diagnose infections caused by bacteria, viruses, or fungi, although it may provide indirect information about the host's ability to contain or eliminate these pathogens.
* **Option B:** The test is not a measure of neutrophil count or morphology, which can be assessed through other methods such as a complete blood count (CBC).
* **Option C:** While the NBT test is related to the function of neutrophils, it is not used to diagnose conditions such as neutropenia or neutrophilia, which are characterized by abnormal neutrophil counts.
**Clinical Pearl / High-Yield Fact**
The NBT test is a useful diagnostic tool for identifying patients with chronic granulomatous disease (CGD), a condition that increases the risk of severe and recurrent infections, particularly with catalase-positive bacteria such as Staphylococcus aureus.
**Correct Answer: C. Chronic granulomatous disease.**