The calculator in question is the Diagnostic Criteria for Common Variable Immunodeficiency (CVID). This calculator is used in the clinical setting to diagnose patients with suspected CVID, a disorder characterized by low levels of most or all of the immunoglobulin (Ig) classes, poor antibody response to vaccines or absent isohemagglutinins, and increased susceptibility to infections.

The patient population for this calculator includes individuals who present with recurrent infections, particularly of the respiratory and gastrointestinal systems, and those with a family history of immunodeficiency disorders. 

The clinical utility of this calculator is to aid in the diagnosis of CVID, which can then guide appropriate treatment strategies, such as immunoglobulin replacement therapy. Early diagnosis can also help prevent complications associated with CVID, such as chronic lung disease, gastrointestinal disease, and malignancies.

Exclusion criteria for the use of this calculator include patients with secondary causes of hypogammaglobulinemia, such as protein-losing enteropathy, nephrotic syndrome, and use of certain medications. Additionally, patients with other primary immunodeficiency diseases, such as X-linked agammaglobulinemia or selective IgA deficiency, should not be evaluated using the CVID diagnostic criteria.

Diagnostic criteria for common variable immunodeficiency

# Criteria
 A. Decreased serum immunoglobulin G and A at least 2 SD below the mean for age
B. Decreased serum immunoglobulin in any of the isotype (M, G and A) at least 2 SD below the mean for age
C. Onset of immunodeficiency at >= 2 years of age
D. Absent isohemagglutinins and/or poor response to vaccines
E. Exclusion of defined causes of hypogammaglobulinemia

# Conditions
- A and C and D and E
- B and C and D and E
- true

Diagnostic criteria for primary immunodeficiencies. Representing PAGID (Pan-American Group for Immunodeficiency) and ESID (European Society for Immunodeficiencies).