NBT test: assessment of neutrophil oxidative burst

The spontaneous nitroblue tetrazolium test is a cytochemical method for assessing neutrophil oxidative metabolism without external stimulation in vitro. Its principle is based on the fact that upon formation of superoxide anion, a soluble yellow dye is reduced to an insoluble dark formazan, which can be visualized in the cell under a microscope. Essentially, the test evaluates the phagocyte's ability to initiate the oxygen-dependent stage of the microbicidal response.

Biologically, this test is linked to the function of the neutrophil's nicotinamide adenine dinucleotide phosphate oxidase complex. When the complex is functioning normally, the cell produces reactive oxygen species and can reduce nitroblue tetrazolium to formazan. If the system is damaged, the number of stained cells decreases or disappears. This is why the NBT test has historically played an important role in the diagnosis of chronic granulomatous disease. [1]

It's important to differentiate the two methods. The spontaneous test assesses neutrophil baseline activity without the addition of a standard stimulator. The activated, or stimulated, test is performed after exposure to a substance that triggers an oxidative burst, such as phorbol myristate acetate or another stimulus. Clinically, these are not the same thing: the spontaneous test is more sensitive to current neutrophil activation, while the stimulated test is more sensitive to the system's reserve. [2]

Historically, laboratories have relied on low, unambiguous values for formazan-positive neutrophils in healthy individuals for spontaneous testing. However, the current problem is that NBT testing methods have evolved for a long time without full standardization, and therefore there is no universal, equally validated modern reference interval for different laboratories. This is one of the reasons why test interpretation has always been difficult. [3]

From a practical standpoint, the spontaneous NBT test does not indicate "general immunity" or "the presence of infection." It reflects only one specific aspect of the innate immune response—the ability of a neutrophil to generate reactive oxygen species under the conditions of a given test system. Therefore, it should never be used alone as the sole diagnostic criterion. [4]

Table 1. What exactly does the NST test evaluate?

Parameter	Spontaneous NST test	Activated test with NST
What is being checked?	Neutrophil basal oxidative activity	Oxidative burst reserve after stimulation
Biological meaning	Background readiness of the cell to form reactive oxygen species	The ability to dramatically increase the respiratory burst
The main historical role	Neutrophil activation assay, an old infectious and immunological marker	Screening for oxidase system defects, especially in chronic granulomatous disease
Reading method	Microscopy of formazan-positive cells	Microscopy or semiquantitative assessment after stimulation
The main modern problem	Very low specificity	Inferior to flow cytometry with dihydrorhodamine

The table summarizes the differences between spontaneous and stimulated NST approaches. [5]

The historical significance of the test and what has changed in modern practice

The NBT test was a key method for studying neutrophil function in the 1960s and 1970s. It made it possible to clinically identify oxidative burst defects in patients with chronic granulomatous disease. This was a significant advance at the time, as the test allowed for the detection of phagocytic bactericidal defects even before the advent of modern flow-through technologies. [6]

Over time, it became clear that the spontaneous version of the method was too dependent on external and internal factors. Older reviews explicitly stated that both spontaneous and stimulated NST tests suffered from numerous protocol modifications, a lack of strict standardization, and limited reproducibility. This did not render the test useless, but it severely limited its reliability as a broad diagnostic tool. [7]

In modern immunology and clinical hematology, the NBT test has been superseded in most centers by the dihydrorhodamine flow cytometric test. GeneReviews notes that the dihydrorhodamine test is superior to other tests in chronic granulomatous disease, and a 2024 review calls it the current gold standard for assessing nicotinamide adenine dinucleotide phosphate oxidase function. [8]

The reasons for this shift are clear. The dihydrorhodamine assay is quantitative, more sensitive, easier to standardize, and evaluates the signal in each individual cell. It better detects residual oxidase activity, better recognizes mosaic populations in carriers of the X-linked form, and is better suited for monitoring after transplantation and in more complex genetic variants of the disease. [9]

However, the NST test has not completely disappeared. A 2024 review emphasizes that it may still be useful in resource-limited settings where flow cytometry is unavailable, results are needed quickly, and local expertise in performing the test exists. This does not make it equal to the current standard, but it explains why the method continues to be used in some laboratories. [10]

