Quantitative Immunoglobulins

Immunoglobulins are glycoprotein antibodies made by plasma cells. IgG is the dominant class and crosses the placenta, IgA protects mucosal surfaces, IgM is the first antibody made in an immune response, IgE mediates allergy and parasite defence, and IgD has a minor role. Serum IgG, IgA and IgM are measured by immunoturbidimetry or nephelometry in g/L, IgE by immunoassay in kU/L (1 kU/L is 2.4 ng/mL), and IgG subclasses (IgG1 to IgG4) by immunoassay.

The test quantifies humoral immunity. It is used to diagnose primary and secondary antibody deficiency and to decide on immunoglobulin replacement, to detect polyclonal rises in chronic infection, liver and autoimmune disease, to characterise paraproteins, and to support the diagnosis of IgG4-related disease and allergic bronchopulmonary aspergillosis.

Primary antibody deficiencies are the commonest primary immunodeficiencies. Selective IgA deficiency affects about 1 in 300 to 700 people of European descent and is usually asymptomatic. Common variable immunodeficiency (CVID) affects about 1 in 25 000 to 50 000. Secondary hypogammaglobulinaemia from drugs and malignancy is more frequent than all primary causes combined.

Indications. Recurrent or severe infection (sinopulmonary, bacterial, unusual organisms), failure to thrive with infections, bronchiectasis, suspected immunodeficiency before and during rituximab or other B cell depleting therapy, chronic lymphocytic leukaemia, myeloma and lymphoma, raised total protein with low albumin, autoimmune hepatitis and liver disease, IgA nephropathy, suspected IgG4-related disease, eosinophilia and allergic lung disease, and coeliac testing in suspected IgA deficiency.

Specimen. Serum. No fasting is needed. Stable for 7 days at 2 to 8 °C. Avoid sampling within 4 weeks of immunoglobulin infusion when assessing the patient's own production. Age specific ranges are essential in children.

Adult ranges are assay specific, and local ranges take precedence.

IgG. Range: 7 to 16 g/L. Subclasses: IgG1 (about 60 to 70%), IgG2 (20 to 25%), IgG3 (5 to 8%) and IgG4 (under 4%). Raised: chronic infection (HIV, hepatitis, tuberculosis, visceral leishmaniasis, malaria), autoimmune hepatitis (often over 1.1 times the upper limit), systemic lupus erythematosus, Sjögren syndrome, sarcoidosis, cirrhosis, lymphoproliferative disease, monoclonal IgG.

Lowered: primary antibody deficiency (X-linked agammaglobulinaemia, CVID), secondary deficiency (chronic lymphocytic leukaemia, myeloma, lymphoma, nephrotic syndrome, protein losing enteropathy, rituximab, glucocorticoids, mycophenolate, antiepileptics), and transient hypogammaglobulinaemia of infancy.

An IgG under 4 g/L with recurrent infection suggests severe antibody deficiency, and a level under 3 g/L with poor vaccine response commonly leads to replacement therapy.

IgA. Range: 0.7 to 4.0 g/L. Raised: IgA nephropathy (raised in up to half of patients), alcoholic cirrhosis, coeliac disease, Crohn disease, chronic infection, monoclonal IgA. Lowered: selective IgA deficiency (under 0.07 g/L with normal IgG and IgM, in a person over 4 years), CVID, ataxia telangiectasia, and drugs (phenytoin, sulfasalazine).

IgA deficiency lowers the sensitivity of IgA-based coeliac serology, and IgG-based tests are then needed. Rarely, IgA-deficient patients have anti-IgA antibodies that cause anaphylaxis with blood products.

IgM. Range: 0.4 to 2.3 g/L. Raised: acute infection, Waldenström macroglobulinaemia and IgM paraprotein, primary biliary cholangitis, hyper-IgM syndrome (with low IgG and IgA), cryoglobulinaemia, and rheumatoid arthritis. Lowered: CVID, selective IgM deficiency, chronic lymphocytic leukaemia, and splenectomy.

