Comparison of three parathyroid hormone assays

Transcription

1 Ann Clin Bichem 198; 23: Cmparisn f three parathyrid hrmne assays JIFR A ISBT Frm the Department f Chemical Pathlgy, St. Gerge's Hspital and Medical Schl, Lndn SW17, UK SUMMARY. The efficiency f three parathyrid hrmne (PTH) assays in the diagnsis f primary hyperparathyridism has been investigated. Tw assays used cmmercial reagents and measured the intact hrmne and middle regin f parathyrid hrmne and the third assay emplyed the antiserum AS 211/32. Althugh the intact and mid-mlecule assays shwed a greater increase in PTH abve nrmal than the labratry PTH assay, patients with prven primary hyperparathyridism were nt distinguished frm healthy subjects in every case. The intact and in huse assays gave elevated values in seven ut f nine patients with prven hyperparathyridism and the mid-mlecule assay in eight ut f nine assays. The intact PTH and in-huse assays were mre successful than the mid-mlecule methd in separating patients with primary hyperparathyridism frm thse with hypercalcaemia due t malignancy. Assays fr serum parathyrid hrmne (PTH) using cmmercial reagents have previusly shwn cnsiderable variatin in the frequency f elevated values in primary hyperparathyridism. I Likely causes are the hetergeneity f parathyrid hrmne in the circulatin and the fact that sme PTH immunassays include bilgically inactive fragments. ffrts have therefre been directed at increasing the specificity f PTH assays t see if this imprved their diagnstic capability. Recently, assays fr the detectin f intact PTH and the middle regin f PTH have been reprted.?: 3 The intact PTH assay is based n a tw-step immunchemical methd which shwed excellent separatin between nrmal and hyperparathyrid subjects with the lwest hyperparathyrid PTH value five times greater than the highest nrmal value.? xcellent sensitivity has als been fund fr assays detecting the middle regin f PTH. 3 Reagents fr these tw assays are nw available cmmercially. In this study their efficiency in the diagnsis f primary hyperparathyridism has been evaluated. Hypercalcaemic patients either suspected f having primary hyperparathyridism r with a knwn malignancy were investigated and the results cmpared with thse frm an in-huse assay in which the antiserum has been reprted t measure predminantly the amin-terminal f PTH. 4 Materials and methds Subjects studied were 19 healthy hspital staff wh were nrmcalcaemic and 45 hypercalcaemic patients with ttal calcium greater than 2 7 mmlll n at least ne previus ccasin. The hypercalcaemic grup cmprised nine patients with prven primary hyperparathyridism, 17 patients suspected f having primary hyperparathyridism and 19 patients with hypercalcaemia due t malignancy. The rati f male subjects t female subjects was 1:3 in the cntrl and hyperparathyrid grup and 1:1 in the malignant grup. Ages ranged frm 24-5 years, years and years in the cntrl, hyperparathyrid and malignant grups respectively. In the hyperparathyrid grup seven patients had renal calculi and the remainder either had clinical evidence f hyperparathyridism r had n bvius cause t accunt fr their hypercalcaemia. In the grup with hypercalcaemia f malignancy, tumurs were lcated in the lung (n=5), breast (n=5), prstate (n=l), cln (n=l) and bne (n=l). Three patients had myelma, ne had a squamus cell carcinma and in tw the site was unknwn. Bld was cllected int DTA tubes fr intact and mid-mlecule methds and int lithium heparin tubes fr the labratry assay. Plasma prtins were stred frzen within 30 min f cllectin f the bld samples. A 429

2 430 isbet further prtin f fresh plasma was assayed fr calcium' and albumin." The intact and mid-mlecule PTH kits were btained frm the Immun-uclear Crpratin (Stillwater, Minnesta, USA). The intact PTH methd used a sample vlume f 2 5 ml and invlved immunextractin and cncentratin with slid phase antibdies directed against hpthl-34, fllwed by radiimmunasay f eluted PTH with anti-mid-mlecule antiserum. The mid-mlecule assay emplyed an antiserum sensitive t the 44-8 regin f hpth and bpth37-84 as tracer. ach prcedure was cmpleted In ne day. The in-huse assay emplyed the antiserum AS 211/32 at a final dilutin f , 1st IRP hpth as standard, bth supplied by the atinal Institute f Bilgical Standards and Cntrl (Lndn W3) and PTH tracer frm Burrughs Wellcme. A nn-equilibrium system was used with incubatin f samples with antiserum fr 3 days at 4 C, fllwed by a further 3-day incubatin with tracer (3000 cpm) and a plyethylene glycl accelerated secnd antibdy separatin f the bund and free fractins. PTH was estimated by the in-huse methd as requests were received and by the intact and mid-mlecule methds at intervals ver a -mnth perid. During similar perids n trends have been detected in PTH values f human plasma stred frzen when measured by the in-huse methd. All assays were carried ut in LP3 plystyrene tubes (Luckam Ltd, Sussex, UK) (SB <2% fr all assays). Tw quality cntrl materials were prepared. A nrmal range cntrl was btained by cllecting bld frm a nrmcalcaemic subject, with either lithium heparin r DTA as anticagulant, the plasma frzen and prtins included in each batch alng with the kit quality cntrl sera. A high quality cntrl was als prepared by supplementing pled serum with human PTH standard fr the intact and midmlecule methds and bvine PTH standard fr the in-huse methd. The PTH data (mean and standard deviatin) btained fr the nrmal quality cntrl were 5 5±0 8 pml/l (n=15), 48±7 3 pml/l (n=15), and 0 37±0 037!Jg/L (n=28) fr the intact, midmlecule and in-huse methds respectively. Fr the high cntrl, mean and standard deviatin were 41 1±4 1 pml/l (n=), 110 8± 7 pml/l (n=4), and 0 83±0 08!JglL (n=35), fr the intact, mid-mlecule and inhuse methds respectively. The quality cntrl supplied with the kits gave results within u.5...; :l..c:.5..l :::. :l 'u "@u 1i: ac ac ",co O.... C!'<:"l 01, \C r- ac <Ii "t:l..c: U Q en + c: "C.. OIl c: "@ Ẹ.. c:. ::J.cU.. "C ::l C"

