The MAGLUMI 2000 FT4 assay is for in vitro diagnostic use in the quantitative determination of free thyroxine (FT4) in human serum. The measurement of FT4 is used in the diagnosis of thyroid disorders.

Device Description

MAGLUMI 2000 FT4 kit consists of the following reagents: Magnetic Microbeads- coated with T4 antigen, BSA, NaN3(<0.1%) Calibrator Low-Containing BSA and T4 antigen, NaN3 (<0.1%) Calibrator High- Containing BSA and T4 antigen, NaN3 (<0.1%) Buffer- Containing BSA and NaN3 (<0.1%) ABEI Label- Anti-T4 monoclonal antibody labeled with ABEI, containing BSA, NaN3(<0.1%) Control 1- Containing BSA and T4 antigen, NaN3 (<0.1%) Control 2- Containing BSA and T4 antigen, NaN3 (<0.1%)

AI/ML Overview

Here's an analysis of the acceptance criteria and study findings for the MAGLUMI 2000 FT4 device, based on the provided text:

1. Table of Acceptance Criteria and Reported Device Performance

Performance Characteristic	Acceptance Criteria (Implicit)	Reported Device Performance
Precision	CV% within acceptable ranges for clinical chemistry devices at various concentrations (implied by CLSI EP5-A2). For example, Total CV for Control 1: ≤ 10% (often a general guideline for low-concentration analytes). Total CV for other controls/samples: ≤ 8-10% depending on concentration. Specific pre-defined limits are not explicitly stated, but the study aims to demonstrate acceptable precision.	Control 1 (1.006 ng/dL): Total CV 9.34%Control 2 (1.9952 ng/dL): Total CV 7.81%Control 3 (3.9978 ng/dL): Total CV 5.04%Calibrator low (0.3333 ng/dL): Total CV 9.99%Calibrator high (7.1929 ng/dL): Total CV 3.20%Serum Pools/Patient Pools: Total CVs ranged from 3.76% to 9.96%
Linearity	The assay should demonstrate linearity across its claimed measuring range (0.19 to 10.0 ng/dL) with a strong correlation (R² close to 1) and a slope close to 1, and y-intercept close to 0 (implied by CLSI EP6-A).	Linear between 0.19 and 10.0 ng/dL. Observed = 0.9898 (Expected) + 0.01364, R² = 0.9991.
Stability (Reagents)	Reagents, calibrators, and controls should be stable for a specified shelf life at recommended storage conditions (e.g., 2-8°C for 12 months for accelerated stability, confirmed by real-time studies).	Accelerated stability at 37°C showed controls, calibrators, and reagents are stable for 12 months at 2-8°C. Real-time stability is on-going.
Detection Limit (Limit of Blank - LOB)	95th percentile value from analyte-free samples (implied by CLSI EP17-A guidelines). A low LOB is desired.	0.087 ng/dL (highest of 3 lots).
Detection Limit (Limit of Detection - LOD)	Lowest analyte concentration that can be detected (implied by CLSI EP17-A guidelines). A low LOD is desired.	0.143 ng/dL (highest of 3 lots).
Detection Limit (Limit of Quantitation - LOQ)	Lowest analyte concentration that can be reproducibly measured with an intermediate precision CV of ≤ 20% (as per CLSI EP17-A guidelines).	0.190 ng/dL (highest of 3 lots).
Interference (Cross-Reactivity)	% Cross-reactivity for relevant compounds should be very low (e.g., <0.02% as shown for the specified substances). No significant interference (recovery ± 10% of initial value) from common drugs and endogenous substances at specified concentrations.	All tested cross reactants (Monoiodotyrosine, L-Triiodothyronine, Diiodotyrosine, 3,5-Diiodo-L-thyronine, Reverse Triiodothyronine) showed <0.02% cross-reactivity. No significant interference (recovery ± 10%) from listed common drugs (e.g., Bilirubin, Hemoglobin, Triglycerides, Cefoxitin) and endogenous interferents (HAMA, RF, Total protein) at specified concentrations.
Method Comparison	Strong correlation and good agreement between the candidate device and the predicate device, demonstrated by a linear regression equation with an R² close to 1 (e.g., >0.95 or >0.98), a slope close to 1, and a y-intercept close to 0 (implied by CLSI EP9-A2).	Y = 1.0451X - 0.05375, R² = 0.9919, for MAGLUMI FT4 (y) vs. ADVIA CENTAUR FT4 (x).

