standardization of thyroid function tests, 05 05 - 2017

Standardization of Thyroid Function Tests

Ola H. ElgaddarMD, PhD, MBA, CPHQ - Lecturer of Chemical Pathology

Medical Research Institute - Alexandria [email protected]

Clinical laboratory tests performedby different laboratories havecomparable results

1. Imprecision +2.systematic differences in method results, which

can arise from inherent differences in method design +++ (Very obvious in immunoassays)

Is this a problem??

Immunoassays are critical to many clinicaldecisions, and many parameters are presentin several guidelines…..... Assuming thatthere is no difference between test resultsassayed in different settings!

UK NEQAS [Edinburgh], Department of Laboratory Medicine, Royal Infirmary of Edinburgh, Edinburgh EH16 4SA, UK

Harmonization of Measurements

Any process that enables the establishmentof equivalence of reported values producedby different measurement procedures for thesame measurand, (i.e. the quantityintended to be measured)

Harmonization is achieved via:

1) Method #1 = Standardization:

The conventional approach which dependson the application of a well-understoodreference measurement procedure (RMP)and commutable reference materials toestablish the calibration for each availableroutine measurement procedure.

Commutability

The ability of a reference material to haveinter-assay properties comparable to theproperties demonstrated by authenticclinical samples when measured by morethan one measurement procedure

The core component of standardization isthe “establishment of metrologicaltraceability” which requires the availability ofan ISO conform reference measurementsystem (RMS)

Components of ISO conform RMS:

ØDefined measurand,ØUnits for expression of measurement

results (ideally the Système Internationald’Unités or SI),

ØDifferent hierarchical levels of materialsand measurement procedures.

Metrological Traceability

Accordingly, the provided lab results in thelower part of the hierarchy (routine IVDmethods), will provide measurementstraceable to the top of the hierarchy, withinstated uncertainty constraints.

Joint Committee for Traceability in Laboratory Medicine (JCTLM)

Harmonization is achieved via:

2) Method #2:Considered only when RMPs are notavailable and is based on application ofsound statistical methodology to producearbitrary equivalence of otherwise disparateresults generated by the various non-standardized routine measurementprocedures (i.e: surrogate RMPs)

ØThyroid function tests(TFTs) are among the mostcommonly used tests inboth primary and secondaryhealthcare settings

ØMany factors affect theinterpretation, and hencethe clinical decision to betaken based on the TFTsresults.

ØThese factors could berelated to the physiology ofthe hypothalamic – pituitary– thyroid axis, and hencethe distribution of TH in thethe tissue.

ØSome physiological / pathological /pharmacological cofounders could be present aswell affecting TH results….

ØAnd finally, the difference in TH results could beattributed to the lab itself due to a standardizationproblem

Till the end of this presentation, all datais obtained either from the C-STFTofficial website, or from their progressreports released periodically!

Aim of the project

The C-STFT aims at globalstandardization of thyroid function testingand works towards this with theinternational in vitro diagnostic (IVD)industry.

Aim of the project

The overall expected benefit:

ØProduce equivalent laboratory data sothat common reference intervals and/orclinical practice decision limits can beused;

ØInclude laboratory data in electronicpatient records;

Aim of the project

The overall expected benefit:

ØDevelop evidence-based clinicalpractice guidelines for application ofconsistent standards of medical care;

ØCombine laboratory data across studies totranslate research into patient care &disease prevention activities.

Total T3 & T4

Standardization of TT4 and TT3

The mission was easy as there is an existingSI- traceable reference measurementsystem that could be recommended by theC-STFT to the IVD industry forimplementation in accordance with the ISO17511.

Such RMS is composed of:qPrimary calibrators (IRMM 468 & 469)qReference methods (isotope dilution-mass

spectrometry, ID/MS)qReference laboratories offering

measurement servicesThe committee refers to the database of theJoint Committee for Traceability inLaboratory Medicine (JCTLM)

Free T3 & T4

Standardization of FT4 & FT3

When the C-STFT started its activities in2005, no reference measurement systemwas available for free thyroid hormones(FT4, FT3). Therefore, the C-STFT startedfrom scratch with defining the measurand,and proposing a reference measurementprocedure (RMP)

Definition of the measurand

According to the International Union of Pure andApplied Chemistry (IUPAC)/IFCC format, thecomponent was ‘‘T4 that is not bound to proteins’’(present in the water fraction of plasma or serum),the kind-of-quantity to measure the ‘‘amount-of-substance concentration’’ and the system ‘‘plasmaor serum at physiological conditions (pH 7.40,temperature 37.0°C)’’. The preferred unit forexpression of measurement results was pmol/L.