3 Cmparisn f three PTH assays 431 the manufacturer's quted ranges. Fr the intact methd fur values fr quality cntrl, assayed nly in the radiimmunassay stage f the assay ranged between +24% and +47% f the mean, and tw values fr quality cntrl, supplied with later kits and assayed thrughut the entire methd, were + 1% and +20% frm the mean value. In the case f the mid-mlecule methd, kit quality cntrl results ranged between -9% and + 14% f the mean value. Ttal calcium was adjusted fr albumin in samples with albumin values belw 40 gil accrding t an expressin derived fr ur methds [i.e. crrected calcium ttal calcium + (0 01 x (40 - albumin)]. In samples with albumin values greater than 40 gil, the adjustment was cnsidered unnecessary [crrected calcium = ttal calcium (O 007x(albumin -40)]. Results PTH data btained by all methds in nrmal and hypercalcaemic subjects are shwn in Table.I. Mean PTH values by the kit methds in the surgically-prven grup were six t seven times higher than mean values in the nrmal grup and three times higher in the in-huse methd. In the grup with hypercalcaemia f malignancy, mean PTH values were less than thse in the nrmal grup fr the intact and in-huse methds but slightly higher in the mid-mlecule methd. The incidence f PTH values abve the nrmal range fr each methd is shwn in Table 2. In the prven hyperparathyrid grup, tw patients with renal calculi and in ne case hypercalcaemia lasting 10 years, had nrmal intact and in-huse PTH values. In ne patient, calcium was 3 19 mml/l, intact PTH 7 8 prnl/l, mid-mlecule PTH 74 pml/l, and in-huse PTH value 0 29 ug/l, and in the ther calcium, intact, mid-mlecule and in-huse PTH values were 3 07 mml/l, 7 8 pml/l, 175 pml/l and 0 29 ug/l respectively. In the suspect hyperparathyrid grup, eight patients had elevated PTH values by bth intact and mid-mlecule methds. One patient was cnsidered t have primary hyperparathyridism, and three thers were still underging investigatin. One patient admitted fr remval f a neurfibrma was fund t be hypercalaemic. A rare assciatin between primary hyperparathyridism and neurfibrma has been previusly reprted." Tw patients died, ne f whm had calcium 3 21 mml/l, intact PTH 17 3 prnl/l, mid-mlecule PTH 210 pml/l and in-huse PTH 0 31 ug/l and n evidence f parathyrid adenmas r hyperplasia at pst-mrtem. Tw patients in this grup had elevated PTH values by the mid-mlecule methd, and nrmal values by the intact methd. One f these with mid-mlecule PTH 190 pml/l and intact PTH 7 8 pml/l als had nrmal parathyrid glands at pst-mrtem. Only three patients in this grup had in-huse PTH results abve the upper limit f the nrmal grup. Seven patients had nrmal PTH results by all methds. Three f these patients had renal calculi, and ne had evidence f hypercalcaemia 10 years previusly. In the malignancy grup, ne patient with carcinma f the lung had intact PTH 11 5 pml/l, mid-mlecule PTH 19 pml/l, and in-huse PTH 0 29 ug/l, but died befre any further investigatin culd be carried ut. With the exceptin f this patient, there was little verlap between the hyperparathyrid and the hypercalcaemia f malignancy grups in the case f the intact and in-huse methds. These tw grups were less clearly distinguished by the mid-mlecule methd as 14 ut f 19 patients had PTH values which verlapped with thse in the suspected hyperparathyrid grup. Discussin In this study the assays measuring intact and middle regin f PTH shwed a greater increase in PTH abve nrma', than the in-huse assay. Hwever, despite this, the intact assay did nt identify any mre cases f primary TABL 2. Incidence f hyperca1caemic patients with PTH values abve the nrmal range Methd Intact Mid-mlecule In-huse Prven hyperparathyrid Suspect hyperparathyrid Malignancy 7/9 H/17 IIIH H/9 10/17 3/19 7/9 3/17 0/19

