Holotranscobalamin — a sensitive marker of cobalamin malabsorption

European Journal of Clinical Investigation (1999) 29, 321–329

Holotranscobalamin – a sensitive marker of cobalaminmalabsorption

A. Lindgren* , A. Kilander† E. Bagge† and E. Nexø‡*Boras Central Hospital and †Sahlgrenska University Hospital, Gothenburg, Sweden, and‡KH Aarhus University Hospital, Denmark

Abstract Background No simple and reliable method of identifying patients with cobalamin malab-sorption is available at present. The measurement of plasma holotranscobalamin, i.e. themetabolically active cobalamins bound to the transport protein transcobalamin, has beensuggested as a means of fulfilling such criteria.

Design We describe a method that directly quantifies cobalamins attached to transcobala-min. The method is evaluated in patients referred for gastrointestinal examination because ofsuspected cobalamin malabsorption.

Results Of the 101 patients referred, all 48 with gastrointestinal conditions compatible withcobalamin malabsorption had plasma holotranscobalamin below 35 pmol L¹1 (interval of35–160pmol L¹1). None of the patients with plasma holotranscobalamin above the lowerreference limit had conditions compatible with cobalamin malabsorption.

Conclusion The values obtained for plasma holotranscobalamin showed a better correla-tion with possible malabsorption than the values obtained for plasma cobalamins. Thespecificity of the test, however, needs to be elucidated further.

Keywords Cobalamin, holotranscobalamin, homocysteine, methylmalonic acid, vitaminB12 deficiency.Eur J Clin Invest 1999; 29 (4): 321–329

Introduction

Identifying patients with conditions giving rise to cobala-min (vitamin B12) deficiency can be performed on twolevels. The first and most attractive approach, albeit lesscommon in clinical practice, is to identify patients withcobalamin malabsorption before they have become defi-cient. However, the second and most frequently usedapproach is to diagnose patients who have already devel-oped deficiency in the form of haematological signs andneurological disease. In these patients, the diagnosis can beverified by a decreased concentration of plasma cobalaminsor an increased concentration of the metabolites methyl-malonic acid [1,2] and homocysteine [3,4] in serum. The

metabolites accumulate when the two cobalamin-dependent reactions are impaired.

The traditional Schilling test diagnoses patients withsevere gastric body atrophy when intrinsic factor (IF)secretion has ceased. Patients subjected to previous partialgastric resections and patients with gastric body atrophybut with sufficient remaining IF production cannot bediagnosed using the Schilling test. Instead, a protein-bound absorption test must be used [5–7]. However, sucha test is laborious and is dependent on accurate urinecollection. Moreover, the sensitivity of the test is low [8].

To date, nobody has evaluated whether patients withcobalamin malabsorption can be identified by means of ablood sample, i.e. whether patients who are likely todevelop cobalamin deficiency at a later stage can bedetected by a simple test.

We have chosen measurement of the fraction of cobala-mins attached to the transport protein transcobalamin as acandidate for such an analysis. Only 6–20% of the plasmacobalamins are attached to transcobalamin (holotransco-balamin) [9–11]. This is the metabolically active fractionthat ensures the internalization of cobalamins to the cells ofthe body [9,12]. The remainder is attached to a protein,haptocorrin, the function of which is unknown. In 1988,

Q 1999 Blackwell Science Ltd

Departments of Internal Medicine, Boras Central Hospital (A.Lindgren), Boras, Sweden; Department of Internal Medicine,Sahlgrenska University Hospital, Gothenburg, Sweden(A. Lindgren, A. Kilander, E. Bagge); Department of ClinicalBiochemistry, KH Aarhus University Hospital, Denmark(E. Nexø).

Correspondence to: A. Lindgren, Department of InternalMedicine, Boras Central Hospital, S-501 82 Boras, Sweden.