Table 2. Historical NST test and modern dihydrorhodamine test

Characteristic	NST test	Dihydrorhodamine test
Technology	Cytochemical, microscopic	Flow cytometry
Assessment type	Mainly qualitative or semi-quantitative	Quantitative
Partial defect sensitivity	Below	Higher
Evaluation of carriers of the X-linked form	Limited	Much better
Evaluation of residual oxidase function	Limited	Good
Current status	Historical and auxiliary method	The preferred modern method

Table based on GeneReviews and the 2024 review of chronic granulomatous disease diagnostics.[11]

Why the spontaneous NST test cannot be used as a universal marker of infection

In older clinical literature, the spontaneous NBT test was actively studied as a possible way to differentiate bacterial infection from nonbacterial conditions. The logic was clear: neutrophils activated by bacterial infection should restore the dye more frequently and produce more formazan-positive cells. However, experience has shown that this idea is too unstable. [12]

A 1975 review, still considered an important summary of the method, explicitly stated that the NST test cannot be considered an absolute method for distinguishing between bacterial and nonbacterial infections or noninfectious diseases, as false-positive and false-negative results were common. In other words, even historically accumulated data did not confirm the expected specificity for the spontaneous variant. [13]

Work in dermatology was also very revealing: in patients with infectious and non-infectious inflammatory dermatoses, spontaneous NST was often elevated, but it was poor at distinguishing between bacterial and viral skin processes. Furthermore, the test also increased in a number of non-infectious inflammatory diseases, further demonstrating its non-specificity. [14]

This is explained biologically. The spontaneous test responds not only to the microbe but also to the overall level of neutrophil activation. Such an increase is possible in allergic, autoimmune, inflammatory, and other conditions where there is stimulation of innate immunity without direct bacterial infection. Therefore, interpreting a high spontaneous NST as a "sign of bacteria" is too crude and clinically dangerous. [15]

In modern practice, neither sepsis, nor bacterial pneumonia, nor fever of unknown origin are diagnosed based on the spontaneous NST test. It is absent from current routine algorithms for infectious diagnostics, and its historical use in this role is now best viewed as a stage in the development of laboratory immunology rather than as a current standard. [16]

Table 3. Why high or low spontaneous NST does not equal a diagnosis

Result	What could this mean?	Why this is not enough
Promotion	Neutrophil activation during infection or inflammation	An increase is possible even without a bacterial infection.
Normal result	Lack of pronounced baseline activation	Does not exclude infection and does not exclude a phagocyte defect
Decrease	Severe neutrophil dysfunction, immunodeficiency, or laboratory influences	Confirmatory functional tests are needed
Sharply low if chronic granulomatous disease is suspected	Possible defect in the oxidase system	A modern confirmatory test is needed
High in allergies or autoimmune inflammation	Nonspecific activation of innate immunity	Doesn't say anything about the nature of the process

The table reflects the main limitation of the spontaneous NST test: non-specificity. [17]

Where the test really matters: chronic granulomatous disease and oxidative burst defects

The primary classical application of the NBT test is chronic granulomatous disease, a congenital disorder of phagocytic function in which neutrophils are unable to generate superoxide and other reactive oxygen species normally. These patients experience recurrent severe bacterial and fungal infections, granulomas, and inflammatory complications. [18]

GeneReviews emphasizes that, under normal stimulation in healthy individuals, more than 95% of neutrophils reduce nitroblue tetrazolium to formazan, whereas in chronic granulomatous disease, cells with this reaction are either absent or their numbers are sharply reduced. It is this phenomenon that has made the NBT test a recognizable historical diagnostic method for the disease. [19]

But a very precise clarification is important here: for chronic granulomatous disease, it is the stimulated test, not the purely spontaneous one, that is clinically significant. Spontaneous NBT assesses baseline activation and, by itself, is not the optimal tool for proving an innate defect of the oxidative burst. For this purpose, stimulated NBT and, even better, the dihydrorhodamine test are far more informative. [20]