4 432 isbet hyperparathyridism than the in-huse assay. Further the separatin between primary hyperparathyrid and nrmal subjects in the case f intact PTH assay was nt as gd as expected.i but this may nly reflect the lesser severity f disease in the patients studied as cmpared with Lindall et al. 2 The mid-mlecule assay assigned elevated PTH values t three patients in the hyperparathyrid grup wh were fund t be nrmal by the intact methd. On the ther hand, there was an increased incidence f elevated values in patients with hypercalcaemia f malignancy s that the apparent increase in sensitivity is prbably less useful diagnstically. One patient with hypercalcaemia due t malignancy appeared t have ectpic PTH secretin as shwn by an increase in intact and mid-mlecule PTH values. Otherwise the separatin between the hyperparathyrid and malignancy grups in the intact and in-huse methds was gd. This suggests that the tw patients in the prven grup with nrmal values by these methds and the majrity f patients in the suspect hyperparathyrid grup had intact and in-huse PTH values which were inapprpriately elevated fr the plasma calcium value. The verall precisin f the kit assays as measured by the labratry quality cntrl was nt as gd as the manufacturer's perfrmance data. Quted cefficients f variatin fr the intact and mid-mlecule assays were 7% and 9 4% respectively. There was n trend in the intact PTH values f the labratry quality cntrl during the study s that instability f the material is unlikely t be the cause f the imprecisin. Kit quality cntrl results fr the intact assay indicated that the intact PTH values were high, althugh all the values fr the nrmal grup were well belw the manufacturer's upper limit. The specificity f the antiserum AS 211/32, used in the in-huse assay, has been reprted t be directed predminantly twards the aminterminal f PTH,4 bth the amin and middle regins f PTH M and t depend n the nature f tracer used. M Studies with gel chrmatgraphy f uraemic serum indicated that AS 211/32 recgnised nly intact PTH.') In this investigatin, PTH results frm the in-huse assay crrelated better with thse frm the intact assay than with the mid-mlecule assay. xperience f ther wrkers with these cmmercial assays seems t be limited as judged by published data. levated midmlecule PTH values, 2-3 times the upper limit f nrmal, were btained in fur ut f fur patients with primary hyperparathyridism. III The mid-mlecule assay was fund t be n better than the C-terminal assay in the differential diagnsis f hypercalcaemia. II In cnclusin, assays using cmmercial reagents measuring intact and mid-mlecule PTH levels, did nt discriminate cmpletely in this study between healthy subjects and patients with prven primary hyperparathyridism. As frequently reprted by ther wrkers, the separatin f these tw grups is likely t be imprved, particularly fr the intact PTH methd, if the PTH result is related t the plasma calcium value. Acknwledgements I am grateful t all wh helped with the cllectin f bld samples and t the staff f the Department f Chemical Pathlgy, St Gerge's Hspital, fr their assistance with this study. References Raisz LG, Chaitanya HY, Bckman RS, Bwer BF. Cmparisn f cmmercially availahle parathyrid hrmne immunassays in the differential diagnsis f hypercalcaemia due t primary hyperparathyridism r malignancy. Ann lnt Med 1979; 91: Lindall AW, lting J, lls J. Rs BA. stimatin f bilgically intact parathyrid hrmne in nrmal and hyperparathyrid sera hy sequential -terminal immunreactin and mid-regin radiimmunassay. J Clin ndcrinl Metab 1983; 57: Mallette LC, Tuma S, Berger R. Kirkland JL. Radiimmunassay fr the middle regin f human parathyrid hrmne using an hmlgus antiserum with a carhxy-terminal fragment f bvine parathyrid hrmne as radiligand. J Clin ndcrinl Metab 1982; 54: W J, Singer FR. Radiimmunassay fr human parathyrid hrmne. Clin Chim Acta 1974; 54: Technicn methd. S4-()(KJ3FJ4, Septemher Pinnell A, rtham B. ew autmated dyehinding methd fr serum alhumin determinatin with brmcresl purple (mdified), Clin Chem 1978; 24: Freimanis MG, Rdgers RW, Sarnaan A. eurfibrmatsis and primary hyperparathyridism. Suth Med J 1984; 77: Mallette L. Sensitivity f the anti-hvine parathrid serum 211/32 t synthetic fragments f human parathyrid hrmne. J Clin ndcrinl Metah 1980; 50:

5 Cmparisn f three PTH assays Oldham SB, Finck J, Singer FR. Parathyrid hrmne clearance in man. Metablism 1978; 27: Francini G, ami R, Gnnelli S, Grppa A, Mntagnani M, Gennari C. Radiimmunassay fr -terminal, C-terminal and middle regin f human parathyrid hrmne in bne diseases. Quad Sclav Diagn 1982; 18: Ward GJ, Richards RJ, Kunze H. Cmparisn f carbxyl terminal and middle mlecule parathrmne assays (abstract). The Clinical Bichemist 1983; 4: 94. Accepted fr publicatin vember 1985