When suspected, the diagnosis can be confirmed by laboratory measurement of IgA level in the blood. SigAD is an IgA level < 7 mg/dL with normal IgG and IgM levels (reference range 70–400 mg/dl for adults; children somewhat less).

Diagnosis of X-SCID is possible through lymphocyte cell counts, lymphocyte function tests, and genetic testing. A healthy immune system should contain large amounts of lymphocytes, but individuals with X-SCID will contain unusually small amounts of T-cells, non-functional B-cells, and some natural killer cells.

Individuals with X-SCID often have decreased lymphocyte function. This can be tested through the introduction of agents to the immune system; the reaction of the lymphocytes is then observed. In X-SCID, Antibody responses to introduced vaccines and infections are absent, and T-cell responses to mitogens, substances that stimulate lymphocyte transformation, are deficient. IgA and IgM immunoglobulins, substances that aid in fighting off infections, are very low.

The absence of a thymic shadow on chest X-rays is also indicative of X-SCID. In a normal child, a distinctive sailboat shaped shadow near the heart can be seen. The thymus gland in normal patients will gradually decrease in size because the need for the thymus gland diminishes. The decrease in the size of the thymus gland occurs because the body already has a sufficient number of developed T-cells. However, a patient with X-SCID will be born with an abnormally small thymus gland at birth. This indicates that the function of thymus gland, of forming developed T-cells, has been impaired.

Since the mutation in X-SCID is X-linked, there are genetic tests for detecting carriers in X-SCID pedigrees. One method is to look for family-specific IL2RG mutations. Finally, if none of those options are available, there is an unusual pattern of nonrandom X-chromosome inactivation on lymphocytes in carriers, thus looking for such inactivation would prove useful.

If a mother is pregnant and the family has a known history of immunodeficiency, then doctors may perform diagnostic assessment in-utero. Chorionic Villus Sampling, which involves sampling of the placental tissue using a catheter inserted through the cervix, can be performed 8 to 10 weeks into gestation. Alternatively, Amniocentesis, which entails extracting a sample of the fluid which surrounds the fetus, can be performed 15 to 20 weeks into gestation.

Early detection of X-SCID (and other types of SCID) is also made possible through detection of T-cell recombination excision circles, or TRECs. TRECs are composed of excised DNA fragments which are generated during normal splicing of T-cell surface antigen receptors and T-cell maturation. This maturation process is absent across all SCID variants, as evidenced by the low counts of T-lymphocytes. The assay is performed using dried blood from a Guthrie card, from which DNA is extracted. Quantitative PCR is then performed and the number of TRECs determined. Individuals who have the SCID phenotype will have TREC counts as low as <30, compared to approximately 1020 for a healthy infant. A low TREC count indicates that there is insufficient development of T-cells in the thymus gland. This technique can predict SCID even when lymphocyte counts are within the normal range. Newborn screening of X-SCID based on TREC count in dried blood samples has recently been introduced in several states in the United States including California, Colorado, Connecticut, Delaware, Florida, Massachusetts, Michigan, Minnesota, Mississippi, New York, Texas, and Wisconsin. In addition, pilot trials are being performed in several other states beginning in 2013.

XLA diagnosis usually begins due to a history of recurrent infections, mostly in the respiratory tract, through childhood. This is due to humoral immunodeficiency. The diagnosis is probable when blood tests show the complete lack of circulating B cells (determined by the B cell marker CD19 and/or CD20), as well as low levels of all antibody classes, including IgG, IgA, IgM, IgE and IgD.

When XLA is suspected, it is possible to do a Western Blot test to determine whether the Btk protein is being expressed. Results of a genetic blood test confirm the diagnosis and will identify the specific Btk mutation, however its cost prohibits its use in routine screening for all pregnancies. Women with an XLA patient in their family should seek genetic counseling before pregnancy.Although the symptoms of a XLA and other primary immune diseases (PID) include repeated and often severe infections, the average time for a diagnosis of a PID can be up to 10 years.

Early diagnosis of Severe Combined Immunodeficiency is rare because doctors do not routinely count each type of white blood cell in newborns.

Health professionals must look at a person's history, symptoms, physical exam and laboratory test in order to make a diagnosis. If the results show patients with low levels of lymphocytes, absence of granulocytes or absence of thymus then the patient may be suspected to have RD.

About half of US states are performing screening for SCID in newborns using real-time quantitative PCR to measure the concentration of T-cell receptor excision circles. Wisconsin and Massachusetts (as of February 1, 2009) screen newborns for SCID. Michigan began screening for SCID in October 2011. Some SCID can be detected by sequencing fetal DNA if a known history of the disease exists. Otherwise, SCID is not diagnosed until about six months of age, usually indicated by recurrent infections. The delay in detection is because newborns carry their mother's antibodies for the first few weeks of life and SCID babies look normal.