Note on Acceptance Criteria: The document often refers to CLSI guidelines (e.g., EP5-A2, EP6-A, EP17-A, EP9-A2) for performance characteristic studies. While explicit numerical acceptance criteria are not always stated directly in the text, compliance with these guidelines implicitly means that the performance falls within industry-accepted ranges and statistical thresholds. For precision, for instance, a total CV of less than 10% (or even lower for higher concentrations) is generally considered acceptable in clinical chemistry. For linearity and method comparison, an R² value close to 1, a slope near 1, and a y-intercept near 0 are standard indicators of good performance.

2. Sample Size Used for the Test Set and Data Provenance

Precision Study:
- Sample Size: 240 measurements per control/calibrator/pool (N=240, from 80 samples analyzed per level on each of 3 instruments).
- Data Provenance: Not explicitly stated, but implied to be patient serum pools and native patient samples from clinical settings, likely domestic to the manufacturer's testing location or within industry standard practices for clinical evaluation. The study was conducted on "three controls, two calibrators, four spiked patient serum pools and four native patient sample pools."
Linearity Study:
- Sample Size: 11 levels with FT4 concentrations from 0.19 to 10.0 ng/dL, each measured in quadruplicate on 3 lots of reagent.
- Data Provenance: Samples prepared by spiking T4 USP Standard into T4-free human serum samples.
Detection Limit Study (LOB, LOD, LOQ):
- LOB: 80 measurements of T4 free human serum samples.
- LOD: Four levels of low samples, measured in 80 replicates over 5 days per sample.
- LOQ: Six low serum samples, in six replicates per run, one run per day, over 5 days.
- Data Provenance: T4 free human serum samples and low serum samples.
Interference Study:
- Sample Size: Variable, involves preparing solutions, and for common drugs/interfering substances, three serum samples (low, medium, high FT4) per substance. For HAMA/RF/Protein, specific FT4 human serum samples.
- Data Provenance: Spiked solutions, human serum pools, FT4 human serum samples.
Method Comparison Study:
- Sample Size: 224 human serum samples.
- Data Provenance: Human serum samples. Provenance (e.g., country of origin, retrospective/prospective) is not explicitly stated.
Expected Values/Reference Range Study:
- Sample Size: 157 serum samples.
- Data Provenance: Normal, apparently healthy adult individuals (22 years and older). Provenance (e.g., country of origin, retrospective/prospective) is not explicitly stated.

3. Number of Experts Used to Establish the Ground Truth for the Test Set and Their Qualifications

This information is not provided in the document. For an in vitro diagnostic device like the MAGLUMI 2000 FT4, the "ground truth" for analytical performance tests (precision, linearity, detection limits, interference) is typically established by the reference methods, spiked concentrations, or consensus values derived from the testing procedures themselves, rather than human expert opinion. For method comparison, the "ground truth" is typically the result from the established predicate device.

4. Adjudication Method for the Test Set

The concept of an "adjudication method" (like 2+1 or 3+1 for clinical diagnoses based on expert review) is not applicable to the analytical performance studies described for this in vitro diagnostic device. The studies rely on quantitative measurements and statistical analysis rather than subjective expert interpretation of results for ground truth.

5. If a Multi-Reader Multi-Case (MRMC) Comparative Effectiveness Study was done

No, an MRMC comparative effectiveness study was not done. This type of study is primarily relevant for medical imaging devices or other diagnostic tools where human readers interpret results, and the AI system acts as an aid to improve human performance. For an automated in vitro diagnostic assay like MAGLUMI 2000 FT4, the performance is evaluated through direct analytical comparisons.

6. If a Standalone (i.e., algorithm only without human-in-the-loop performance) was done

Yes, the studies described are for the standalone performance of the MAGLUMI 2000 FT4 device. These are analytical performance studies (precision, linearity, detection limits, interference, method comparison) that evaluate the device's ability to accurately and precisely measure FT4 concentrations in human serum, independent of human interpretation or intervention in the measurement process itself. The "algorithm" here refers to the immunoassay's chemistries and the instrument's measurement principles.

