Commutability of Reference Materials for a-fetoprotein in Human Serum

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1 Commutbility of Reference Mterils for -Fetoprotein in Humn Serum Yuhong Yue, MM; Shunli Zhng, MD; Zhenzhen Xu, MM; Xi Chen, BSc; Qingto Wng, MM Context. Relible quntifiction of -fetoprotein (AFP) is criticl for clinicl dignosis. Accurcy in AFP nlysis relies on trcebility to reference mterils with confirmed commutbility. Objective. To ssess the commutbility of the reference mterils for AFP. We screened for pproprite reference mterils for the clibrtion of clinicl AFP nlysis nd for ppliction in n externl qulity ssessment scheme. The fesibility of using wter to dilute reference mteril from the World Helth Orgniztion ws lso evluted. Design. Ptient serum smples with vrious levels of AFP were rndomly interspersed mong AFP reference mterils from the World Helth Orgniztion, the Beijing Center for Clinicl Lbortories, nd Beijing Controls nd Stndrds Biotechnology nd qulity controls from Bio- Rd. The smples were nlyzed on 5 different pltforms to ssess the comprbility of the results nd commutbility of the reference mterils. Results. Significnt vritions in AFP mesurement were observed mong the 5 instrument pltforms. The Beijing Center for Clinicl Lbortories nd Beijing Controls nd Stndrds Biotechnology reference mterils were commutble cross ll the instrument pltforms. The World Helth Orgniztion AFP 72/225 reference mteril diluted with distilled wter ws lso commutble t high concentrtions. The Bio-Rd qulity control mterils for AFP were commutble mong 4 out of 5 instrument pltforms. Conclusions. Our results suggested tht the Beijing Center for Clinicl Lbortories nd Beijing Controls nd Stndrds Biotechnology mterils were commutble cross ll 5 instrument pltforms, wheres the Bio-Rd qulity controls were limited by the concentrtion of AFP nd the instrument pltforms used. Cution needs to be tken in using wter to dilute the World Helth Orgniztion 72/225 reference mteril becuse its commutbility is limited to high concentrtions. (Arch Pthol Lb Med. 2017;141: ; doi: /rp OA) etoprotein (AFP) is one of the most commonly used -F mrkers in the clinicl dignosis of cncer. Its expression is highly specific for liver, nd it is commonly used in screening of high-risk popultions for primry liver cncer nd in dignosis nd monitoring of therpeutic outcome nd tumor recurrence. In ddition, AFP levels in mternl blood nd mniotic fluid re used for screening of developmentl bnormlities in the fetus. Accurte mesurement of AFP, therefore, weighs hevily Accepted for publiction Jnury 19, Published s n Erly Online Relese August 2, Supplementl digitl content is vilble for this rticle t www. rchivesofpthology.org in the October 2017 tble of contents. From the Deprtment of Clinicl Lbortory, Beijing Center for Clinicl Lbortories, Beijing Choyng Hospitl, Beijing, Chin (Drs Yue, Zhng, nd Wng nd Ms Chen); nd the Deprtment of Clinicl Lbortory, Beijing Choyng Hospitl Jing-Xi Cmpus, Cpitl Medicl University, Beijing, Chin (Dr Xu). Drs Yue nd Zhng contributed eqully to this mnuscript. The uthors hve no relevnt finncil interest in the products or compnies described in this rticle. This study ws supported by the Ntionl High Technology Reserch nd Development Progrm of Chin (2011AA02A116) nd the Ntionl Clinicl Key Specilty Construction Project. Reprints: Qingto Wng, MM, Deprtment of Clinicl Lbortory, Beijing Center for Clinicl Lbortories, Beijing Choyng Hospitl, CpitlMediclUniversity,No.8GongtiSRd,ChoyngDistrict, Beijing , Chin (emil: wqt36@163.com). in decision mking for dignosis nd ptient mngement. 