The basic tests performed when an immunodeficiency is suspected should include a full blood count (including accurate lymphocyte and granulocyte counts) and immunoglobulin levels (the three most important types of antibodies: IgG, IgA and IgM).

Other tests are performed depending on the suspected disorder:

- Quantification of the different types of mononuclear cells in the blood (i.e. lymphocytes and monocytes): different groups of T lymphocytes (dependent on their cell surface markers, e.g. CD4+, CD8+, CD3+, TCRαβ and TCRγδ), groups of B lymphocytes (CD19, CD20, CD21 and Immunoglobulin), natural killer cells and monocytes (CD15+), as well as activation markers (HLA-DR, CD25, CD80 (B cells).

- Tests for T cell function: skin tests for delayed-type hypersensitivity, cell responses to mitogens and allogeneic cells, cytokine production by cells

- Tests for B cell function: antibodies to routine immunisations and commonly acquired infections, quantification of IgG subclasses

- Tests for phagocyte function: reduction of nitro blue tetrazolium chloride, assays of chemotaxis, bactericidal activity.

Due to the rarity of many primary immunodeficiencies, many of the above tests are highly specialised and tend to be performed in research laboratories.

Criteria for diagnosis were agreed in 1999. For instance, an antibody deficiency can be diagnosed in the presence of low immunoglobulins, recurrent infections and failure of the development of antibodies on exposure to antigens. The 1999 criteria also distinguish between "definitive", "probable" and "possible" in the diagnosis of primary immunodeficiency. "Definitive" diagnosis is made when it is likely that in 20 years, the patient has a >98% chance of the same diagnosis being made; this level of diagnosis is achievable with the detection of a genetic mutation or very specific circumstantial abnormalities. "Probable" diagnosis is made when no genetic diagnosis can be made, but the patient has all other characteristics of a particular disease; the chance of the same diagnosis being made 20 years later is estimated to be 85-97%. Finally, a "possible" diagnosis is made when the patient has only some of the characteristics of a disease are present, but not all.

The diagnosis is based on clinical features, with a concomitant decreased blood adenosine deaminase level supporting the diagnosis.

Serology (detection on antibodies to a specific pathogen or antigen) is often used to diagnose viral diseases. Because XLA patients lack antibodies, these tests always give a negative result regardless of their real condition. This applies to standard HIV tests. Special blood tests (such as the western blot based test) are required for proper viral diagnosis in XLA patients.

It is not recommended and dangerous for XLA patients to receive live attenuated vaccines such as live polio, or the measles, mumps, rubella (MMR vaccine). Special emphasis is given to avoiding the oral live attenuated SABIN-type polio vaccine that has been reported to cause polio to XLA patients. Furthermore, it is not known if active vaccines in general have any beneficial effect on XLA patients as they lack normal ability to maintain immune memory.

XLA patients are specifically susceptible to viruses of the Enterovirus family, and mostly to: polio virus, coxsackie virus (hand, foot, and mouth disease) and Echoviruses. These may cause severe central nervous system conditions as chronic encephalitis, meningitis and death. An experimental anti-viral agent, pleconaril, is active against picornaviruses. XLA patients, however, are apparently immune to the Epstein-Barr virus (EBV), as they lack mature B cells (and so HLA co-receptors) needed for the viral infection. Patients with XLA are also more likely to have a history of septic arthritis.

It is not known if XLA patients are able to generate an allergic reaction, as they lack functional IgE antibodies.There is no special hazard for XLA patients in dealing with pets or outdoor activities. Unlike in other primary immunodeficiencies XLA patients are at no greater risk for developing autoimmune illnesses.

Agammaglobulinemia (XLA) is similar to the primary immunodeficiency disorder Hypogammaglobulinemia (CVID), and their clinical conditions and treatment are almost identical. However, while XLA is a congenital disorder, with known genetic causes, CVID may occur in adulthood and its causes are not yet understood.

XLA was also historically mistaken as Severe Combined Immunodeficiency (SCID), a much more severe immune deficiency ("Bubble boys").A strain of laboratory mouse, XID, is used to study XLA. These mice have a mutated version of the mouse Btk gene, and exhibit a similar, yet milder, immune deficiency as in XLA.