Received 16 June 1998; accepted 30 October 1998

322 A. Lindgren et al.

Herzlich & Herbert [13] suggested that a decreased con-centration of plasma holotranscobalamin could be an earlyindicator of negative cobalamin balance. An extensiveclinical evaluation of this interesting hypothesis has, toour knowledge, not been performed, probably becausethe test has not yet been developed for routine use. Amajor problem has been that most methods described todate measure cobalamins bound to transcobalamin as thedifference between total plasma cobalamins and the amountof cobalamins attached to haptocorrin, i.e. the value isobtained by subtracting two large values from each other.

In this paper we describe a method that directly quan-tifies cobalamins attached to transcobalamin. The methodis used to study holotranscobalamin and transcobalaminsaturation in a group of patients examined extensivelyfor suspected cobalamin deficiency. We conclude thatmeasurement of holotranscobalamin may be a usefulmethod in the diagnosis of cobalamin malabsorption.

Methods

Patients were recruited from medical and neurologicalclinics, private practitioners and primary care centres inthe catchment area of Sahlgrenska University Hospital inGothenburg. All patients with suspected cobalamin defi-ciency and at least one plasma cobalamin concentrationvalue less than 200 pmol L¹1 were invited to take part in thestudy. Only patients with serum creatinine within theinterval of reference were included. The symptoms andsigns that gave rise to the suspicion of cobalamin deficiencywere evaluated only by the referring physicians and not bythe authors of this study. A total of 101 consecutive patientswere included (35 men and 66 women, mean and medianages 53 and 52 years, respectively, range 18–80 years). Thepatients arrived at our laboratory after fasting, and anupper gastrointestinal endoscopy was performed, bloodsamples were drawn, and the dual-isotope Schilling testwas started. Within 2 weeks of the examination the patientshad received five injections, each containing 1 mg of hydro-xocobalamin (Beheyan, Kabi Pharmacia, Sweden), intra-muscularly. New blood samples were drawn about 4 weeksafter the first examination.

As several drugs have been reported to interfere with theabsorption of cobalamins [14–16] and as antibiotics havebeen reported to reverse abnormal protein-bound absorp-tion test results [17], all patients in the study were askedabout their use of drugs during the period from the firstexamination at the referring unit until the first visit to ourlaboratory.

Laboratory tests

Plasma cobalamins (method I) used for the inclusion ofpatients in the study as well as for the evaluation of thepatients at first visit to our laboratory were determined withpurified hog intrinsic factor as the binder and an alkaline

pH, no-boil procedure for extraction (Diagnostic Products,Los Angeles, CA, USA). The coefficient of variation(CV) was 0·15 (mean ¼ 112 pmol L¹1) and 0·10 (mean ¼

580 pmol L¹1). The reference interval was 130–740 pmol L¹1.

Plasma cobalamins (method II) were determined usingan isotope dilution method using pure human intrinsicfactor as the binding protein and boiling at an acid pH inorder to extract the cobalamins from the binding protein[18,19]. This method was found to be more suitable for thecomparison with plasma holotranscobalamin (and holo-haptocorrin), because the determination of these concen-trations was based on the same method. The CV was 0·04(mean ¼ 311 pmol L¹1, n ¼ 24), 0·06 (mean 205 pmol L¹1,n ¼ 26), and the interval of reference used was 200–600 pmol L¹1 [20].