IgE. Range: under about 120 kU/L in adults (age dependent, and higher in atopic people). Raised: atopic disease, allergic bronchopulmonary aspergillosis (often over 1000 kU/L), parasitic infection, eosinophilic granulomatosis with polyangiitis, hyper-IgE syndromes (often over 2000 kU/L), Hodgkin lymphoma, drug reactions, and rarely IgE myeloma. Lowered: no clinical use, though absent IgE can occur in ataxia telangiectasia.

A normal total IgE does not exclude allergy, and specific IgE to allergens is needed to identify sensitisation.

IgG4. Raised in IgG4-related disease (over 1.35 g/L in about 70 to 80% of cases), and also in vasculitis, eosinophilic disease, atopic dermatitis and pancreatic cancer. A normal level does not exclude the disease, and the diagnosis requires histology with a high IgG4 plasma cell count.

Antibody deficiency.

• X-linked agammaglobulinaemia: all classes very low (IgG under 2 g/L), absent B cells, onset in boys after maternal antibody wanes at 3 to 6 months.
• CVID: low IgG with low IgA and/or IgM, poor vaccine response, onset commonly in the second to fourth decade.
• Selective IgA deficiency: low IgA only, with some patients developing recurrent infection or autoimmunity.
• IgG subclass deficiency: a low subclass with normal total IgG is of uncertain significance unless there is a poor specific antibody response.
• Hyper-IgM syndrome: raised or normal IgM with low IgG, IgA and IgE.

Assessment of functional antibody response (tetanus, Haemophilus influenzae type b, and pneumococcal polysaccharide titres before and 4 to 8 weeks after vaccination) is part of the diagnosis.

Secondary hypogammaglobulinaemia. Most common causes are lymphoproliferative malignancy, B cell depleting drugs (rituximab, which can cause prolonged low IgG), glucocorticoids, nephrotic syndrome, and protein losing enteropathy. Baseline levels before B cell depleting therapy identify patients at risk.

Polyclonal rise. Raised IgG, IgA and IgM together, with a broad gamma band, point to chronic infection or inflammation. A raised single class with the other two suppressed (immune paresis) points to a monoclonal protein.

Paraprotein check. A raised class with suppression of the others needs electrophoresis and immunofixation.

Critical values and findings needing immediate action.

• IgG under 3 g/L with severe or recurrent bacterial infection.
• Absent immunoglobulins in an infant with severe infection.
• IgM over 30 g/L with hyperviscosity symptoms.
• Hypogammaglobulinaemia in a patient about to receive a live vaccine or B cell depleting therapy.
• Suspected anti-IgA antibodies before blood product or immunoglobulin transfusion.

Interferences and pitfalls.

• Recent immunoglobulin replacement, plasma exchange or blood product transfusion alters results.
• Nephelometry may under read a very high monoclonal concentration (antigen excess), and dilution is then needed.
• Paraproteins can cause falsely high or low results for the involved class with some assays.
• Lipaemia and turbidity interfere with turbidimetry.
• Pregnancy, age and assay variation shift reference ranges.
• Normal immunoglobulin levels do not exclude a functional antibody defect.
• Transient low levels occur in infection and after drugs, so repeat measurement is needed before diagnosing a primary deficiency.

Special groups.

• Neonates: IgG is of maternal origin, and IgA and IgM are low. Raised IgM in cord blood indicates congenital infection.
• Infants: IgG nadirs at 3 to 6 months (about 3 to 4 g/L) and rises over the first years.
• Children: age specific ranges apply, and adult ranges are reached in adolescence.
• Pregnancy: IgG falls modestly.
• Older adults: IgG and IgA are mildly higher, and a monoclonal band is more frequent.
• Regions: chronic malaria, visceral leishmaniasis, schistosomiasis, HIV and tuberculosis produce marked polyclonal rises. In children with HIV, paradoxical hypergammaglobulinaemia can precede immunodeficiency.

Follow up tests. Serum protein electrophoresis with immunofixation, serum free light chains, lymphocyte subsets (CD3, CD4, CD8, CD19, NK cells), vaccine antibody titres, complement, HIV test, full blood count, IgG subclasses, specific IgE panel, Aspergillus IgE and IgG, IgG4 histology, liver function tests, coeliac serology, genetic testing for immunodeficiency, chest imaging for bronchiectasis.