Recent reviews specifically emphasize that the NST test is less effective at detecting partial forms of chronic granulomatous disease, residual systemic activity, and mixed cell populations in carriers of the X-linked form. It is in these clinically challenging scenarios that the dihydrorhodamine test has a clear advantage. [21]

There's another important nuance. In severe myeloperoxidase deficiency, the dihydrorhodamine test can produce a false-semblance of chronic granulomatous disease, while the NST in such patients remains normal. Therefore, in complex cases, the older NST test sometimes retains additional differential value, but this is a highly specialized task, rather than a widespread use of the spontaneous variant. [22]

Table 4. When to think about chronic granulomatous disease

Clinical situation	Why is this alarming?
Recurrent deep abscesses	Characteristic of a defect in phagocytic killing of microbes
Invasive fungal infections, especially Aspergillus	Typical of chronic granulomatous disease
Pneumonia, lymphadenitis, osteomyelitis, liver and skin abscesses	Classic infectious spectrum
Granulomas of the gastrointestinal or genitourinary tract	Frequent inflammatory manifestation
Family history of similar infections	Supports hereditary nature
Suspicious NST or dihydrorhodamine test	Requires immunological and genetic confirmation

Table based on GeneReviews, ESID and 2024 review.[23]

How to properly examine a patient today if the old NST test is abnormal

If a spontaneous or stimulated NBT test is abnormal, the correct modern approach is not to repeat it indefinitely, but to move on to a more accurate functional level of diagnosis. In most centers, this level is considered to be the dihydrorhodamine flow cytometric test. ARUP explicitly recommends this test as a screening test for chronic granulomatous disease. [24]

The next step after confirming the oxidative burst defect is to clarify its nature. A 2024 review indicates that after functional confirmation of the absence or decreased activity of nicotinamide adenine dinucleotide phosphate oxidase, the expression of individual proteins of the complex can be examined and molecular diagnostics can be performed to determine the specific genetic cause. [25]

If the dihydrorhodamine test result is suggestive of chronic granulomatous disease, but the clinical picture is unusual, differential diagnoses should be considered. The most important of these is myeloperoxidase deficiency, which can produce an atypical dihydrorhodamine test result, as well as certain forms of severe glucose-6-phosphate dehydrogenase deficiency and rare acquired neutrophil dysfunction. [26]

Tests of this level must be performed on live neutrophils and under strict pre-analytical conditions. ARUP emphasizes that the dihydrorhodamine test requires fresh whole blood, room temperature for transport, and no refrigeration or freezing, as the test depends on the intact function of living cells. [27]

Thus, a pathological NST test today is not a definitive answer, but rather a reason to quickly move on to more reliable functional and genetic diagnostics. This is a crucial difference between the modern article and the old laboratory logic, where the NST test itself was sometimes perceived as almost a definitive diagnosis. [28]

Table 5. Modern algorithm after a pathological NST test

Stage	What to do
1	Assess the clinical context: infections, granulomas, family history
2	Perform a dihydrorhodamine test using flow cytometry
3	If the defect is confirmed, study the proteins of the nicotinamide adenine dinucleotide phosphate oxidase complex and genetics
4	Eliminate mimics, especially myeloperoxidase deficiency
5	Refer the patient to a clinical immunologist or hematologist with experience in the management of congenital phagocyte defects

Table based on GeneReviews, ARUP and 2024 review.[29]

Limitations of the spontaneous NST test and why it has fallen out of routine practice

The main methodological limitation of the NST test is its weak standardization. Even a classic 1975 review emphasized the numerous modifications of the method, leading to significant variability in results between laboratories. For the modern clinician, this means a simple thing: the same percentage of formazan-positive cells can be interpreted differently depending on the protocol. [30]

The second limitation is its semi-quantitative nature. The NBT test is read microscopically and essentially relies on manual assessment of the number of stained cells and the intensity of the reaction. Compared to flow cytometry, it is less accurate, less reproducible, and less suitable for detecting weak, partial, and mosaic defects. [31]

The third limitation is the low specificity of the spontaneous variant. It can be elevated in a number of inflammatory and non-infectious conditions and decreased in various scenarios of secondary immune suppression. Therefore, spontaneous NST does not provide a reliable answer to either the question "is there a bacterial infection" or the question "is there a primary immunodeficiency" when used alone. [32]