Plasma holotranscobalamin was determined by a mod-ification of the method described by Jacob et al. [21]. Thismodification has been described by Toft et al. [22]. Themodification involves measuring holotranscobalamindirectly and not as a value obtained by subtracting cobala-mins attached to haptocorrin from plasma cobalamins. Inbrief, 300 mL of silica FK 383 Philadelphia (Bie andBerntsen, Denmark) and 80 mg mL¹1 of 0·9% sodiumchloride was mixed with 600 mL of serum and 5 mL of0·9% sodium chloride. After 10 min the samples werecentrifuged for 10 min at 3200 ×g at 48C and the super-natant was discarded. The precipitate was dissolved in300 mL of 0·1 mol L¹1 phosphate buffer, pH 8·0, 0·1%human albumin, ORHA 20/21 (Behringwerke, Marburg,Germany), and the amount of cobalamins present wasdetermined using the method described above [18] with aslight modification in order to include silica gel in thecalibration standards. Silica precipitate was prepared in thesame way as the serum samples except that a phosphatebuffer was used instead of serum. The precipitate wasdissolved in 300 mL of calibration standard (0, 42, 83, 167,333, 500, 666 pmol L¹1) and further processed in parallelwith the serum samples. The concentration of holotransco-balamin in the serum samples was calculated as half thevalues read from the calibration curve. The CV of the assaywas 0·07 (mean¼ 38 pmol L¹1, n ¼ 10) and 0·05 (mean-¼ 73 pmolL¹1, n ¼ 10). The interval of reference was deter-mined to be 35–160 pmolL¹1 based on an analysis of twopopulations, a group of blood donors (n ¼ 51), and samplesobtained from a population study (n ¼ 96) [22].

Saturation of transcobalamin with cobalamins was cal-culated as the ratio between plasma holotranscobalaminand the sum of plasma holotranscobalamin and theunsaturated cobalamin binding capacity of transcobalamin.

The unsaturated cobalamin binding capacity of trans-cobalamin was determined as previously described [23].

Serum methylmalonic acid (MMA) was determinedusing a modification of a stable-isotope dilution techniquewith solid-phase sample extraction and gas chromato-graphy, using a mass spectrometer in the selected ion-monitoring mode [24]. The upper reference limit usedwas 0·4 mmol L¹1. The CV of the assay was 0·07 for therange of values determined.

Q 1999 Blackwell Science Ltd, European Journal of Clinical Investigation, 29, 321–329

Holotranscobalamin 323


Serum total homocysteine was measured using a modi-fication of a method described previously with reversed-phase high-performance liquid chromatography [25]. Inthe text the analysis will be designated serum homocys-teine. The upper reference limit used was 13 mmol L¹1.The CV of the assay was 0·05 (mean ¼ 21 mmol) and 0·10(mean ¼ 9 mmol).

All patients with an elevated serum methylmalonic acidand/or serum total homocysteine that normalized ordecreased significantly after treatment with cobalaminwere considered to have functional cobalamin deficiency.In all patients the reductions in the initial values aftertreatment were at least 45% for methylmalonic acid andat least 25% for homocysteine.

Schilling tests were performed using 1 mg of [57Co]-cyanocobalamin and 0·46 mg of [58Co]-cyanocobalamin(each containing approximately 20 kBq) as a modificationof a dual isotope test described by Doscherholmen et al.[5], and modified by Lindgren et al. [8] ([57Co]- and[58Co]-cyanocobalamin, Amersham International, UK)with the simultaneous measurement of the absorption of‘crystalline’, i.e. free, [57Co]-cobalamin and protein-bound[58Co]-cobalamin. Urinary excretion < 10% of the admi-nistered dose of labelled crystalline and < 5 % of protein-bound cobalamin was considered abnormal.

Morphological classification

The gastric body mucosa was classified according to theSydney system [26], with inflammation and atrophy gradedas absent, mild, moderate or severe. The duodenal mucosawas classified as normal or villous atrophy. A minimum ofthree biopsy specimens were taken from each location(duodenum and the gastric body). The biopsies from thegastric mucosa were paraffin embedded and those from theduodenum were embedded in plastic resin and all biopsieswere stained with haematoxylin–eosin and PAS (periodicacid–Schiff). The histological assessment of coded sectionswas made by the same pathologist.

Statistical analysis

The discriminant analysis, comparing the methods ofmeasuring plasma cobalamins and plasma holotranscoba-lamins, was made using a multivariate logistic regression.Student’s t-test was used for the comparison of meansbetween groups.

