37 - Freelite immunoassay instrumentation

Chapter 37

Freelite® sFLC immunoassays:

  • Are optimised for use on Binding Site analysers: SPAPLUS®, Optilite®, and MININEPHPLUS®, all of which have prozone parameters.
  • Can also be used on the majority of other nephelometric and turbidimetric analysers.

37.1. Introduction

Freelite immunoassays for the measurement of free light chains (FLCs), are available for the majority of nephelometric and turbidimetric laboratory instruments. The assays all utilise latex enhancement to allow detection of FLCs at low concentrations. Turbidimeters and nephelometers have similar levels of sensitivity and precision, but instruments vary in their ability to handle samples, clean reaction cuvettes, identify antigen excess, etc.

37.2. Binding Site SPAPLUS

The Binding Site SPAPLUS is an automated, bench-top, batch mode turbidimeter with host interface capability, barcoded sample identification and reagent management systems (Figure 37.1). Precision is maintained through a combination of acid/alkali cuvette washing and an innovative reaction cuvette mixing system (Table 37.1). Air pressure is used in place of stirrers to mix the reaction mixture in a U-shaped cuvette. No physical contact is made with the reaction mixture, thereby removing any possibility of carry-over on a stirrer. Calibration curves are made from calibrator sets and validated by assay of control fluids supplied with the kits. Samples are initially measured at the standard programmed sample dilution and if out of range, the instrument automatically re-measures the samples at the appropriate alternative dilutions. All dilutions are made with the instrument’s pipetting system, which is capable of dilutions between 1/10 and 1/100. The assays include prozone parameters (Section 7.4). The instrument has a good overall performance compared with other analysers, and provides good throughput for patient samples in spite of its relatively small size.

κ sFLC λ sFLC
Range at 1/10 4.0 - 180.0 mg/L 4.5 - 165.0 mg/L
Sensitivity at 1/1 0.4 mg/L 0.5 mg/L
Assay time 15 min 15 min
Precision: within-run 3.3% at 7.2 mg/L 3.4% at 10.4 mg/L
1.6% at 35.7 mg/L 2.4% at 35.1 mg/L
1.8% at 123.8 mg/L 2.0% at 142.1 mg/L
Precision: between-run 4.2% at 7.2 mg/L 2.2% at 10.4 mg/L
1.9% at 35.7 mg/L 0.0% at 35.1 mg/L
2.3% at 123.8 mg/L 2.4% at 142.1 mg/L

Table 37.1. Freelite assay performance on the Binding Site SPAPLUS.

Antigen excess (prozone) parameters exist for IgMκ and IgMλ Hevylite SPAPLUS assays, and are discussed further in Section 11.4. Samples detected as being in antigen excess are automatically flagged by the instrument with a prozone (P) flag and the sample is automatically re-assayed at the higher re-dilution.

37.3. Binding Site Optilite

The Binding Site Optilite is an automated, bench-top, random access turbidimeter allowing host interface capability with barcoded parameter entry, sample identification and reagent management systems (Figure 37.2). Precision is good due to the use of disposable cuvettes (Figure 37.2). Single vial calibrators are used that are diluted automatically by the instrument to generate the calibration curves. Starting sample dilutions are 1/10 for κ sFLC assays and 1/8 for λ sFLC assays. Samples reporting outside of the standard assay measuring range are automatically re-diluted, with one lower and three higher sample dilutions programmed within the assay parameters. Very high level samples may still require further off-line dilutions. The assays include prozone parameters, with a higher automatic re-dilution being performed for samples that prompt the “prozone detected” data flag.

κ sFLC λ sFLC
Range 1/10 (κ); 1/8 (λ) 2.9 - 127.0 mg/L 5.2 - 139.0 mg/L
Sensitivity (undiluted) 0.6 mg/L 1.3 mg/L
Assay time 15 min 15 min
Precision: within-run 2.6% at 4.9 mg/L 2.1% at 4.7 mg/L
1.5% at 23.2 mg/L 1.8% at 19.8 mg/L
3.3% at 71.8 mg/L 1.8% at 71.1 mg/L
Precision: between-run 2.9% at 4.9 mg/L 3.2% at 4.7 mg/L
1.8% at 23.2 mg/L 2.1% at 19.8 mg/L
1.8% at 71.8 mg/L 2.4% at 71.1 mg/L

Table 37.2. Freelite assay performance on the Binding Site Optilite.

37.4. Binding Site MININEPHPLUS

The Binding Site MININEPHPLUS is a small, manual nephelometer designed for sFLC analysis (Figure 37.3). The calibration curves are stored on magnetic swipe-cards. Once loaded into the instrument's memory, curve validity is confirmed by assaying control samples. Patient samples are analysed individually by semi-automated addition of reagents into cuvettes within the instrument. The assays show good precision and linearity, and include prozone parameters (Figure 37.3). The instrument may find use in laboratories with low workloads (<10 samples per day) or when the cost of a larger instrument cannot be justified.

κ sFLC λ sFLC
Range at 1/20 3.0 - 72.4 mg/L 4.9 - 98.3 mg/L
Sensitivity at 1/20 3.0 mg/L 4.9 mg/L
Assay time 5 min 5 min
Precision: within-run 7.3% at 55.8 mg/L 5.1% at 80.5 mg/L
5.7% at 18.7 mg/L 3.9% at 29.4 mg/L
4.9% at 4.8 mg/L 3.2% at 7.6 mg/L
Precision: between-run 8.9% at 55.8 mg/L 6.7% at 80.5 mg/L
6.8% at 18.7 mg/L 3.8% at 29.4 mg/L
5.0% at 4.8 mg/L 7.4% at 7.6 mg/L

Table 37.3. Freelite assay performance on the Binding Site MININEPHPLUS.

37.5. Other analytical platforms

In addition to the Binding Site instruments listed above, Freelite immunoassays are also available for the majority of nephelometric and turbidimetric laboratory instruments including: 1) Beckman Coulter AU® (400, 640, 2700 and 5400); 2) Beckman Coulter IMMAGE® and IMMAGE 800; 3) Roche Cobas® c501, c502 and Integra (400,400 plus and 800); 4) Roche Hitachi 911/912/917 and Modular P; and 5) Siemens ADVIA® (1650, 1800 and 2400), BNII and BN ProSpec®.


  1. Which Binding Site instrument provides random-access sample measurement?


1. The Optilite instrument offers random-access sample measurement (Section 37.3).