1 Achieving ccurcy within clinicl meningful limits is chllenging, s mesurement procedures nd instruments in different clinicl lbortories differ. Though interntionl stndrds hve helped in the stndrdiztion nd hrmoniztion of clinicl mesurement, their use is rther limited to certin mesurnds becuse of the lck of commutbility. Accurcy in AFP nlysis relies on trcebility to interntionl stndrd mterils becuse of the unvilbility of reference mesurement procedure. The AFP reference mteril 72/225, provided by the World Helth Orgniztion (WHO), hs been used for clibrtion by the mjority of instrument pltforms; however, the commutbility of this reference mteril is yet to be confirmed. Prticipting in externl qulity ssessment schemes (EQAS) helps improve the relibility of the mesurement of AFP. Results of EQAS from the College of Americn Pthologists, the Beijing Center for Clinicl Lbortories (BCCL), nd Stichting Kwliteitsbewking Medische Lbortoriumdignostiek of the Netherlnds indicte significnt vribility in the mesurement of AFP using different instrument pltforms. 2 This suggests tht commutbility of the EQA mterils nd the WHO reference stndrd is questionble, nd this my potentilly ffect the comprbility of the results. Arch Pthol Lb Med Vol 141, October 2017 Commutbility of Reference Mterils for AFP Yue et l 1421

2 Tble 1. Comprbility of the Anlyticl Results of -Fetoprotein on 5 Instrument Pltforms Architect Cobs DXI Centur A2000 Medin, ng/ml Minimum, ng/ml Mximum, ng/ml Residul (S res ) Correltion coefficient (r) Slope (b) Intercept () CV% Abbrevition: CV, coefficient of vrition. The 5 instruments included Architect (Abbott Dignostics, Lke Forest, Illinois), Cobs (Roche Dignostics, Indinpolis, Indin), DXI (Beckmn Coulter, Bre, Cliforni), Centur (Siemens, Erlngen, Germny), nd A2000 (Autobio, Zhengzhou, Chin). A totl of 24 humn serum specimens were included. In the present study, the comprbility of AFP mesurements using 5 different instrument pltforms ws evluted using 24 humn serum specimens. Reference mterils from WHO, BCCL, nd Beijing Controls nd Stndrds Biotechnology (BCSB) nd qulity control mterils from Bio-Rd (Hercules, Cliforni) were used in the commutbility study so s to screen for pproprite reference mterils for EQAS. In ddition, the min fctors contributing to the vrition were explored to guide further stndrdiztion. MATERIALS AND METHODS Ptient Smples Twenty-four humn serum specimens with low, medium, nd high levels of AFP were collected from Beijing Cho-yng Hospitl, Cpitl Medicl University (Beijing, Chin), fter pprovl from its humn ethics committee (document ). All 24 specimens were nonhemolyzed nd nonlipemic, with AFP levels rnging from to ng/ml s determined using the Abbott Architect i2000 (Abbott Dignostics, Lke Forest, Illinois). For every serum smple, minimum of 2.5 ml ws collected, mixed, liquoted into 5 prts, nd stored t 808C until nlysis. Reference Mterils Stndrd mterils for AFP nlysis included WHO 72/225 ( ng/ml), purchsed from Ntionl Institute for Biologicl Stndrds nd Control (Hertfordshire, United Kingdom); 3 cndidte mterils with different concentrtions of AFP from BCCL (Beijing, Chin); certified AFP stndrd (GBW: E090088) from BCSB (Beijing, Chin); nd 3 qulity control mterils with low, medium, nd high levels of AFP supplied by Bio-Rd Lyphochek Tumor Mrker Plus Control (54581, 54582, nd 54583, respectively). Instrument Pltforms nd Anlysis The 5 nlyticl systems used in this study included the Architect i2000sr Immunossy Anlyzer (Abbott Dignostics), the UniCel DXI 800 