IMMULITE® 2000 Assays Perform!

Introduction
On March 3, 1998, DPC launched the IMMULITE 2000, a new, high throughput, continuous random access immunoassay system. The IMMULITE 2000 continues in the IMMULITE® tradition with a wide menu of high-quality assays based on the same alkaline phosphatase label and ultrasensitive chemiluminescent substrate, and employing a centrifugal wash method. The launch menu includes 18 assays; a total of 40 are planned by the end of this year. The results for a sampling of currently available IMMULITE 2000 assays presented here demonstrate their reliable performance on this new platform.

Thyroid Function Testing
Modern thyroid function testing paradigms demand TSH assays which are classified as third generation, that is, which maintain inter-assay coefficients of variation of 20% or less at a concentration range of 0.01 to 0.02 µIU/mL. DPC's IMMULITE Third Generation TSH assay - the first such assay to be fully automated - has now been replicated on the IMMULITE 2000 system. The results of a comparison study, depicted in Figure 1, demonstrate the equivalence of the two methods. In a precision study based on NCCLS EP5 guidelines, a sample with a mean concentration of 0.015 µIU/mL was analyzed by the IMMULITE 2000 Third Generation TSH procedure over a period of 20 days. Analysis of the resulting data yielded the following statistics.

Intraassay coefficient of variation = 11.7%
Total coefficient of variation = 15.2%

Figure 1. Linear regression analysis of results obtained in a comparison study using 108 patient samples with TSH values ranging from 0.004 to 70 µIU/mL yielded the equation IMMULITE 2000 = 0.94 (IMMULITE) + 0.23 µIU/mL with a correlation coefficient of 0.985.

A precision-dose profile obtained for multiple samples appears in Figure 2. These data clearly indicate that the IMMULITE 2000 Third Generation TSH assay yields precision performance which places it well within the parameters of the third generation TSH definition.

Figure 2. Precision-dose profile for IMMULITE 2000 Third Generation TSH assay, including results for samples processed in accordance with the NCCLS EP5 guideline. Circles: within-run CVs.

Squares: run-to-run ("total") CVs. Dilutional parallelism constitutes another means for evaluating assay sensitivity on low-concentration samples. Figure 3 shows observed versus expected concentrations for a low sample that was serially diluted to the sensitivity of the assay.

Figure 3. Observed values for a sample containing a low TSH concentration, serially diluted to the sensitivity of the IMMULITE 2000 Third Generation TSH assay. The straight line indicates expected values.

Such assay sensitivity imparts practicality to the reflexive testing feature available on the IMMULITE 2000, whereby an option may be chosen to reflex automatically to another test if results fall outside the reference range. Samples with high or low TSH values, for example, can automatically undergo testing for free T4. This feature can enhance the efficiency of algorithms such as the one employed at Mayo Clinic, in which third generation TSH serves as the frontline test followed by free T4.

Rounding out the available thyroid function testing parameters are IMMULITE 2000 Total T4, Thyroid Uptake, Free T4, and Total T3. Future additions to the thyroid menu will be Free T3, Anti-TPO Ab, Anti-TG Ab, TBG, and Thyroglobulin.

Fertility/Infertility Testing
One of the hallmarks of DPC is its expansive menu of products designed to evaluate fertility and infertility parameters. DPC's history in this testing area began with manual RIAs that later made the technology transfer to the automated chemiluminescent IMMULITE system. These same high-quality products now have counterparts on the new IMMULITE 2000 platform. The available assays include Estradiol, Progesterone, LH, FSH, Total HCG, Prolactin, and DHEA-SO4, with Total Testosterone and SHBG to follow.

IMMULITE 2000 HCG exemplifies the synchronicity of instrumentation and assay design features, which combine to provide an amplified assay range of up to 5,000 mIU/mL with no high-dose hook effect even at HCG levels exceeding 1,000,000 mIU/mL. (See Figure 4.) Furthermore, the unique feature of autodilution allows samples to be run and diluted only if they are above the calibration range. IMMULITE 2000 will automatically dilute and reassay these samples if requested, without operator intervention.

Figure 4. IMMULITE 2000 HCG assay shows no high-dose hook effect even at HCG concentrations exceeding 1,000,000 mIU/mL.

Tumor Markers
The worldwide cancer diagnostic market is approximately $1 billion, with PSA testing alone accounting for nearly a third of this figure. The IMMULITE system provided the first automated third generation PSA assay, a test that is now also available on the IMMULITE 2000 platform. An IMMULITE 2000 precision-dose profile appears in Figure 5. These assays have a functional sensitivity of 0.01 ng/mL (with a detection limit of 0.003 ng/mL), and provide a tremendous time advantage when monitoring prostate cancer patients. Other IMMULITE assays for PSA, one with an extended range (up to 150 ng/mL) and one that detects free PSA,* have also been replicated on the IMMULITE 2000 system. The use of the Free PSA assay in combination with either of the other two assays for PSA can help elucidate the significance of elevated PSA levels.

Figure 5. Precision-dose profile for IMMULITE 2000 Third Generation PSA assay, including results for samples processed in accordance with the NCCLS EP5 guideline. Circles: within-run CVs. Squares: run-to-run ("total") CVs.

Other tumor marker assays available on the IMMULITE 2000 system include CEA and, shortly, AFP,* BR-MA (CA15-3),* GI-MA (CA19-9),* and OM-MA (CA125).*

Other Assays
Another special IMMULITE 2000 feature is the ability to run certain tests according to barcoded priorities. Whenever the instrument detects that intact PTH assays, for example, are to be performed, it processes these tests on all samples on board first (with the exception of STAT samples, which take precedence), to avoid losses of this temperature-sensitive analyte. Figure 6 displays the agreement between the IMMULITE 2000 and IMMULITE assays for intact PTH, as demonstrated in a method comparison study.

Figure 6. Linear regression analysis of results obtained in a comparison study using 118 patient serum samples with intact PTH values ranging from 10 to 2,200 pg/mL yielded the equation IMMULITE 2000 = 1.05 (IMMULITE) + 1.5 pg/mL with a correlation coefficient of 0.995.

IMMULITE 2000 Ferritin is also available as one of the initial launch assays. Additional assays under development include ACTH, Beta-2 Microglobulin, Cortisol, C-Peptide, and Total IgE.

Conclusion
Assay results from beta sites demonstrate the successful replication of IMMULITE assays on the new IMMULITE 2000 system. IMMULITE 2000 offers an expanding menu and state-of-the-art solutions for operational simplicity, systems integration, and assay performance.

*Available outside the US.