A primary immunodeficiency blood test panel is not one universal package. It is a staged evaluation chosen from the patient’s infections, immune complications, age, family history, medicines, and examination. The modern term inborn error of immunity reflects a broader reality: these disorders can cause recurrent infection, but they can also present with autoimmunity, uncontrolled inflammation, severe allergy, unusual viral disease, lymphoproliferation, or malignancy. Initial blood tests often include a complete blood count, quantitative immunoglobulins, vaccine antibody measurements, lymphocyte subsets, and complement screening. Results then direct specialized studies of B cells, T cells, natural killer cells, neutrophils, cytokine pathways, or genes. Normal screening tests do not exclude every disorder, and one abnormal number does not establish a genetic diagnosis. Timing during infection, age-specific reference ranges, recent vaccination, immunosuppressive treatment, protein loss, and specimen handling all affect interpretation. This article explains what the major test groups show, how result patterns narrow the differential diagnosis, and when urgent specialist evaluation is needed.
- There is no single blood panel that can rule in or rule out all inborn errors of immunity.
- Screening usually begins with cell counts, immunoglobulins, antibody function, and pathway-based complement tests.
- Age-specific ranges are essential, especially for lymphocyte counts, immunoglobulins, and vaccine responses in children.
- Acquired causes such as medicines, infection, protein loss, cancer, or malnutrition must be evaluated alongside genetic causes.
- Severe lymphopenia, profound neutropenia, absent immunoglobulins, or a strongly suggestive clinical pattern may require urgent immunology care.
Table of Contents
- Start with the clinical phenotype
- First-line cell counts and blood smear
- Immunoglobulin quantity and antibody quality
- Lymphocyte subsets and cellular function
- Complement and phagocyte testing
- Secondary causes that mimic primary disease
- When specialized and genetic tests are added
- Turning results into a diagnosis and plan
Start with the clinical phenotype
The most efficient immune evaluation begins before blood is drawn. The clinician defines what is abnormal about the illness pattern and then chooses tests that examine the relevant arm of immunity.
Recurrent bacterial ear, sinus, and lung infections suggest an antibody problem, impaired splenic function, complement deficiency, airway disease, or an anatomic cause. Deep abscesses, poor wound healing, invasive mold infection, or infection with catalase-positive organisms raises concern for neutrophil number or function. Persistent thrush, severe viral infection, Pneumocystis pneumonia, chronic diarrhea, or opportunistic disease can point toward T-cell or combined immunodeficiency.
Other presentations are not dominated by infection. Early or multiple autoimmune diseases, unexplained low blood counts, granulomas, enlarged lymph nodes or spleen, recurrent fever, severe eczema, inflammatory bowel disease, unusual allergy, and lymphoma can all be features of an inborn error of immunity. The 2024 International Union of Immunological Societies classifications include more than 550 disorders spanning immune deficiency and dysregulation.
Age of onset helps prioritize diagnoses but does not determine them. Severe combined immunodeficiency often presents in infancy, whereas common variable immunodeficiency may become apparent in adolescence or adulthood. Somatic variants and anti-cytokine autoantibodies can produce phenocopies later in life.
A detailed history records:
- infection sites, organisms, cultures, imaging, and antibiotic courses;
- hospitalization, intravenous therapy, sepsis, meningitis, or organ damage;
- growth, chronic diarrhea, oral thrush, skin disease, and dental history;
- vaccine reactions and whether routine vaccines were completed;
- autoimmune, inflammatory, allergic, and malignant conditions;
- consanguinity, infant deaths, and similarly affected relatives;
- travel, occupational exposure, smoking, and household contacts; and
- medicines, transfusions, immunoglobulin therapy, and transplant history.
Frequency alone can mislead. Young children in group care commonly have viral infections, while one episode of invasive meningococcal disease may justify complement testing. Recurrent pneumonia in the same lung segment may be structural rather than immunologic. The test plan should match severity, organism, site, and complications.