Immunossy System (Beckmn Coulter, Bre, Cliforni), the Cobs e601 Anlyzer (Roche Dignostics, Indinpolis, Indin), the Advi Centur XP Immunossy System (Siemens, Erlngen, Germny), nd the Lumo A2000 Immunossy System (Autobio, Zhengzhou, Chin), ech with the mnufcturer s own regents nd clibrtors. The instruments were clibrted ccording to ech mnufcturer s specifictions in the instructions for use. In ddition, the WHO stndrd mteril 72/225 ws diluted by distilled wter, which is commonly used in the dilution of the clibrtors, into 10 different levels (966.2, 724.1, 482.9, 241.7, 180.8, 120.7, 60.5, 30.1, 18.1 nd 6.0 ng/ml). These stndrds, together with the 3 reference mterils from BCCL, 1 from BCSB, nd 3 qulity control mterils from Bio-Rd, were rndomly interspersed mong the 24 ptient serum smples. The AFP levels in these smples/stndrds were then mesured in the sme nlyticl sequence in triplicte using the 5 different instrument pltforms to ssess the possibility of using wter to dilute this widely used stndrd. Sttisticl Anlysis Microsoft Excel 2010 (Microsoft, Redmond, Wshington) ws used to process the dt, using formuls provided in the Clinicl nd Lbortory Stndrds Institute (CLSI) guideline EP30-A (formerly CLSI C53-A). 3 Results from 1 pltform were individully compred with those from the other 4 systems, nd generl liner regression nlysis ws performed to clculte the slopes (b), intercepts (), correltion coefficient (r), nd residuls (S res ). The coefficient of vrition (CV) of the verge of ll 24 humn serum specimens cross ll 5 pltforms ws used s stndrd. If 2CV ws lower thn the llowble totl error (TE), the results were regrded s comprble. Anlysis of vrince (rndomized block Tble 2. Commutbility of Reference Mterils BCSB b BCCL c Bio-Rd d Test Mterils Assy Pir 72/225 e WHO Architect-Cobs Architect-DXI Architect-Centur Architect-A Cobs-DXI Cobs-Centur Cobs-A DXI-Centur DXI-A Centur-A Abbrevitions: BCCL, Beijing Center for Clinicl Lbortories; BCSB, Beijing Controls nd Stndrds Biotechnology; WHO, World Helth Orgniztion. 1 nd 0 denote commutble nd noncommutble, respectively. Commutbility ws ssessed by compring nlyticl results obtined from 5 instrument pltforms: Architect (Abbott Dignostics, Lke Forest, Illinois), Cobs (Roche Dignostics, Indinpolis, Indin), DXI (Beckmn Coulter, Bre, Cliforni), Centur (Siemens, Erlngen, Germny), nd A2000 (Autobio, Zhengzhou, Chin). b BCSB (Beijing, Chin). c Three levels of BCCL (Beijing, Chin) stndrd were tested t 27.8, nd ng/ml, respectively. d Bio-Rd (Hercules, Cliforni) Lyphochek Tumor Mrker Plus Control for -fetoprotein (seril Nos , 54582, nd 54583) were tested. e Only low-vlue (6.0 ng/ml) mterils were tested for the WHO 72/ 225 (Ntionl Institute for Biologicl Stndrds nd Control, Hertfordshire, United Kingdom) stndrd Arch Pthol Lb Med Vol 141, October 2017 Commutbility of Reference Mterils for AFP Yue et l

3 Tble 3. Bises of the Mesurements (% of Certified Vlues) on Different Instrument Pltforms When World Helth Orgniztion (WHO) Stndrd 72/225 Ws Diluted AFP, ng/ml Architect b Architect c Cobs b Cobs c DXI b DXI c A2000 b A2000 c Centur b,d NA Abbrevitions: AFP, -fetoprotein; NA, result ws out of nlyticl mesurement rnge. World Helth Orgniztion stndrd 72/225 supplied by Ntionl Institute for Biologicl Stndrds nd Control (Hertfordshire, United Kingdom). Mnufcturers of instruments: Architect, Abbott Dignostics (Lke Forest, Illinois); Cobs, Roche Dignostics (Indinpolis, Indin); DXI, Beckmn Coulter (Bre, Cliforni); Centur, Siemens (Erlngen, Germny); nd A2000, Autobio (Zhengzhou, Chin). b Distilled wter