Laboratory Features of PID

Initial Screening And Level 1 Testing

Basic laboratory screening must begin with a complete blood count with differential analysis.
This initial test is essential to identify baseline hematologic abnormalities.
Clinicians must look for severe neutropenia, persistent lymphopenia, or marked leukocytosis.
A peripheral blood smear should be reviewed for distinct morphological clues.
The presence of giant lysosomal granules is characteristic of Chédiak-Higashi syndrome.
Howell-Jolly bodies are indicative of isolated congenital asplenia.
Newborn screening programs facilitate the early detection of Severe Combined Immunodeficiency.
These programs utilize a quantitative polymerase chain reaction assay.
The assay measures T-cell receptor excision circles.
T-cell receptor excision circles serve as an accurate marker for quantifying recent thymic emigrants.
Many screening programs concurrently assay kappa excision circles.
This allows for the simultaneous identification of infants with severe B-cell defects.

Initial humoral evaluation requires the quantitative measurement of serum immunoglobulin levels.
Levels of IgG, IgA, IgM, and IgE must be compared to age-matched normal values.
Evaluating specific antibody titers in response to previous routine vaccinations is critical.
This assesses functional humoral immunity against protein and polysaccharide antigens.
Flow cytometry is mandatory for identifying and enumerating circulating B-cell populations.
Specific surface markers such as CD19 and CD20 are utilized.
Advanced flow cytometric analysis enumerates the specific percentages of naive, memory, and class-switched memory B-cells.

Immunodeficiency	Characteristic Laboratory Findings
Congenital Agammaglobulinemia	Complete absence of CD19+ B-cells in the peripheral blood.
Hyper-IgM Syndrome	Very low IgG, IgA, and IgE alongside normal or elevated IgM.
Selective IgA Deficiency	Isolated profound decrease in IgA with normal IgG and IgM.
Wiskott-Aldrich Syndrome	Low IgM, elevated IgA and IgE, and normal or slightly decreased IgG.

Flow cytometry is used to precisely quantify total T-lymphocytes and their subsets.
Markers such as CD3, CD4, and CD8 are utilized.
The ratio of CD4+ to CD8+ T cells is analyzed to indicate underlying immune dysregulation.
Flow cytometry differentiates naive T cells from memory T cells by evaluating specific CD45 isoforms.
Functional T-cell competence is evaluated in vitro via lymphocyte blast transformation assays.
Peripheral blood mononuclear cells are stimulated with specific mitogens or recall antigens.
A proliferative response to phytohemagglutinin that is less than 10 percent of a normal control confirms Severe Combined Immunodeficiency.
Natural killer cells are evaluated by enumerating their absolute count using CD16 and CD56 markers.

Patients presenting with deep tissue abscesses require functional evaluation of the neutrophil respiratory burst.
The dihydrorhodamine reduction assay is the gold standard test.
It measures oxidant production through increased fluorescence when oxidized by intracellular hydrogen peroxide.
In cases of severe myeloperoxidase deficiency, the assay for neutrophils may yield a false-positive result mimicking Chronic Granulomatous Disease.
Evaluating the patient's eosinophils reliably differentiates these two conditions.

Defect Type	Confirmatory Flow Cytometry Finding
Leukocyte Adhesion Deficiency Type 1	Absence or severe reduction of CD11b and CD18 on neutrophils.
Leukocyte Adhesion Deficiency Type 2	Absence of the sialyl Lewis X antigen on neutrophils.

The functional integrity of the complement system is initially assessed using global hemolytic assays.
The CH50 assay evaluates the total functional activity of the classical complement pathway.
The AH50 assay specifically screens the functional activity of the alternative complement pathway.
Specific immunochemical tests are utilized to definitively quantify single complement components if initial screening reveals an abnormality.
Complement activation products are quantified to distinguish between a primary genetic deficiency and active acquired consumption.
The evaluation of Paroxysmal Nocturnal Hemoglobinuria relies on flow cytometry to detect abnormally reduced surface levels of CD55 and CD59.

Establishing the precise genetic etiology through mutation analysis is the most definitive diagnostic method.
Targeted gene sequencing is routinely employed following abnormal functional laboratory results.
Genetic diagnosis is strictly necessary in conditions categorized as primary immune regulatory disorders.
Definitive identification of specific pathogenic variants dictates the feasibility of definitive cellular therapies.
It also provides guidance for targeted biologic agents and is essential for accurate prenatal genetic counseling.