7. The Type of Ground Truth Used

Precision: Reference materials (controls, calibrators) with established target values, and pooled human serum samples where the mean serves as a statistical "ground truth" for variability assessment.
Linearity: Gravimetrically or volumetrically prepared "expected" concentrations of T4 in serum.
Stability: Initial measurements at time 0, with subsequent measurements compared to these baselines.
Detection Limits (LOB, LOD, LOQ): Analyte-free samples and low-concentration samples measured extensively to statistically determine minimum detectable/quantifiable levels.
Interference (Cross-Reactivity): Known amounts of potential interferents/cross-reactants.
Method Comparison: Results from the predicate device (Siemens ADVIA Centaur FT4) are used as the reference "ground truth" for comparison.
Expected Values/Reference Range: Statistical distribution of measurements from a reference population (157 apparently healthy adults) to establish a normal range.

8. The Sample Size for the Training Set

This information is not provided. The document describes performance characteristic studies (test set), but does not detail the development or "training" specific to an AI algorithm for this device. For in vitro diagnostics, "training set" would typically refer to samples used during the assay development and optimization phase to establish parameters like calibration curves, reagent formulations, or instrument settings. The document focuses on validation studies.

9. How the Ground Truth for the Training Set Was Established

This information is not provided as a "training set" isn't explicitly defined in the context of this submission. If referring to the development/optimization phase of a typical immunoassay, ground truth would be established through:

Reference materials: Highly purified FT4 standards.
Known concentrations: Samples prepared with precise, known concentrations of FT4.
Comparative data: Initial comparisons against existing reference methods or well-characterized assays during the research and development phase.
Clinical samples: Use of a large number of clinical samples to optimize the assay's dynamic range and specificity.

Summary

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October 4, 2018

Shenzhen New Industries Biomedical Engineering Co., Ltd % Joe Shia Scientific Reviewer LSI International Inc 504E Diamond Ave., Suite F Gaithersburg, MD 20877

Re: K182423

Trade/Device Name: MAGLUMI 2000 FT4 Regulation Number: 21 CFR 862.1695 Regulation Name: Free thyroxine test system Regulatory Class: Class II Product Code: CEC Dated: August 31, 2018 Received: September 6, 2018

Dear Joe Shia:

We have reviewed your Section 510(k) premarket notification of intent to market the device referenced above and have determined the device is substantially equivalent (for the indications for use stated in the enclosure) to legally marketed predicate devices marketed in interstate commerce prior to May 28, 1976, the enactment date of the Medical Device Amendments, or to devices that have been reclassified in accordance with the provisions of the Federal Food, Drug, and Cosmetic Act (Act) that do not require approval of a premarket approval application (PMA). You may, therefore, market the device, subject to the general controls provisions of the Act. Although this letter refers to your product as a device, please be aware that some cleared products may instead be combination products. The 510(k) Premarket Notification Database located at https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfpmn/pmn.cfm identifies combination product submissions. The general controls provisions of the Act include requirements for annual registration, listing of devices, good manufacturing practice, labeling, and prohibitions against misbranding and adulteration. Please note: CDRH does not evaluate information related to contract liability warranties. We remind you, however, that device labeling must be truthful and not misleading.

If your device is classified (see above) into either class II (Special Controls) or class III (PMA), it may be subject to additional controls. Existing major regulations affecting your device can be found in the Code of Federal Regulations, Title 21, Parts 800 to 898. In addition, FDA may publish further announcements concerning your device in the Federal Register.

Please be advised that FDA's issuance of a substantial equivalence determination does not mean that FDA has made a determination that your device complies with other requirements of the Act or any Federal statutes and regulations administered by other Federal agencies. You must comply with all the Act's

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requirements, including, but not limited to: registration and listing (21 CFR Part 807); labeling (21 CFR Part 801 and Part 809); medical device reporting of medical device-related adverse events) (21 CFR 803) for devices or postmarketing safety reporting (21 CFR 4, Subpart B) for combination products (see https://www.fda.gov/CombinationProducts/GuidanceRegulatoryInformation/ucm597488.html; good manufacturing practice requirements as set forth in the quality systems (OS) regulation (21 CFR Part 820) for devices or current good manufacturing practices (21 CFR 4, Subpart A) for combination products; and, if applicable, the electronic product radiation control provisions (Sections 531-542 of the Act); 21 CFR 1000-1050.