ws used s dilution gent. c The dilution gent provided by the instrument supplier ws used. d Centur does not supply dilution gent. design) ws performed to ssess the significnce in vrition mong pltforms using SPSS (version 17.0; SPSS, Chicgo, Illinois). The commutbility of AFP reference mterils ws evluted by Deming regression, which ws crried out following CLSI guideline EP30-A, to obtin the regression nd the 95% prediction intervls. The reference mterils tht fell within the 95% prediction intervls were regrded s commutble between 2 instrument pltforms. 3,4 To estimte the mtrix-relted bises for noncommutble mterils, the predicted logrithms were bck trnsformed nd the vritions of mesured vlues from the predicted vlues were clculted. 4 Effect of the diluents distilled wter nd the provided diluents from individul instrument suppliers on the ccurcy of AFP mesurement ws ssessed using WHO AFP stndrd 72/225. The certified concentrtions of AFP nd the detected levels on individul pltforms were compred to clculte the percentge devitions nd liner regression. RESULTS Comprbility of the Anlyticl Results of AFP The levels of AFP in 24 ptient serum smples determined by the 5 different instrument pltforms re summrized in Tble 1 nd detiled in Supplementl Tble 1 (see supplementl digitl content contining 3 tbles t www. rchivesofpthology.org in the October 2017 tble of contents). The medin vlues of the 24 smples were clculted for individul nlyticl systems. The results suggested tht the mesurements of AFP levels by the Cobs pltform were generlly high, nd those by the A2000 pltform were low. The verge CV in the detected vlues for ll 24 smples on 5 pltforms ws 12.7% (Supplementl Digitl Content Tble 2); thus, 2CV ws 25.4%, which ws higher thn TE of 21.9%. The TE ws selected bsed on the biologicl vrition. 5 The results suggested significnt vritions in the detected vlues of AFP cross different pltforms (P,.05). To evlute the performnce of individul instrument pltforms, the detected vlues for 24 smples were compred between 2 systems, covering ll possible combintions, by liner regression nlysis. Thus, for ech pltform, 4 pirs of comprisons were performed nd verge S res, r, b, nd vlues clculted (Tble 1; Supplementl Digitl Content Tble 3). The dt indicted pprent system devitions (2%~13%) mong 5 pltforms with verge residuls rnging from to Commutbility of Reference Mterils The vritions in the nlyticl results of AFP on 5 commonly used instrument pltforms interntionlly necessitted the screening for reference mterils, which idelly should mnifest robust equivlences of the mthemticl reltionships mong the results obtined using different mesuring protocols. We thus tested reference mterils from BCSB, BCCL, Bio-Rd, nd WHO, nd their commutbility scores re detiled in Tble 2. Commutbility ws evluted by pirwise comprison between ny 2 mesurement procedures of the mesured vlues of the reference mterils with known AFP concentrtions, nd the plots re shown in Figure 1, A through J. The results indicted tht the reference mterils from BCCL nd BCSB were commutble cross ll 5 instrument pltforms, with 10 commutble mrks out of 10 pirwise comprisons (Tble 2; Figure 1, A through J). The Bio-Rd reference mteril ws commutble with 4 of the 5 pltforms tested (Tble 2; Figure 1, A, B, C, E, F, nd H). The WHO AFP stndrd 72/225 diluted with distilled wter showed substntil commutbility t high concentrtion except t the low level (6.0 ng/ml), nd cceptble equivlence in its mesured vlues ws limited to 3 comprison pirs (Tble 2; Figure 1, E, F, nd I). The mtrix bis rnge ws 20% to 66%. Effects of the Diluents on the Anlyticl Results of AFP Poor concordnce of the clinicl nlysis results from different mesurement procedures cn lso be ttributed to the mtrix differences for the reference mterils, the diluents used, nd the impct of dilution on the mesurnd in solution. 