First-line cell counts and blood smear
A complete blood count with differential is inexpensive and can immediately identify important clues. The absolute values are more useful than percentages because a normal-looking percentage can hide a low cell number.
The absolute lymphocyte count is especially important in infants. Infants normally have more circulating lymphocytes than adults, so an adult reference range can miss dangerous lymphopenia. Persistently low lymphocytes in a baby with infection, diarrhea, thrush, poor growth, or an abnormal newborn severe combined immunodeficiency screen warrants urgent evaluation.
The absolute neutrophil count helps detect congenital, cyclic, autoimmune, medication-related, or infection-associated neutropenia. A repeat count may distinguish a persistent defect from a temporary viral suppression. When cyclic neutropenia is suspected, serial counts over several weeks may be needed because one normal result does not exclude the pattern.
Eosinophilia can occur with severe atopy, parasitic infection, drug reactions, and selected immune disorders. Monocytopenia is a clue in certain marrow and dendritic-cell syndromes. Thrombocytopenia with unusually small platelets can suggest Wiskott-Aldrich syndrome, while large platelets point toward other diagnoses. Anemia and multilineage cytopenias may reflect marrow failure, immune destruction, malignancy, infection, or systemic inflammation.
A peripheral blood smear adds morphology. It can confirm platelet size, identify abnormal or immature white cells, reveal Howell-Jolly bodies suggesting absent or impaired splenic function, and expose laboratory artifacts. Flow cytometry and genetic testing should not replace this basic review when the blood count is abnormal.
Results obtained during acute illness may change rapidly. Viral infections can lower lymphocytes or neutrophils, bacterial inflammation can raise neutrophils, and corticosteroids redistribute lymphocyte subsets. A clinically stable repeat is often appropriate, but severe abnormalities should not be postponed simply to obtain a “baseline.”
Basic metabolic and organ tests can reveal consequences or alternative causes. Albumin and total protein help identify protein loss or poor production. Liver and kidney tests matter because immunoglobulins and complement proteins are produced, distributed, and cleared through these systems. Inflammatory markers can provide context but do not diagnose an immune defect.
Laboratory logistics are part of diagnostic accuracy. Flow-cytometry and lymphocyte-function specimens may need to arrive while cells remain viable, sometimes within a narrow weekday shipping window. Complement-function samples require rapid processing and freezing, whereas serum immunoglobulins are comparatively stable. Before a rare abnormality is accepted, the ordering team should confirm that the right tube was used, the sample was not clotted or delayed, and the laboratory’s age-specific interval was applied. Results should also be compared with earlier complete blood counts and immunoglobulin measurements. A stable lifelong pattern carries different implications from an abrupt change after medication, infection, weight loss, or kidney disease. Repeating a mild abnormality is useful when clinically safe, but repetition should not delay treatment of an infant or severely ill patient whose phenotype suggests a time-critical disorder.
Immunoglobulin quantity and antibody quality
Quantitative IgG, IgA, and IgM are core screening tests for recurrent bacterial respiratory infection. IgE is added when severe allergy, eczema, parasitic disease, or a hyper-IgE phenotype is suspected. Results must be compared with age-specific laboratory ranges.
A profoundly low IgG with low IgA and IgM suggests major antibody-production failure, especially when B cells are absent or very low. A modestly low IgG has a broader differential that includes common variable immunodeficiency, transient developmental patterns, medication effects, lymphoid malignancy, nephrotic loss, intestinal protein loss, and laboratory variation.
Selective IgA deficiency requires an appropriately low or absent IgA with otherwise preserved major immunoglobulins in a person old enough for the diagnosis. An isolated low IgM or IgG subclass is less straightforward and should be confirmed. A high immunoglobulin value can reflect infection, autoimmunity, liver disease, chronic inflammation, or a monoclonal protein rather than effective immunity.
Quantity does not equal function. A person can have normal total immunoglobulins but fail to make useful antibodies to selected antigens. Functional humoral testing therefore includes antibody concentrations to previous vaccines.