Also, please note the regulation entitled, "Misbranding by reference to premarket notification" (21 CFR Part 807.97). For questions regarding the reporting of adverse events under the MDR regulation (21 CFR Part 803), please go to http://www.fda.gov/MedicalDevices/Safety/ReportaProblem/default.htm.

For comprehensive regulatory information about medical devices and radiation-emitting products, including information about labeling regulations, please see Device Advice (https://www.fda.gov/MedicalDevices/DeviceRegulationandGuidance/) and CDRH Learn (http://www.fda.gov/Training/CDRHLearn). Additionally, you may contact the Division of Industry and Consumer Education (DICE) to ask a question about a specific regulatory topic. See the DICE website (http://www.fda.gov/DICE) for more information or contact DICE by email (DICE@fda.hhs.gov) or phone (1-800-638-2041 or 301-796-7100).

Sincerely,

Kellie B. Kelm -S

Courtney H. Lias, Ph.D. for Director Division of Chemistry and Toxicology Devices Office of In Vitro Diagnostics and Radiological Health Center for Devices and Radiological Health

Enclosure

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Indications for Use

510(k) Number (if known) K182423

Device Name MAGLUMI 2000 FT4

Indications for Use (Describe)

Type of Use (Select one or both, as applicable)
-------------------------------------------------

|X Prescription Use (Part 21 CFR 801 Subpart D)

| | Over-The-Counter Use (21 CFR 801 Subpart C)

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K182423

510(k) SUMMARY

This summary of 510(k) safety and effectiveness information is submitted in accordance with the
requirements of 21 CFR 807.92

1. Date: October 2, 2018 2. Submitter: Shenzhen New Industries Biomedical Engineering Co., Ltd. No.16, Jinhui Road, Pingshan New District, Shenzhen China 518122 3. Contact person: Joe Shia LSI International Inc. 504 East Diamond Ave., Suite F Gaithersburg, MD 20878 Telephone: 240-505-7880 Fax: 301-916-6213 Email:shiajl@yahoo.com
1. Device Name: MAGLUMI 2000 FT4

Classification:

Class II (assay)

Product Code	CFR #	ProductAbbreviation	Product Name
CEC	862.1695	FT4	Free Thyroxine Test System

Predicate Devices: K080167, Siemens ADVIA Centaur FT4

6. Intended Use:

7. Device Description:

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Control 2- Containing BSA and T4 antigen, NaN3 (<0.1%)

1. Standard/Guidance Documents
  Clinical and Laboratory Standards Institute EP5-A2 – Evaluation of Precision Performance of Clinical Chemistry Devices-Approved Guideline-Second Edition.

Clinical and Laboratory Standards Institute EP6-A – Evaluation of the Linearity of Quantitative Analytical

Clinical and Laboratory Standards Institute EP17-A2: Evaluation of detection Capability for Clinical Laboratory Measurement Procedures

Clinical and Laboratory Standards Institute EP7-A2 – Interference Testing in Clinical Chemistry Clinical and Laboratory Standards Institute EP9-A2 – Method Comparison and Bias Estimation Using Patient Samples

1. Substantial Equivalence Information

Item	Predicate Device	Subject Device
Intended Use/Indication for Use	The ADVIA Centaur FT4 Immunoassay is for invitro diagnostic use in the quantitativedetermination of free thyroxine (FT4) in serumor plasma (heparinized or EDTA) using theADVIA Centaur and ADVIA Centaur XPsystems. Measurement of free thyroxine areused in the diagnostic and treatment of thyroiddiseases.	The MAGLUMI 2000 FT4assay is for in vitrodiagnostic use in thequantitative determination offree thyroxine (FT4) inhuman serum. Themeasurement of FT4 is usedin the diagnosis of thyroiddisorders.
Specimen	Serum , heparinized plasma, EDTA plasma	serum
Measured Analyte	Free thyroxine	same
Measurement	Quantitative	same
Test principle	Competitive immunoassay	same
Capture Antibody	Biotin-labeled polyclonal anti-T4 bound to avidinparamagnetic particles	T4 antigen coated Magneticmicrobeads
Detection Antibody	Acridium ester labeled T4	ABEI labeled monoclonalanti-T4 antibody
Measuring range	0.1-12.0 ng/dL	0.19-10.0 ng/dL
Sample size	25µL	40 µL
Calibration	2 Point	same
Calibrator	Calibrator A	Low and high Calibrators, 2levels, ready for use
Calibratorspackaging	Provided separately	Provided with reagent kit
Automated	Yes	same

Assay Similarities and Differences

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10. Test Principle

The FT4 assay is a competitive chemiluminescence immunoassay using FDA previously cleared MAGLUMI 2000 instrument (K162698).