The diagnosis of T cell deficiency can be ascertained in those individuals with this condition via the following:

- Delayed hypersensitivity skin test

- T cell count

- Detection via culture(infection)

X-linked SCID is a known pediatric emergency which primarily affects males. If the appropriate treatment such as intravenous immunoglobulin supplements, medications for treating infections or a bone marrow transplant is not administered, then the prognosis is poor. The patients with X-linked SCID usually die two years after they are born. For this reason, the diagnosis of X-linked SCID needs to be done early to prevent any pathogens from infecting the infant.

However, the patients have a higher chance of survival if the diagnosis of X-linked SCID is done as soon as the baby is born. This involves taking preventative measures to avoid any infections that can cause death. For example, David Vetter had a high chance of having X-linked SCID because his elder sibling had died due to SCID. This allowed the doctors to place David in the bubble and prevented infections. In addition, if X-linked SCID is known to affect a child, then live vaccines should not be administered and this can save the infants life. Vaccines, which are pathogens inserted into the body to create an immune response, can lead to death in infants with X-linked SCID. Moreover, with proper treatments, such as a bone marrow transplant, the prognosis is good. The bone marrow transplant has been successful in treating several patients and resulted in a full immune reconstitution and the patient can live a healthy life. The results of bone marrow transplant are most successful when the closest human leukocyte antigen match has been found. If a close match is not found, however, there is a chance of graft-versus-host-disease which means the donor bone marrow attacks the patient's body. Hence, a close match is required to prevent any complications.

Complete or partial deficiency

- "Complete insufficiency" of T cell function can result from hereditary conditions (also called primary conditions) such as severe combined immunodeficiency (SCID), Omenn syndrome, and cartilage–hair hypoplasia.

- "Partial insufficiencies" of T cell function include acquired immune deficiency syndrome (AIDS), and hereditary conditions such as DiGeorge syndrome (DGS), chromosomal breakage syndromes (CBSs), and B-cell and T-cell combined disorders such as ataxia-telangiectasia (AT) and Wiskott–Aldrich syndrome (WAS).

- "Primary (or hereditary) immunodeficiencies" of T cells include some that cause complete insufficiency of T cells, such as severe combined immunodeficiency (SCID), Omenn syndrome, and Cartilage–hair hypoplasia.

- "Secondary causes" are more common than primary ones. Secondary (or acquired) causes are mainly:

In order to ascertain if an individual has activated PI3K delta syndrome, usually one finds atypical levels of immunoglobulins. Methods to determine the condition are the following:

- Genetic testing

- Laboratory findings

- Symptoms exhibited

The following types of CVID have been identified, and correspond to mutations in different gene segments.

According to a European registry study, the mean age at onset of symptoms was 26.3 years old. As per the criteria laid out by ESID (European Society for Immunodeficiencies) and PAGID (Pan-American Group for Immunodeficiency), CVID is diagnosed if:

- the person presents with a marked decrease of serum IgG levels (<4.5 g/L) and a marked decrease below the lower limit of normal for age in at least one of the isotypes IgM or IgA;

- the person is four years of age or older;

- the person lacks antibody immune response to protein antigens or immunization.

Diagnosis is chiefly by exclusion, i.e. alternative causes of hypogammaglobulinemia, such as X-linked agammaglobulinemia, must be excluded before a diagnosis of CVID can be made.

Diagnosis is difficult because of the diversity of phenotypes seen in people with CVID. For example, serum immunoglobulin levels in people with CVID vary greatly. Generally, people can be grouped as follows: no immunoglobulin production, immunoglobulin (Ig) M production only, or both normal IgM and IgG production. Additionally, B cell numbers are also highly variable. 12% of people have no detectable B cells, 12% have reduced B cells, and 54% are within the normal range. In general, people with CVID display higher frequencies of naive B cells and lower frequencies of class-switched memory B cells. Frequencies of other B cell populations, such as IgD memory B cells, transitional B cells, and CD21 B cells, are also affected, and are associated with specific disease features. Although CVID is often thought of as a serum immunoglobulin and B cell-mediated disease, T cells can display abnormal behavior. Affected individuals typically present with low frequencies of CD4, a T-cell marker, and decreased circulation of regulatory T cells and iNKT cell. Notably, approximately 10% of people display CD4 T cell counts lower than 200 cells/mm; this particular phenotype of CVID has been named LOCID (Late Onset Combined Immunodeficiency), and has a poorer prognosis than classical CVID.

Prognosis is excellent, although there is an association with autoimmune disease. Of note, selective IgA deficiency can complicate the diagnosis of one such condition, celiac disease, as the deficiency masks the high levels of certain IgA antibodies usually seen in celiac disease.

As opposed to the related condition CVID, selective IgA deficiency is not associated with an increased risk of cancer.