Protein-antigen responses may be assessed with tetanus or diphtheria antibodies. Polysaccharide responses are often evaluated with serotype-specific pneumococcal IgG before and after an appropriately selected vaccine challenge. Interpretation depends on age, baseline titers, vaccine type, timing, laboratory method, and the proportion of serotypes responding.
A common staged approach is:
- Measure quantitative immunoglobulins and available baseline vaccine antibodies.
- Confirm that routine vaccination history and dates are reliable.
- If needed, give a clinician-selected vaccine challenge.
- Repeat antibody testing at the planned interval, often about four to eight weeks.
- Assess absolute concentrations, fold change, and response breadth together.
Specific antibody deficiency describes impaired polysaccharide responses in a person whose major immunoglobulin concentrations are usually normal. The diagnosis also requires a compatible infection history. Low baseline pneumococcal titers alone are not enough because antibodies naturally wane.
Serum protein electrophoresis and immunofixation may be added when low immunoglobulins, unexplained high globulins, kidney disease, neuropathy, anemia, or a monoclonal process is possible. These tests answer a different question from quantitative immunoglobulins.
Lymphocyte subsets and cellular function
Flow cytometry counts major lymphocyte populations. A standard panel commonly includes total T cells marked by CD3, helper T cells marked by CD4, cytotoxic T cells marked by CD8, B cells marked by CD19 or CD20, and natural killer cells marked by CD16 and CD56.
The pattern can rapidly narrow severe combined immunodeficiency. For example, some genetic defects produce T-cell-negative, B-cell-positive, NK-cell-negative disease, while others produce different combinations. The subset pattern guides urgent treatment and molecular testing, but it does not identify every gene by itself.
Absolute subset counts should be used, not percentages alone. Pediatric reference intervals change sharply with age. A low CD4 count in an adult requires evaluation for HIV and acquired causes as well as inherited disease. A normal number of T cells does not prove normal T-cell function.
B-cell presence also needs context. People with X-linked agammaglobulinemia typically have very few mature B cells, whereas many patients with common variable immunodeficiency have B cells but impaired maturation and antibody production. Expanded B-cell phenotyping can measure class-switched memory B cells, transitional B cells, plasmablasts, and CD21-low populations.
T-cell functional testing measures proliferation after stimulation with mitogens, antigens, or receptor-directed agents. It can reveal profound functional impairment despite a measurable T-cell count. Results are vulnerable to shipping delay, temperature, recent transfusion, immunosuppressive treatment, and severe illness, so specialized laboratory instructions are critical.
Additional assays may assess T-cell receptor diversity, naïve and memory populations, recent thymic emigrants, cytotoxic granule release, perforin expression, or cytokine signaling. These are selected from the phenotype. For example, recurrent hemophagocytic lymphohistiocytosis or severe Epstein-Barr virus disease prompts a different panel from chronic mucocutaneous candidiasis.
Natural killer cell counts are not the same as natural killer function. Functional cytotoxicity or degranulation studies are used when the history suggests an NK-cell or cytotoxic-lymphocyte disorder. A mildly low NK percentage on one panel is rarely diagnostic.
Newborn screening for severe combined immunodeficiency measures T-cell receptor excision circles from a dried blood spot. A positive screen requires urgent confirmatory testing; a normal historical newborn screen does not exclude every later-onset cellular disorder.
Complement and phagocyte testing
Complement evaluation is pathway based. CH50 measures the functional integrity of the classical and terminal pathways. AH50 measures the alternative and terminal pathways. A lectin-pathway functional assay may be added when mannose-binding lectin or a related defect is suspected.
Interpretive patterns include:
| CH50 | AH50 | Possible direction |
|---|---|---|
| Very low | Normal | Early classical component defect, such as C1, C2, or C4, or classical-pathway consumption |
| Normal | Very low | Alternative pathway defect involving factors such as B, D, or properdin |
| Very low | Very low | C3 or terminal component defect, broad consumption, or a degraded specimen |
| Normal | Normal | Major defects in tested pathways are less likely; lectin-pathway or regulatory disorders may remain |
C3 and C4 concentrations help distinguish inherited absence from inflammatory consumption, although normal quantities do not prove normal function. Specific component assays, complement activation markers, properdin testing, or genetic analysis follow an abnormal screen.