6 We therefore exmined the effects on the mesurnds cused by dilution using distilled wter or diluents provided by the suppliers of individul instrument pltforms. The WHO AFP stndrd 72/225 ws used in this nlysis. The impcts were ssessed by clculting the devition of the mesured vlues from the certified levels, nd the verges of 3 replictes re shown in Tble 3. The Arch Pthol Lb Med Vol 141, October 2017 Commutbility of Reference Mterils for AFP Yue et l 1423

4 Figure 1. Commutbility of reference mterils for -fetoprotein (AFP) on 5 instrument pltforms. The unit for both x- nd y-xis is log (1003 ng/ ml). Humn serum smples (blck circles) include 24 specimens. Mterils re s follows: Beijing Controls nd Stndrds Biotechnology (BCSB) mteril (green tringles) is qulity control products from BCSB (Beijing, Chin); Beijing Center for Clinicl Lbortories (BCCL) mteril (red tringles) is mteril prepred by BCCL (Beijing, Chin); Bio-Rd mteril (blck crosses) is 3 qulity control smples from Lyphochek Tumor Mrker Plus Control (Bio-Rd, Hercules, Cliforni); nd World Helth Orgniztion (WHO) mteril (blue squres) is WHO 72/225, purchsed from 1424 Arch Pthol Lb Med Vol 141, October 2017 Commutbility of Reference Mterils for AFP Yue et l

5 Tble 4. Mtrix-Relted Bises for Noncommutble Mterils Commutbility-Relted Bis, % Bio-Rd Mterils, Seril No. b WHO Assy Pir /225 c Architect-Cobs 25 Architect-DXI 27 Architect-Centur 20 Architect-A Cobs-DXI 66 Cobs-Centur 58 Cobs-A DXI-A Centur-A Abbrevition: WHO, World Helth Orgniztion. Empty cells denote commutbility between 2 instrument pltforms. Mnufcturers of instruments: Architect, Abbott Dignostics (Lke Forest, Illinois); Cobs, Roche Dignostics (Indinpolis, Indin); DXI, Beckmn Coulter (Bre, Cliforni); Centur, Siemens (Erlngen, Germny); nd A2000, Autobio (Zhengzhou, Chin). b Bio-Rd Lyphochek Tumor Mrker Plus Control for AFP (Hercules, Cliforni). c WHO stndrd 72/225 (6.0 ng/ml dilution; Ntionl Institute for Biologicl Stndrds nd Control, Hertfordshire, United Kingdom). results indicted obvious devitions in the mesured vlues obtined on ll 5 pltforms t certified level of 6.0 ng/ml. Using distilled wter s diluent generlly exggerted the devition, except for pltform A2000, which underestimted AFP concentrtions irrespective of the diluent used (Tble 3). At higher concentrtions of AFP, the Architect system overestimted the AFP level when the smples were diluted with distilled wter, but underestimted when its respective diluent ws used. Other instruments were not sensitive to the chnge of diluent (Tble 3; Figure 2, A through I). DISCUSSION The dignosis of some liver diseses, bnorml pregnncy, nd certin mlignncies relies on ccurte mesurement of serum AFP level. 7 Achieving comprble results regrdless of instrument pltforms nd mesurement procedures hs been fundmentl gol of lbortory medicine to void misdignosis. 1 The strtegy of compring 2 mesurement procedures hs been recommended by CLSI guideline EP09, 8 nd is prticulrly pplicble to scenrio in which vlidted mesurement procedure is used to evlute nother protocol. However, it is chllenging to use this scheme for mesurnd for which there is no high-order mesurement procedure vilble. Thus, in this study, we ssessed the comprbility of AFP mesurements on 5 instrument pltforms. Although nlyticl performnce specifictions bsed on direct or indirect clinicl outcomes re highly recommended for the ssessment of betweenmethod comprbility, these cn be ffected by current mesurement qulity, chrcteristics of the investigted popultion, nd helth cre setting. 