The conventional Reference Measurement Procedure (cRMP)

qIdeally, metrological traceability of assays shouldbe established with a trueness-based RMP askey element

qSince free thyroid hormones have to include aphysical step for separation of the free frombound fraction (by equilibrium dialysis (ED) orultrafiltration (UF), it is by no means possible tounequivocally demonstrate that the T4concentration in dialysate or ultrafiltrate isidentical to the true free hormone concentrationin the original sample


qTherefore, the C-STFT decided to proposean international “conventional” RMP(cRMP)

qIt should be based on ED and be combinedwith direct determination of the T4concentration in the dialysate with atrueness-based RMP utilizing ID-liquidchromatography/ tandem MS (ID-LC/tandem MS) as measurement principle.


This implied that according to this proposalthe measurand was operationally defined as“T4 in the dialysate from ED of serumprepared under defined conditions”. Theconvention did not apply to the ID-LC/tandemMS part of the cRMP, provided it is calibratedwith the certified T4 primary calibrator IRMM468, already available in the total T4reference measurement system

No CRM present when you search for FT4

Harmonization of TSH

qTSH is a glycoprotein circulating in thebloodstream as mixture of componentscomprising disease-specific glycoforms.

qCurrent TSH immunoassays all aretraceable to the same WHO IRP TSH80/558, and express their measurements inmIU/L.

Harmonization of TSH

qThe WHO IRP TSH 80/558 is from humancadaver pituitary and may compriseglycosylation forms that differ from thosetypically present in serum TSH. Therefore,the above issues of non commutabilityapplies, and hence, IRP-traceable TSHmeasurements do not give equivalent orinterchangeable results.

Before defining the TSH measurand

TSH measurement is mixture analysis. Thisimplies that each component in the mixturemust contribute significantly to thediagnostic application of the assay used formixture analysis, and that the latter shouldmeasure the components in the mixture inan “equimolar” way, but to the diagnosticrelevant extent.

Before defining the TSH measurand

qThis means that the important changes in overallconcentration of the mixture in health anddisease should outweigh the minor changes incomposition of the mixture in individual samples.

qThis requirement of equimolar measurement isof utmost relevance for TSH, since glycoformsare different in euthyroidism versus subclinicaland overt hypothyroidism, and because there isevidence that immunoassays may be sensitiveto the differences

Definition of TSH measurand

The C-STFT defined the measurand as“TSH, intact, total, glycosylationencountered in diagnostic applications whichshould be specified”.This definition requires in first instance thatTSH assays examined for harmonizationdeliver a measure for “total TSH”. It alsoimplies that harmonization should be basedon the glycoforms present in native samples.

The “All Procedure Trimmed Mean APTM” as surrogate RMP for TSH

ØCurrent TSH assays are expressed in IUs(concentrations in mIU/L). However, it is toexpect that one day it will be possible toestablish the relation between the IU and massunits (SI) for TSH.

ØFrom this point of view, the reference measurement system for TSH and its realization should be viewed as continuum from discovery of the analyte to final translation into the SI, with different needs at different stages of the continuum


ØTherefore, the C-STFT decided to preserve thetraceability of TSH assays to the WHO IRP intheir proposed harmonization process, andrecommends the “All Procedure Trimmed Mean”(APTM, expressed in IU) as surrogate RMP. It isproposed to statistically derive the APTM fromdata of a dedicated method comparison studywith native human samples, in which as manyTSH assays as possible participate


ØAccording to this proposal, the APTMtargeted panel becomes the key tool in theharmonization process.

ØIt has the big advantage that it is composedof samples that contain the typicalglycoforms of TSH in the blood circulation,thus are as commutable as possible.


ØAfter being assigned with the APTM(expressed in mIU/L) derived from themeasurement values of the WHO traceableassays that participated in the methodcomparison study, the panel is intendedto serve as a new measurementstandard, in other words, at that point inthe harmonization process, the IU of theWHO IRP is transferred to the panel.


Note that for the process to be successful,proof of sufficient correlation of theconcerned assays to the APTM is aprerequisite. Also the new standard shouldbe sustainable. This requires that the IU istransferred from the first panel to the follow-up panels via analogous method comparison

studies.

standardization of thyroid function tests, 05 05 - 2017

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