Patients with Selective IgA deficiency are at risk of anaphylaxis from blood transfusions. These patients should receive IgA free containing blood products and ideally blood from IgA-deficient donors.

In terms of diagnosis of "humoral immune deficiency" depends upon the following:

- Measure "serum immunoglobulin levels"

- B cell count

- Family medical history

The diagnosis of Nezelof syndrome will indicate a deficiency of T-cells, additionally in ascertaining the condition the following is done:

Gene therapy is a relatively new concept in the field of SCID. This therapy is currently undergoing clinical trial and has cured a small number of children suffering from X-linked SCID and recessive allele SCID. Gene therapy aims to correct the underlying genetic abnormality in SCID. In the case of RD, the genetic abnormality would be AK2 malfunction. Stem cells are taken from an affected child's blood or bone marrow. Then in laboratory conditions the stem cells are manipulated and corrected with gene technology. They are then injected back into the patient. Similarly, in bone transplant, stem cells are able to find their way back through tracking mechanisms.

Bone marrow transplant may be possible for Severe Combined Immune Deficiency and other severe immunodeficiences.

Virus-specific T-Lymphocytes (VST) therapy is used for patients who have received hematopoietic stem cell transplantation that has proven to be unsuccessful. It is a treatment that has been effective in preventing and treating viral infections after HSCT. VST therapy uses active donor T-cells that are isolated from alloreactive T-cells which have proven immunity against one or more viruses. Such donor T-cells often cause acute graft-versus-host disease (GVHD), a subject of ongoing investigation. VSTs have been produced primarily by ex-vivo cultures and by the expansion of T-lymphocytes after stimulation with viral antigens. This is carried out by using donor-derived antigen-presenting cells. These new methods have reduced culture time to 10–12 days by using specific cytokines from adult donors or virus-naive cord blood. This treatment is far quicker and with a substantially higher success rate than the 3–6 months it takes to carry out HSCT on a patient diagnosed with a primary immunodeficiency. T-lymphocyte therapies are still in the experimental stage; few are even in clinical trials, none have been FDA approved, and availability in clinical practice may be years or even a decade or more away.

The differential diagnosis for this condition consists of acquired immune deficiency syndrome and severe combined immunodeficiency syndrome

Treatment for "B cell deficiency"(humoral immune deficiency) depends on the cause, however generally the following applies:

- Treatment of infection(antibiotics)

- Surveillance for malignancies

- Immunoglobulin replacement therapy

Although MPO deficiency classically presents with immune deficiency (especially candida albicans infections), the majority of individuals with MPO deficiency show no signs of immunodeficiency.

The lack of severe symptoms suggest that role of myeloperoxidase in the immune response must be redundant to other mechanisms of intracellular killing of phagocytosed bacteria.

Patients with MPO deficiency have a respiratory burst with a normal nitro blue tetrazolium (NBT) test because they still have NADPH oxidase activity, but do not form bleach due to their lack of myeloperoxidase activity. This is in contrast to chronic granulomatous disease, in which the NBT test is 'negative' due to the lack of NADPH oxidase activity (positive test result means neutrophils turn blue, negative means nitroblue tetrazolium remains yellow).

Patients with MPO deficiency are at increased risk for systemic candidiasis.

Once a diagnosis is made, each individual's treatment is based on an individual’s clinical condition. Hematopoietic stem cell transplant is a possible treatment of this condition but its effectiveness is unproven.

Additionally, magnesium supplementation is a promising potential treatment for XMEN. One of the consequences of loss of "MAGT1" function is a decreased level of unbound intracellular Mg2+. This decrease leads to loss of expression of an immune cell receptor called "NKG2D", which is involved in EBV-immunity. Remarkably, Mg2+ supplementation can restore "NKG2D" expression and other functions that are abnormal in patients with XMEN. Early evidence suggests continuous oral magnesium threonate supplementation is safe and well tolerated. Nonetheless, further research is needed to evaluate the use of Mg2+ as a treatment for XMEN. It remains unclear if such supplementation will protect against the development of lymphoma in patients with XMEN. Investigators at the National Institute of Allergy and Infectious Diseases at the US National Institutes of Health currently have clinical protocols to study new approaches to the diagnosis and treatment of this disorder.

On September 1990, the first gene therapy to combat this disease was performed by Dr. William French Anderson on a four-year-old girl, Ashanti DeSilva, at the National Institutes of Health, Bethesda, Maryland, U.S.A.

In April 2016 the Committee for Medicinal Products for Human Use of the European Medicines Agency endorsed and recommended for approval a stem cell gene therapy called Strimvelis, for children with ADA-SCID for whom no matching bone marrow donor is available.