9 Therefore, the desirble TE vlue ws bsed on biologicl vrition, which is more generl pproch to recommending nlyticl specifictions 5,10 : ech instrument pltform ws compred with 2 times the verge CV of the verge mesured vlues of ll smples on the 5 instrument pltforms to ssess the comprbility of nlyticl results. Although ll 5 mesurement procedures tested in this study re believed to hve clibrtion trceble to the WHO AFP stndrd 72/225, the 2CV vlue (25.4%) ws higher thn the desirble TE (21.9%), which ws clculted bsed on previously used formul 10 [TE ¼ CV I þ 0.25(CV I2 þ CV G2 ) 0.5 ]. Significnt system error ws observed in the mesured vlues nd poor correltions were found between pltforms. The poor comprbility is result not only of clibrtion, but lso of the questionble specificity of the mesurement procedures. Antibodies used by different suppliers my recognize different epitopes of AFP, s previously observed for other nlytes. 11,12 Comprbility of the clinicl nlysis relies on the stndrdiztion of the mesurement procedures, nd trcebility of the stndrd used in clibrtion is required. Poor commutbility of the reference mterils my result in noncomprble results cross different pltforms. The CLSI guideline EP14 recommends the evlution of protocol for mtrix effect bsed on reference procedure. 13 In this study, we selected 5 mesurement procedures commonly used for AFP nlysis nd ssessed the commutbility of reference mterils following the CLSI EP30-A protocol. 3,13 The results indicte tht the reference mterils from BCCL nd BCSB re commutble cross ll 5 procedures, representing idel reference mterils for future stndrdiztion of AFP mesurement nd for EQAS. However, the qulity control mterils from commercil suppliers, which hve been frequently used in EQAS, were found to be commutble only between certin mesurement procedures (Tble 4; Figure 1, A through C, E, F, nd H). Precise determintion of low levels of AFP in blood is prticulrly importnt for clinicl dignosis nd monitoring of therpeutic outcomes. 2 The commonly used reference mteril nd the interntionl stndrd for AFP, WHO 72/ 225, origintes from humn cord blood, 14 nd is prepred t high concentrtion ( ng/ml). Minimizing the impct of the dilution on AFP quntifiction determines the ccurcy of clibrtion of the mesurement procedures nd the nlysis of ptient smples. Our dt suggest tht dilution with distilled wter led to poor commutbility t low level (6.0 ng/ml) between different mesurement protocols, with mtrix bis of 20% to 66% (Tble 4; Figure 1, E, F, nd I). The diluents recommended by the supplier of the individul instrument pltforms were not idel, either, becuse of the devitions from the certified vlue observed. It is, therefore, importnt in clinicl nlysis to determine the mtrix effect on the commutbility of the WHO stndrd 72/225 in the rnge of concentrtions exmined. Ntionl Institute for Biologicl Stndrds nd Control (Hertfordshire, United Kingdom). Commutbility ws ssessed by compring nlyticl results from 2 instruments in the following pirings: A, Architect (Abbott Dignostics, Lke Forest, Illinois) versus Cobs (Roche Dignostics, Indinpolis, Indin). B, Architect versus DXI (Beckmn Coulter, Bre, Cliforni). C, Architect versus Centur (Siemens, Erlngen, Germny). D, Architect versus A2000 (Autobio, Zhengzhou, Chin). E, Cobs versus DXI. F, Cobs versus Centur. G, Cobs versus A2000. H, DXI versus Centur. I, DXI versus A2000. J, Centur versus A2000. Arch Pthol Lb Med Vol 141, October 2017 Commutbility of Reference Mterils for AFP Yue et l 1425

