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Chapter 11

As with any laboratory test, Hevylite results should be interpreted alongside a patient’s clinical details and other laboratory tests. When serial monitoring data is available, results should be compared with previous measurements to check they are consistent and to identify trends.

11.2.1. Normal reference intervals

A number of published normal reference intervals are available for Hevylite assays. These are discussed further in Chapter 10. Users are recommended to either establish their own local ranges or validate an existing reference interval.

11.2.2. Terminology

For a particular Hevylite isotype, both κ and λ assays (e.g. IgAκ and IgAλ) should always be measured, and the HLC ratio calculated (e.g. IgAκ/IgAλ). Definitions of commonly used terms are given in Table 11.5. For serial measurements, either the iHLC, dHLC or the HLC ratio may be used [38][37][238][929]. When an abnormal HLC ratio is present, and the concentration of the uninvolved HLC-pair is below the normal reference interval, this is termed “HLC pair suppression”.

Term Definition Comment For an IgGκ MM tumour...
iHLC Involved HLC The HLC isotype that is produced by the tumour.
IgGκ
uHLC Uninvolved HLC The same heavy chain isotype but alternate light chain type to the iHLC. IgGλ
HLC ratio e.g. IgGκ/IgGλ For a particular immunoglobulin isotype, a ratio of the concentration of Hevylite κ to λ. Indicates monoclonality. IgGκ/IgGλ
dHLC iHLC - uHLC For a particular immunoglobulin isotype, the difference in concentration between iHLC and uHLC. IgGκ - IgGλ
HLC pair suppression When the concentration of the uHLC is below the normal reference interval
The HLC ratio must be abnormal.Suppression of IgGλ

Table 11.5. Summary of HLC terminology.

11.2.3. Result interpretation

A raised concentration of one HLC type, with an abnormal HLC ratio is indicative of monoclonal immunoglobulin production. Renal impairment is not known to influence HLC results, since intact immunoglobulin molecules are not cleared by the kidney (Chapter 3).

11.2.4. The HLC dot plot

In some cases it may be helpful to plot an individual patient’s result (or an entire cohort of patients’ results on a HLC dot plot Figure 11.1). This is a graph displaying Ig’κ HLC concentrations (x-axis) against Ig’λ HLC concentrations (y-axis), both on logarithmic scales. The limits of the normal reference interval for the Ig’κ/Ig’λ HLC ratio are represented by two parallel lines. Each patient result is represented by a single point. If the Ig’κ/Ig’λ HLC ratio is normal, the result will fall within the normal reference interval. A result that lies outside the normal reference interval is consistent with the presence of a monoclonal intact immunoglobulin.

11.2.5. Freelite and Hevylite are independent tumour markers

Intact immunoglobulins (and therefore Hevylite measurements) and FLCs are independent tumour markers. For example, a plot of IgGκ HLC vs. κ sFLC concentrations for 170 IgGκ MM patients showed no correlation between the two parameters (Figure 11.2). This important point is discussed further in Section 17.2.

11.2.6. Biological variation

Knowledge of the biological variation of HLC in serum is useful when interpreting monitoring data for individual patients. There is currently only limited information on the biological variation of HLC measurements in healthy subjects. Finlay et al. [239] studied the biological variation of IgG and IgA HLC in 15 healthy blood donors. Samples were collected on the same day of the week, every 2 weeks for 6 weeks. The laboratory’s analytical performance (CVA) for all methods was acceptable, ranging from 3.4 to 9.4% (Table 11.6). For all HLC specificities, the within-subject variation (CVI) was less than the within-group variation (CVG) (Table 11.6).

The reference change value (RCV) defines the minimum difference between two consecutive measurements from the same individual, which can be considered significant (p<0.05). In this study the RCV for all analytes was similar, with values of around 30 to 40%. This is similar to the RCV for monoclonal protein measurements determined by SPE, in patients with stable monoclonal gammopathy [174].
Analyte n Mean (g/L) CVA (%) CVI (%) CVG (%) RCV (%)
IgGκ 15 6.0 9.4 12.3 24.9 41
IgGλ 15 3.6 3.4 5.9 16.3 28
IgAκ 15 1.1 5.3 7.2 33.3 30
IgAλ 15 1.0 3.8 8.0 35.8 32

Table 11.6. Biological variation of IgG and IgA HLC [239]. CVA: analytical variation; CVI: within-subject variation; CVG: within-group variation; RCV: reference change value.

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References