Results

Based on the morphological examination of gastrointest-inal biopsies and the Schilling test, the 101 patients wereclassified as having conditions compatible (n ¼ 48) or notcompatible (n¼ 53) with cobalamin malabsorption. Table 1summarizes the results obtained from the two groups of

patients. All parameters analysed showed the highestfrequency of positive results in patients with conditions com-patible with cobalamin malabsorption, but only holotransco-balamin showed a positive result in all of these patients.

The blood haemoglobin concentration was < 100 g L¹1

in only 2 out of 99 patients. The mean cell volume (MCV)was elevated (>102 fL) in 8 out of 98 patients, butdecreased $ 5 fL after treatment with cobalamin in onlyfour patients.

Patients with conditions compatible with cobalaminmalabsorption (Table 1 and Fig. 1)

All the patients with gastric body atrophy had plasmaholotranscobalamin <35 pmolL¹1 (Fig. 1a–c). The lowestvalues were observed in the group with severe gastric bodyatrophy.

All seven patients with villous atrophy of the duodenalmucosa had plasma holotranscobalamin <35 pmol L¹1

(Fig. 1d). Two of the patients had normal serum methyl-malonic acid and elevated serum homocysteine. Delayedresponse to cobalamin treatment and whole-blood folateconcentrations just above the lower reference limitindicated folate rather than cobalamin deficiency.

Two patients are not presented in Table 1 and Fig. 1.One had co-existing moderate gastric body atrophy andvillous atrophy of the duodenal mucosa, and one had beensubjected to partial gastric resection many years previously.They both had functional deficiency and plasma holo-transcobalamin <35 pmol L¹1 (18 and 28 pmol L¹1).

Patients with conditions not compatible withcobalamin malabsorption (Table 1 and Fig. 2)

All patients without atrophy of the gastric body mucosa,normal duodenal mucosa and normal Schilling testresults were grouped as ‘not compatible with cobalaminmalabsorption’.

Fourteen of the 27 patients with chronic body gastritiswithout proven atrophy had plasma holotranscobalaminvalues <35 pmol L¹1 (Fig. 2a).

Of the 26 patients with normal gastrointestinal histology,six had plasma holotranscobalamin values <35 pmol L¹1

(Fig. 2b). The mean value for this group (43 pmol L¹1) wassignificantly higher (P <0·05) than for the group withchronic gastritis (35 pmol L¹1), whereas there was nodifference as regards the Schilling and protein-boundabsorption tests between these two groups (Table 2).None of the patients in these two groups was taking drugsknown to interfere with the absorption of cobalamins.

Saturation of transcobalamin with cobalamins

The results of these determinations were virtually identicalto those for plasma holotranscobalamin in all the groupsdescribed. For this reason, a detailed account of theseresults is not presented.



Tab

le1.

Mor

phol

ogic

alfin

ding

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chill

ing

and

prot

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boun

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tion

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lts,

p-ho

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amin

,p-

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lam

ins

and

func

tion

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ficie

ncy

in99

*pa

tien

tsin

vest

igat

edfo

rsu

s-pe

cted

coba

lam

inde

ficie

ncy.

(a)

Pat

ient

sw

ith

cond

itio

nsco

mpa

tibl

ew

ith

coba

lam

inm

alab

sorp

tion

Sch

illin

gP

BA

TP

-hol

o-T

CP

-Cbl

P-C

blF

unct

iona

lM

orph

olog

ical

Pat

ient

ste

st<

10%

<5%

<35

pmol

L¹

1(m

etho

dI)

(met

hod

II)

defic

ienc

yfin

ding

s(n

)(%

)(%

)(%

)(<

130

pmol

L¹

1)

(<20

0pm

olL

¹1)

(%)

Gas

tric

body

atro

phy

Sev

ere

2665

7710

062

95†

65M

oder

ate

650

5010

00

8367

Mild

70

1410

028

33†

43

Vill

ous

atro

phy

ofth

edu

oden

alm

ucos

a7

7157

100

8610

0†10

0**

(b)

