Quantitative Immunoglobulin Measurement Methods
Recommended Laboratory Technique
Quantitative immunoglobulin measurements should be performed using nephelometry or turbidimetry, which are the standard automated methods for measuring IgG, IgA, and IgM levels in serum. 1
Primary Measurement Approach
Nephelometry and Turbidimetry
- These automated immunochemical methods are the gold standard for quantitative immunoglobulin measurement, providing rapid, precise, and reliable results 1, 2
- Nephelometry can be used in place of serum protein electrophoresis when certain immunoglobulins (such as IgA) are difficult to quantify 1
- Turbidimetric methods using centrifugal analyzers provide excellent precision with day-to-day coefficients of variation ranging from 2.5% to 5.2% 3
- These methods can measure wide concentration ranges: IgG from 0.1-24 g/L, and IgA/IgM from 50-4000 mg/L 3
Technical Specifications
- Polyethylene glycol should be added to enhance antigen-antibody reactions and improve sensitivity 3, 4
- Turbidimetric endpoint readings are typically taken at 340 nm, with timing at 4 minutes for IgG and 12 minutes for IgA and IgM 3
- Kinetic turbidimetric approaches allow quantification across all regions of the immunoprecipitin curve, including samples with antigen excess 4
- Both monoclonal and polyclonal antibodies can be used, though polyclonal antisera remain standard 2
Complementary Testing Required
Essential Concurrent Tests
- Serum protein electrophoresis (SPEP) with immunofixation must be performed to distinguish monoclonal from polyclonal elevations 1, 5
- Serum free light chain (FLC) assay with kappa/lambda ratio should be measured to detect light chain abnormalities 1, 5
- The same FLC assay must be used consistently for monitoring individual patients, as different assays (FreeLite vs N Latex) are mathematically inconvertible 1
Urine Studies
- 24-hour urine collection with urine protein electrophoresis (UPEP) is necessary for complete assessment, as it provides total protein, albumin, and monoclonal component levels 1, 5
- Urine immunofixation should be performed on concentrated 24-hour urine specimens 1
- Urinary light chain assays have not been validated and should not be used to quantify Bence Jones protein 1
Critical Technical Considerations
Sample Requirements
- Serum samples are preferred over plasma for immunoglobulin quantification 6
- All measurements should be performed in duplicate to account for coefficient of variation 1
- Testing should be completed within appropriate timeframes after sample collection to ensure stability
Quality Control
- Each laboratory must establish local reference ranges using at least 50-100 healthy individuals, predominantly women 1
- Cut-off values should be calculated using percentile methods rather than standard deviations, as immunoglobulin distributions are non-Gaussian 1
- The 95th, 97.5th, and 99th percentiles should be documented 1
Common Pitfalls to Avoid
Interpretation Errors
- Do not rely solely on quantitative immunoglobulin levels without SPEP/immunofixation, as this will miss monoclonal gammopathies that require different management 5
- Renal impairment significantly affects free light chain ratios: the normal ratio of 0.26-1.65 can rise to 0.34-3.10 in severe renal impairment (CKD stage 5) 1
- Mildly lipemic sera can be tested without correction when using automated turbidimetric methods 7
Monitoring Considerations
- The same assay platform must be used for serial monitoring of individual patients, particularly for free light chains 1
- If initial results are negative but clinical suspicion remains high, consider checking with an alternative assay platform 1
- In diseases with intact monoclonal immunoglobulins (such as PGNMID), serum immunofixation may be more helpful than free light chain assays 1