Specimen handling is crucial. Complement proteins lose activity if serum is not separated, frozen, and transported correctly. A globally low functional result in a patient without a matching phenotype should prompt review of collection conditions and often repeat testing.
Phagocyte evaluation begins with neutrophil count and infection pattern. The dihydrorhodamine oxidative burst assay measures the ability of neutrophils to generate reactive oxygen species and is the preferred screening test for chronic granulomatous disease. It can also identify carrier patterns in some X-linked families.
Leukocyte adhesion deficiency is investigated with flow cytometric expression of adhesion molecules such as CD18 and associated markers. Very high neutrophil counts, delayed separation of the umbilical cord, poor pus formation, and severe bacterial infections provide clinical clues.
Other specialized tests examine chemotaxis, microbial killing, toll-like receptor signaling, interferon pathways, or anti-cytokine autoantibodies. These tests should be targeted because normal results in broad screening do not exclude highly specific innate immune defects.
Secondary causes that mimic primary disease
Before labeling a result as primary or inherited, clinicians investigate acquired explanations. Some are more common than inborn errors and can produce nearly identical laboratory patterns.
Medicines are a major category. B-cell-depleting antibodies can cause prolonged low B cells and poor vaccine responses. Corticosteroids, antimetabolites, calcineurin inhibitors, chemotherapy, anticonvulsants, and other agents can alter immunoglobulins, lymphocyte counts, or function. The dose, duration, and timing relative to blood collection matter.
Protein loss lowers immunoglobulins without necessarily impairing production. Nephrotic syndrome causes urinary loss, while protein-losing enteropathy causes gastrointestinal loss. Low albumin, edema, urine protein, stool alpha-1 antitrypsin clearance, and the clinical context help identify these conditions.
Hematologic malignancies and marrow disorders may produce hypogammaglobulinemia, abnormal lymphocyte populations, neutropenia, or monoclonal proteins. HIV and other infections can cause lymphocyte abnormalities. Severe malnutrition, major burns, critical illness, and organ failure can broadly suppress host defense.
Anatomic and mechanical problems frequently imitate antibody deficiency. Allergic rhinitis can obstruct sinus drainage. Aspiration, reflux, bronchiectasis, cystic fibrosis, primary ciliary dyskinesia, airway foreign body, or congenital lung lesions can cause repeated respiratory infection. The same organism recurring in one site is an important clue.
Prematurity and normal immune maturation affect infants. Immunoglobulin levels fall after maternal IgG wanes, and vaccine responses develop over time. Interpretation must use gestational and chronological age as well as clinical severity.
A secondary cause does not make the immune problem unimportant. Patients may still need infection prevention and treatment. It changes the diagnosis, prognosis, family counseling, and choice of therapy. Some people also have both an inherited susceptibility and an acquired stressor.
When specialized and genetic tests are added
Specialized tests are chosen when screening results or clinical features point toward a pathway. Ordering every available assay at once increases cost, false positives, and uncertain findings.
Flow cytometry can assess proteins missing in specific disorders, such as BTK, WAS protein, CD40 ligand, or selected adhesion molecules. Cytokine-stimulation assays test signaling through interferon, interleukin, or toll-like receptor pathways. Enzyme assays can identify adenosine deaminase deficiency or purine nucleoside phosphorylase deficiency. Chromosomal studies may be needed when a syndromic deletion is suspected.
Autoantibody testing can identify acquired phenocopies. Neutralizing antibodies against interferon-gamma, type I interferons, interleukin-17, granulocyte-macrophage colony-stimulating factor, or other cytokines can create specific infection or inflammatory patterns without a germline mutation.