Pat

ient

sw

ith

cond

itio

nsno

tco

mpa

tibl

ew

ith

coba

lam

inm

alab

sorp

tion

Sch

illin

gP

BA

TP

-hol

o-T

CP

-Cbl

P-C

blF

unct

iona

lM

orph

olog

ical

Pat

ient

ste

st<

10%

<5%

<35

pmol

L¹

1(m

etho

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(met

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defic

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yfin

ding

s(n

)(%

)(%

)(%

)(<

130

pmol

L¹

1)

(<20

0pm

olL

¹1)

(%)

Gas

tric

body

gast

riti

sw

itho

utat

roph

y27

04

5226

60†

33N

opa

thol

ogic

alfin

ding

s26

04

2312

30†

15

PB

AT

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otei

n-bo

und

coba

lam

inab

sorp

tion

test

;P

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,pl

asm

aco

bala

min

s.*T

wo

furt

her

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wit

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ndit

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com

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ble

wit

hco

bala

min

mal

abso

rpti

onw

ere

exam

ined

butn

otlis

ted

inth

eta

ble:

one

wit

hco

-exi

stin

gm

oder

ate

gast

ric

body

atro

phy

and

villo

usat

roph

yof

the

duod

enal

muc

osa

and

one

pati

ent

subj

ecte

dto

part

ialg

astr

icre

sect

ion

man

yye

ars

prev

ious

ly(s

eete

xt).

**In

two

ofth

epa

tien

tsfo

late

defic

ienc

yw

asth

epr

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leca

use

ofth

efu

ncti

onal

defic

ienc

y.F

unct

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lde

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ncy

¼el

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rum

met

hyim

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icac

idan

d/or

hom

ocys

tein

eth

atno

rmal

ized

orde

crea

sed

cons

ider

ably

afte

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min

trea

tmen

t(s

eeM

etho

ds).

†Ow

ing

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elim

ited

amou

ntof

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ma

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anal

ysis

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perf

orm

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the

follo

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gnu

mbe

rof

pati

ents

(top

tobo

ttom

):21

,6,

6,23

,23

.



The protein-bound absorption test in relation togastric body histopathology

The protein-bound absorption test and the metabolite testsidentified 24 out of 39 (62%) of the patients with mild–severe gastric body atrophy, whereas the Schilling test

identified 20 out of 39 (51%). One patient with gastritiswithout proven atrophy had a low protein-bound absorp-tion test. She had elevated metabolite tests, low plasmacobalamins and a low plasma holotranscobalamin,12 pmol L¹1. She was being treated with clomipramin. Inaddition, one patient in the normal gastrointestinal histol-ogy group had an abnormal protein-bound absorption test.He had elevated metabolite tests, low plasma cobalaminsand plasma holotranscobalamin just above the lower limitof the interval of 38 pmol L¹1).

The clinical usefulness of plasma cobalamins andplasma holotranscobalamin

A discriminant analysis using logistic regression was carriedout to compare the ability of plasma cobalamins and plasmaholotranscobalamin, to identify conditions compatible/notcompatible with cobalamin malabsorption (Fig. 3). Therewas a statistically significant difference in favour of thedetermination of plasma holotranscobalamin comparedwith plasma cobalamin methods I and II in identifyingpatients with conditions compatible/not compatible with

Figure 1 Plasma holotranscobalamin (interval of 35–160 pmol L¹1) and functional cobalamin deficiency in (a) 26patients with severe atrophy of the gastric body, (b) six patientswith moderate atrophy of the gastric body, (c) seven patientswith mild atrophy of the gastric body and (d) seven patients withvillous atrophy of the duodenal mucosa. Functional cobalamindeficiency is defined as an increased level of serum methylmalo-nic acid and/or homocysteine that decrease upon injection ofcobalamin (see Methods).