The sample (or calibrator/control, if applicable), ABEI-labeled anti-T4 monoclonal antibody, buffer and T4 antigen-coated magnetic microbeads are mixed thoroughly and incubated at 37°C. Free T4 present in the sample (or calibrator/control, if applicable) competes with T4 antigen immobilized on the magnetic microbeads for a limited number of binding sites on the ABEI-labeled anti-T4 monoclonal antibody, forming immuno-complexes. After precipitation in a magnetic field, decant the supernatant, and then perform a wash cycle. Subsequently, the Starter 1+2 are added to initiate a chemiluminescent reaction. The light signal is measured by a photomultiplier within 3 seconds as relative light units (RLUs), which is inversely proportional to the concentration of free T4 present in the sample (or calibrator/control, if applicable).

11. Performance Characteristics

1. Analytical Performance

a. Precision
The precision was determined using the CLSI EP5-A2 protocol as a guide. The study was conducted on three different instruments with three controls, two calibrators, four spiked patient serum pools and four native patient sample pools. The data was collected over 20 days in duplicate with 2 runs per day with a total of 80 samples analyzed per level on each instrument. The results (in ng/dL) obtained are summarized in the following tables:

	Mean	Within-Run		Between-Run		Between-Day		Total
Sample	(N=240)	SD	CV	SD	CV	SD	CV	SD	CV
Control 1	1.006	0.0879	8.74%	0.0242	2.41%	0.0226	2.25%	0.094	9.34%
Control 2	1.9952	0.1482	7.43%	0.0391	1.96%	0.0282	1.41%	0.1558	7.81%
Control 3	3.9978	0.1852	4.63%	0.0487	1.22%	0.0632	1.58%	0.2016	5.04%
Calibrator low	0.3333	0.0311	9.33%	0.0083	2.49%	0.0084	3%	0.0333	9.99%
Calibrator high	7.1929	0.2109	2.93%	0.0679	0.94%	0.0625	0.87%	0.2302	3.20%
Serum Pool 1	0.5842	0.0498	8.52%	0.0271	4.64%	0.013	2.23%	0.0582	9.96%
Serum Poo2	0.9016	0.0788	8.74%	0.0276	3.06%	0.0239	3%	0.0868	9.63%
Serum Pool 3	1.71	0.1388	8.12%	0.0383	2.24%	0.0415	2.43%	0.1499	8.77%
Serum Pool 4	5.7965	0.1895	3.27%	0.0785	1.35%	0.0742	1.28%	0.2181	3.76%
Native patient pool 1	0.8375	0.0754	9.00%	0.0246	2.94%	0.0232	2.77%	0.0826	9.86%
Native patient pool 2	1.2837	0.1066	8.30%	0.0241	1.88%	0.0265	2.06%	0.1124	8.76%
Native patient pool 3	1.8821	0.1499	7.96%	0.0555	2.95%	0.0249	1.32%	0.1618	8.60%
Native patient pool 4	4.6233	0.2133	4.61%	0.0539	1.17%	0.0553	1.20%	0.2269	4.91%

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b. Linearity

The linearity of the MAGLUMI FT4 method was determined following the CLSI EP6-A procedure. Samples were prepared by spiking T4 USP Standard into T4-free human serum samples to create 11 levels with FT4 concentrations from 0.19 to 10.0 ng/dL. Each sample was measured in quadruple on 3 lots of reagent. Linearity was evaluated using regression analysis based on CLSI EP6-A.

The assays are linear between 0.19 and 10.0 ng/dL with the following relationship: Observed = 0.9898 (Expected) + 0.01364, R2 = 0.9991

c. Stabilitv
Accelerated stability study at 37°C showed that all controls are stable for 12 months at 2-8ºC. Accelerated stability study at 37ºC showed all that calibrators are stable for 12 months at 2-8°C. Accelerated stability study at 37°C showed that the reagent is stable for 12 months at 2-8℃. The real time stability at 2-8℃ is on-going.
d. Detection Limit
Detection limit studies were performed following CLSI EP17-A guidelines. The limit of blank (LOB) is the 95th percentile value from 80 measurements of T4 free human serum samples using 3 different lots of FT4 reagents over 5 days. The LOB corresponds to the concentration below which analyte-free samples are found with a probability of 95% and was determined to be 0.087 ng/dL (highest of the 3 lots).