The fourth limitation is the difficulty in identifying carriers and partial forms of the disease. GeneReviews explicitly states that NST may be misinterpreted as normal in female carriers of the X-linked form of chronic granulomatous disease and in patients with hypomorphic variants, where residual nicotinamide adenine dinucleotide phosphate oxidase function is partially preserved. [33]

It is precisely these factors that have led to the spontaneous NBT test being viewed today primarily as a historically important but limited tool. It can be used in isolated laboratories and resource-limited settings, but as a modern routine standard for assessing neutrophil oxidative burst, it has given way to more accurate methods. [34]

Table 6. Main limitations of the spontaneous NST test

Limitation	Why is this important?
Poor standardization	It is difficult to compare results between laboratories
Semi-quantitative manual assessment	Lower accuracy and reproducibility
Low specificity	Increases and decreases occur in very different conditions.
Low sensitivity to partial defects	Mild forms of the disease may be missed
Limited evaluation of mosaicism in carriers	The method is worse for X-linked carriers
Displacement by more precise technologies	In most centers, the dihydrorhodamine test is preferred.

The table summarizes the reasons why the test is primarily of historical and auxiliary importance. [35]

Conclusion

The spontaneous nitroblue tetrazolium test is an important historical method for assessing neutrophil oxidative metabolism, which helped shape modern understanding of chronic granulomatous disease and other phagocyte dysfunction. However, its clinical role today is significantly narrower than often indicated by older laboratory manuals. [36]

The most accurate modern formulation is as follows: the spontaneous NBT test is not a reliable standalone test for either the diagnosis of bacterial infection or the definitive confirmation of a congenital defect of the oxidative burst. In modern practice, its results should be considered only as ancillary, and if chronic granulomatous disease is suspected, preference should be given to flow cytometry with dihydrorhodamine and subsequent molecular diagnostics. [37]

FAQ

What is the spontaneous NBT test in simple terms?
It's an old laboratory test that measures whether neutrophils are capable of reducing nitroblue tetrazolium to dark formazan without external stimulation. Essentially, it reflects the basic activity of the phagocyte's oxygen-dependent bactericidal mechanism.

How does a spontaneous test differ from an activated one?
The spontaneous variant evaluates background activity, while the activated one evaluates the reserve after artificial stimulation of the cell. For the diagnosis of congenital oxidative burst defects, stimulated methods are clinically more important, and in modern practice, the dihydrorhodamine test is even more important. [38]

Can a spontaneous NST test diagnose a bacterial infection?
No. Historical clinical studies have shown that the test is too nonspecific and produces many false-positive and false-negative results. It cannot be used as a standalone criterion for bacterial infection. [39]

Does an elevated spontaneous NST always indicate an infection?
No. Elevations are also possible in non-infectious inflammatory conditions, allergic and autoimmune processes, and with general neutrophil activation. Therefore, a high result alone does not determine the cause. [40]

Does a low spontaneous NST always indicate chronic granulomatous disease?
No. It can also occur in secondary immunodeficiency states, severe stress, myelosuppression, and other situations. A more accurate, modern algorithm is needed to confirm chronic granulomatous disease.

What test is currently considered the best for diagnosing chronic granulomatous disease?
The dihydrorhodamine flow cytometric test is considered the most preferred. It is more sensitive, quantitative, and better at detecting partial forms of the disease and carriers of the X-linked variant. [41]

Are there situations where the old NST test is still useful?
Yes. It can be used in resource-limited settings where flow cytometry is unavailable, and as an adjunct method in certain complex differential situations. However, it is no longer the modern standard for mass diagnostics. [42]

Why does the dihydrorhodamine test sometimes produce false-positive results in myeloperoxidase deficiency?
Because the dihydrorhodamine reaction depends on the cellular peroxidase system. In severe myeloperoxidase deficiency, the result can mimic chronic granulomatous disease, although the NST remains normal. [43]

Is genetic testing necessary after a pathological functional test?
Yes, if an oxidative burst defect is confirmed. After functional testing, molecular confirmation of the specific cause of the disease and clarification of the inheritance pattern are usually performed. [44]