Figure 2 Plasma holotranscobalamin (interval of 35–160 pmol L¹1) and functional cobalamin deficiency in (a) 27patients with chronic body gastritis without atrophy and (b) 26patients with no pathological findings.


cobalamin malabsorption (P<0·001). Moreover deter-mination of plasma cobalamins by method II showed abetter discrimination than plasma cobalamins determinedby method I (P <0·001).

Comparison of concentrations of plasmaholotranscobalamin, holohaptocorrin and totalplasma cobalamins (method II) in patients withdifferent degrees of gastric body atrophy

These results are shown in Fig. 4a–c. There was a sig-nificantly lower concentration of plasma holotranscobala-min but not of plasma holohaptocorrin and total plasmacobalamins in the mild gastric body atrophy group com-pared with the normal gastric body mucosa group. Severegastric body atrophy in patients with an impaired Schillingtest result was associated with a significantly lower con-centration of plasma holotranscobalamin, plasma holo-haptocorrin and total plasma cobalamins than in thosewith severe atrophy and a normal Schilling test result.

Discussion

We report that the measurement of plasma holotranscoba-lamin is a sensitive marker of cobalamin malabsorption. Allpatients with conditions compatible with cobalamin malab-sorption had low plasma holotranscobalamin, and all thosewith values within the reference range had conditions notcompatible with cobalamin malabsorption.

The great majority of cases with cobalamin malabsorp-tion are caused by gastric body atrophy. The study hastherefore been focused on the histological assessment of thegastric body mucosa supplemented with the Schilling test.The principal value of the Schilling test in the study hasbeen the identification of the patients with gastric bodyatrophy lacking intrinsic factor and the identification ofpatients, besides those with coeliac disease, with small boweldiseases causing cobalamin malabsorption [27–29]. Basedon the results of these investigations, we have classifiedthe patients referred because of suspected cobalamindeficiency into two groups: one group with and one

group without gastrointestinal pathology consistent withcobalamin malabsorption.

An issue that might give rise to cause some concern inthe present work is the classification of all patients withsome degree of gastric body atrophy as ‘compatible withcobalamin malabsorption’. Our work suggests that such aconcern is unjustified. About half of the patients withmoderate or mild gastric body atrophy had elevated meta-bolite tests (Fig. 1b and c). As such an elevation is usuallythe consequence of a long-standing malabsorption extend-ing over many years, it seems reasonable to suppose thatalso the other half of these patients had malabsorption.Accordingly, cobalamin malabsorption probably starts in


Table 2. Results of the Schilling test and the protein-bound absorption test (PBAT) in relation togastric body histopathology and plasma holotranscobalamin (HoloTC).

Schilling PBAT, Holo-TC,Morphological test, mean (SD) mean (SD) mean (SD)findings (%) (%) (pmol L¹1)

Gastric body atrophySevere (n ¼ 26) 8·5 (9·1) 2·7 (2·5) 18·3 (7·3)Moderate (n ¼ 6) 11·9 (5·3) 4·5 (1·6) 25·3 (4·8)Mild (n ¼ 7) 18·9 (5·9) 7·0 (1·7) 28·9 (5·0)

Gastritis without atrophy (n ¼ 27) 21·2 (5·5) 9·4 (3·0) 35·0 (10·9)

No pathological findings (n ¼ 26) 22·2 (6·0) 9·2 (2·9) 42·6 (12·0)

Figure 3 Comparison of (a) plasma holotranscobalamin(n ¼ 101), (b) plasma cobalamin method II (n ¼ 79) and (c)plasma cobalamin method I (n ¼ 101) for identifying patientswith cobalamin malabsorption. Logistic regression was per-formed after calculating the probability of cobalamin malabsorp-tion for patients with increasing values of the three componentsstudied. The figure shows that plasma holotranscobalamin issuperior to both methods for the determination of plasma coba-lamins in identifying patients with conditions compatible/notcompatible with cobalamin malabsorption (P<0·001). MethodII discriminates better than method I (P<0·001).



mild gastric body atrophy. The finding in this study of asignificantly lower plasma holotranscobalamin concentra-tion in the patients with mild atrophy of the gastric bodymucosa than in those with normal gastric body mucosagives further support to such a view.