6 Figure 2. Comprison of detected nd certified vlues (ng/ml) of World Helth Orgniztion reference mteril 72/225 diluted with distilled wter (A, C, E, G, nd I) or dilution gents from the instrument supplier (B, D, F, nd H). Instruments used in the nlysis include Architect (A nd B; Abbott Dignostics, Lke Forest, Illinois), Cobs (C nd D; Roche Dignostics, Indinpolis, Indin), DXI (E nd F; Beckmn Coulter, Bre, Cliforni), A2000 (G nd H; Autobio, Zhengzhou, Chin), nd Centur (I; Siemens, Erlngen, Germny; Centur does not supply dilution gent) Arch Pthol Lb Med Vol 141, October 2017 Commutbility of Reference Mterils for AFP Yue et l

7 In summry, the stndrdiztion of AFP nlysis is yet to be ccomplished nd further studies re required to identify the optiml reference mterils tht re robustly commutble cross different mesurement procedures. The frozen mixture of humn serum exhibits better commutbility thn the cndidte mterils originted from other sources. If the WHO AFP stndrd 72/225 is used, pproprite diluents re needed to ensure ccurte clibrtion. References 1. Miller WG, Jones GR, Horowitz GL, Weykmp C. Proficiency testing/ externl qulity ssessment: current chllenges nd future directions. Clin Chem. 2011;57(12): Houwert AC, Lock MT, Lentjes EG. Alphfetoprotein in the Dutch Externl Qulity Assurnce progrmme: need for improvement. Ann Clin Biochem. 2012;49(pt 3): Vesper H, Emons H, Gnezd M, et l. Chrcteriztion nd Qulifiction of Commutble Reference Mterils for Lbortory Medicine: Approved Guideline. Wyne, PA: CLSI; Zhng S, Zeng J, Zhng C, et l. Commutbility of possible externl qulity ssessment mterils for crdic troponin mesurement. PLoS One. 2014;9(7): e Trpe J, Frnques J, Sl M, et l. Determintion of biologicl vrition of lph-fetoprotein nd choriogondotropin (bet chin) in disese-free ptients with testiculr cncer. Clin Chem Lb Med. 2010;48(12): Vesper HW, Miller WG, Myers GL. Reference mterils nd commutbility. Clin Biochem Rev. 2007;28(4): Houwert AC, Gilty JC, Lentjes EG, Lock MT. Hereditry persistence of lph-fetoprotein (HPAF P): review of the literture. Neth J Med. 2010;68(11): Peng Z, Mo J, Li W, Jing G, Zhou J, Wng S. Comprison of performnces of five cpillry blood collection tubes. Int J Lb Hemtol. 2015;37(1): Sndberg S, Frser CG, Horvth AR, et l. Defining nlyticl performnce specifictions: consensus sttement from the 1st Strtegic Conference of the Europen Federtion of Clinicl Chemistry nd Lbortory Medicine. Clin Chem Lb Med. 2015;53(6): Frser CG, Hyltoft Petersen P, Libeer JC, Ricos C. Proposls for setting generlly pplicble qulity gols solely bsed on biology. Ann Clin Biochem. 1997;34(pt 1): Rej R, Drke P. The nture of clibrtors in immunossys: re they commutble with test smples?: must they be? Scnd J Clin Lb Invest Suppl. 1991;205: Pnteghini M. Selection of ntibodies nd epitopes for crdic troponin immunossys: should we revise our evidence-bsed beliefs? Clin Chem. 2005; 51(5): Korzun WJ, Nilsson G, Bchmnn LM, et l. Difference in bis pproch for commutbility ssessment: ppliction to frozen pools of humn serum mesured by 8 direct methods for HDL nd LDL cholesterol. Clin Chem. 2015; 61(8): Griffiths BW, Pringle DN, Godrd A. Development of provisionl reference humn cord serum stndrd for lph-fetoprotein determintion. Clin Biochem. 1980;13(6): Arch Pthol Lb Med Vol 141, October 2017 Commutbility of Reference Mterils for AFP Yue et l 1427