The limit of detection (LOD) is determined based on the LOB and the standard deviation of low concentration samples. The LOD corresponds to the lowest analyte concentration which can be detected. Four level of low samples were measured in 80 replicates over 5 days per sample using 3 lots of reagents. LOD was determined to be 0.143 ng/dL (highest of the 3 lots).

The limit of quantitation (LOQ) was determined by measuring six low serum samples, in six replicates per run, one run per day, over 5 days, using 3 lots of reagents. LOQ is defined as the lowest analyte concentration that can be reproducibly measured with an intermediate precision CV of ≤ 20 % and was determined to be 0.190 ng/dL (highest of the 3 lots).

Interference e.
Clinical serum samples may contain substances that could potentially interfere with the test. The cross-reactivity is defined as the ratio of the amount of T4 required to displace 50% of the maximally bound labeled T4 from the anti-T4 antibody, and the amount of the cross-reactant to give the same 50% displacement. Solutions of FT4 and potential cross reactants were prepared by spiking corresponding compounds in

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various concentrations. Concentrations of the 50% displacement were measured for these solutions using 3 lots of reagents. The following table shows cross reactivity of potential cross reactant.

Cross Reactant	%Cross-reactivity
Monoiodotyrosine	<0.02%
L-Triiodothyronine	<0.02%
Diiodotyrosine	<0.02%
3,5-Diiodo-L-thyronine	<0.02%
Reverse Triiodothyronine	<0.02%

The effect of common drugs and interference substances were evaluated using human serum pools. For each substance, three serum samples containing a low, medium and high concentration of FT4 were analyzed. For all substances tested, no significant interference was defined as recovery ± 10% of initial value. The substances and the highest concentration tested which did not cause significant interference are listed below.

Compounds	Concentration
Conjugated bilirubin	20 mg/dL
Unconjugated bilirubin	20 mg/dL
Hemoglobin	1000 mg/dL
Triglycerides	1000 mg/dL
Cefoxitin	6.8 mg/dL
Levodopa	2 mg/dL
Metronidazole	12 mg/dL
Rifampicin	6 mg/dL
Methimazole	40 mg/dL
Propythiouracile	4 mg/dL
Ascorbic Acid	6 mg/dL
Acetaminophen	25 mg/dL
Phenylbutazone	4 mg/dL

The effect of human anti-mouse antibodies (HAMA), rheumatoid factor (RF) and human serum total protein was evaluated using human serum samples. Each potential interferent was added to FT4 human serum samples and tested triplicate using 3 lots of reagents. For all substances tested, no significant interference was defined as recovery ± 10% of initial value. The potential interferents and the highest concentration tested which did not cause significant interference are listed below.

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Interferent	Concentration
HAMA	800 ng/mL
RF	1240 IU/mL
Total protein	12.5 g/dL

1. Comparison Studies
  A method comparison study was performed with 224 human serum samples with concentrations ranging from 0.19 to 10.0 ng/dL. The comparison of the MAGLUMI FT4 assay (y) with the predicate device, ADVIA CENTAUR FT4 assay (x), produced the following linear regression equation:

Y = 1.0451X - 0.05375, R2 =0.9919

1. Expected values/Reference range:
  A total of 157 serum samples from normal, apparently healthy adult (22 years and older) individuals were tested according to the procedure in CLSI C28-A3. The expected normal range is 0.82 – 1.72 ng/dL based on the central 95% of the frequency distribution.
1. Conclusion
  Based on the test principle and acceptable performance characteristics including precision, interference, specificity and method comparison of the device, it is concluded that the MAGLUMI 2000 FT4 is substantially equivalent to the predicate.

Regulation Number and Section

§ 862.1695 Free thyroxine test system.

(a)
Identification. A free thyroxine test system is a device intended to measure free (not protein bound) thyroxine (thyroid hormone) in serum or plasma. Levels of free thyroxine in plasma are thought to reflect the amount of thyroxine hormone available to the cells and may therefore determine the clinical metabolic status of thyroxine. Measurements obtained by this device are used in the diagnosis and treatment of thyroid diseases.(b)
Classification. Class II.