The fact that all patients with conditions compatiblewith cobalamin malabsorption had a low level of plasmaholotranscobalamin, whereas the concentration of holo-haptocorrin remained virtually unchanged in all atrophygroups except for the one with severe gastric body atrophylacking intrinsic factor also gives substantial support to thetheory put forward by Herbert and colleagues [30] aboutthe sequence of biochemical events leading to cobalamindeficiency. As cobalamins bound to haptocorrin constitutethe major part of plasma cobalamins [9–11], this alsoexplains, at least partly, the lack of sensitivity of theplasma cobalamins [31]. The low sensitivity of plasmacobalamins is in accordance with previous data indicatingthat a proportion of patients with plasma cobalamins

within the interval of reference may well have functionaldeficiency [31–33].

Although all patients with conditions compatible withcobalamin malabsorption had a low value for plasmaholotranscobalamin, the results of the protein-boundabsorption test were less convincing. We have previouslyreported that some patients with gastric body atrophy havea normal protein-bound absorption test, even in thepresence of elevated metabolites [8]. Elevated metabolitesdespite normal protein-bound absorption test results havealso been reported by others [34–36].

Unexpectedly, we found a difference in the plasmaholotranscobalamin concentrations between the two groupsexpected to have normal cobalamin absorption: ‘patients withchronic gastritis without atrophy’ and those with ‘no patho-logical findings’. Such a difference could not be found withthe Schilling or protein-bound absorption tests (Table 2). Wehave also reported normal findings of serum pepsinogen Aand gastrin, sensitive indicators of gastric body mucosaatrophy, in all the patients of these two groups [37].

About half of the patients with gastritis had a decreasedlevel of plasma holotranscobalamin, and a high proportion ofthese patients also had an increased concentration of serummethylmalonic acid or homocysteine (both metabolites wereelevated in one patient). The results suggest cobalaminmalabsorption to occur with an increased frequency alsoin the group of patients with body gastritis.

The specificity of a low plasma holotranscobalamincould be judged only from a small group of patients(n ¼ 26). Six of these had a low concentration of plasmaholotranscobalamin. No reasonable explanation, such asthe use of drugs interfering with the absorption of cobala-mins [14–16], could be found. Moreover, the normalSchilling test results in this group exclude most conditionscausing small bowel malabsorption of cobalamins, e.g.bacterial overgrowth [27–29].

However, another possible explanation might be foundin the half-life of plasma holotranscobalamin, which is only1–2 h [38,39]. For a better understanding of the test, theinfluence of shorter periods of low or suspended intake ofcobalamins has to be elucidated. If such studies cannot giveus the explanation, it is possible that only a study thatfollows the patients over time can give us a definite answer.

In conclusion, measurement of holotranscobalaminseems to be a valuable contribution to the diagnostictools used in the evaluation of patients with suspectedcobalamin malabsorption. A concentration of plasma holo-transcobalamin within the normal range very probablyexcludes the presence of such a condition. A decreasedconcentration of plasma holotranscobalamin requiresfurther study in order to identify a possible underlyingcause of malabsorption.

Acknowledgements

We thank Ola Nilsson, Gothenburg, for the histologicalassessment of the gastrointestinal biopsies. The technical

Figure 4 Concentrations of (a) plasma holotranscobalamin,(b) plasma holohaptocorrin and (c) plasma cobalamins (methodII, mean 6 SD) related to the state of the gastric body mucosa.N, normal mucosa; G, gastritis without atrophy; mA, mild atro-phy; MA, moderate atrophy; SAþIF, severe atrophy with normalSchilling test; SA, severe atrophy with abnormal Schilling test.


assistance of Helle Persson, Gothenburg, and Anna-LisaChristensen, Aarhus, is also warmly acknowledged.

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Holotranscobalamin — a sensitive marker of cobalamin malabsorption

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