Genetic options include a focused single-gene test, a multigene inborn-error panel, exome sequencing, genome sequencing, copy-number analysis, and testing for somatic variants. The best choice depends on how specific the phenotype is, whether rapid results are needed, and whether the laboratory can detect deletions, duplications, mosaicism, and difficult genomic regions.
A pathogenic or likely pathogenic variant that fits the phenotype can confirm a diagnosis, guide treatment, identify transplant considerations, and permit family testing. A negative panel does not exclude an inborn error because not every disease gene or variant mechanism is known or detectable.
Variants of uncertain significance are common. They are not diagnoses and should not drive high-risk treatment without supporting functional, segregation, and clinical evidence. Reanalysis may be useful as gene-disease knowledge evolves.
Genetic counseling addresses inheritance, reproductive options, testing of relatives, incidental findings, and emotional implications. In urgent infant cases, testing and treatment planning often proceed in parallel rather than waiting sequentially.
Turning results into a diagnosis and plan
The final interpretation integrates phenotype, screening results, specialized function, and genetics. No single column of numbers should be read in isolation.
A useful report answers several questions:
- Which immune compartment appears abnormal?
- Is the abnormality persistent and age inappropriate?
- Does it explain the infections or immune dysregulation?
- Have common acquired causes been excluded?
- Is functional impairment demonstrated?
- Is a molecular diagnosis confirmed, suspected, or still unresolved?
Some patients meet criteria for a named disorder without an identified gene. Others have a genetic variant before the full phenotype emerges. Diagnostic labels can change as children mature, treatment changes results, or new evidence becomes available.
Urgent referral is appropriate for an infant with profound lymphopenia, persistent thrush and failure to thrive, opportunistic infection, absent T-cell function, or an abnormal severe combined immunodeficiency newborn screen. Profound neutropenia with fever, invasive fungal disease, recurrent meningococcal infection, or suspected hemophagocytic lymphohistiocytosis also requires prompt specialist care.
Management may include vaccination planning, antimicrobial prophylaxis, rapid treatment of infection, immunoglobulin replacement, cytokine or pathway-directed therapy, immunosuppression for dysregulation, hematopoietic stem-cell transplantation, or gene therapy. The correct treatment depends on the precise defect; interventions that help one disorder can harm another.
Patients with less severe findings may be followed over time. Repeat immunoglobulins, infection logs, vaccine responses, lung assessment, and age-related reevaluation can show whether an abnormality resolves, remains stable, or evolves.
The term “panel” is most useful when it describes an organized first step, not a promise of certainty. A normal first-line panel lowers the likelihood of common severe defects but does not erase a compelling clinical phenotype. Conversely, a mildly abnormal result in a healthy person may require confirmation rather than a lifelong diagnosis.
Patients can help by keeping a concise infection record with dates, sites, culture results, imaging, antibiotics, hospital care, and missed work or school. Bringing vaccine records and a current medication list prevents avoidable repeat testing. Because immune findings can evolve, the plan should state which results will be repeated, what symptoms should trigger earlier review, and whether relatives need screening. This turns a complex laboratory evaluation into a traceable clinical decision rather than a collection of disconnected abnormalities.
References
- Diagnostic tests for primary immunodeficiency disorders: Classic and genetic testing. 2024. Review.
- The 2024 update of IUIS phenotypic classification of human inborn errors of immunity. 2025.
- Human inborn errors of immunity: 2024 update on the classification from the International Union of Immunological Societies Expert Committee. 2025.
- Laboratory tests. Immune Deficiency Foundation. Accessed 2026.
- Inborn errors of immunity (primary immunodeficiencies). 2024. Review.
- 2025 Inborn errors of immunity practice parameter. 2026.
Disclaimer
This article is for general educational use and does not diagnose an inborn error of immunity or replace evaluation by a qualified clinician. Test selection, age-specific ranges, specimen requirements, and diagnostic criteria vary, and abnormal results often require confirmation and specialist interpretation. Seek urgent care for severe infection, breathing difficulty, meningitis symptoms, fever with profound neutropenia, or a seriously ill infant.
