GENETIC TESTINGVolume 3, Number 1, 1999Mary Ann Liebert, Inc.

Evaluation of Genetic Tests: APOE Genotyping for theDiagnosis of Alzheimer Disease

LAURA M. McCONNELL,1 GILLIAN D. SANDERS,2 and DOUGLAS K. OWENS3

ABSTRACT

Many studies have now confirmed the association between inheritance of the e4 alíele of the apolipoproteinE (APOE) gene and Alzheimer disease (AD). However, although the medical community holds the near-unan-imous opinion that APOE genotyping should not be used for prediction in asymptomatic individuals, contro-versy remains about whether it should be used for diagnosis in patients who show signs of dementia. We as-sessed critically the recent clinical studies, on the basis of four criteria recommended to ensure safety andeffectiveness of genetic tests. We also developed a formal framework for evaluating the usefulness of APOEgenotyping using decision-theoretic principles. We conclude that neither the presence nor absence of an e4alíele provides diagnostic certainty, and the proper interpretation of either result in heterogeneous popula-tions requires further investigation. The appropriate role of APOE genotyping among elements of a tradi-tional assessment for AD has not been determined. Whether APOE genotyping provides sufficient informa-tion to change patient management decisions has not been determined. APOE genotyping presents foreseeable,significant psychosocial consequences for family members that must be weighed against any psychosocial ben-efits. Therefore, the diagnostic use of APOE genotyping outside research settings is premature until such test-ing is shown to be of practical value.

on Genetic Testing to review genetic testing in the United Statesand make recommendations to ensure the safety and effective-ness of new genetic tests (Holtzman and Watson, 1997). TheTask Force strongly urged that genetic tests meet the followingcriteria before they are unconditionally released, thereby re-

ducing the likelihood of premature clinical use:

(i) The genotypes to be detected must be shown by scien-tifically valid methods to be associated with the occurrence ofa disease.

(ii) The analytical sensitivity and specificity of a genetic testmust be determined before it is made available in clinical prac-tice. For DNA-based tests, this means establishing the proba-bilities that the test will accurately detect the presence or ab-sence of the DNA sequence of interest.

(iii) Data to establish the clinical validity of a genetic testmust be collected under investigative protocols. Such data in-clude measures of sensitivity (the probability that the test willbe positive in people who have the disease), specificity (the

'Stanford Program in Genomics, Ethics, and Society, Palo Alto, CA 94304.department of Health Research and Policy, Stanford University School of Medicine, Stanford, CA 94305.'Department of Veterans Affairs Palo Alto Health Care System, and Associate Professor of Medicine and of Health Research and Policy,

Stanford University School of Medicine, Stanford, CA 94305.

INTRODUCTION

Many studies have now confirmed the association be-tween inheritance of the e4 alíele of the apolipoprotein

E (APOE) gene and Alzheimer disease (AD). Clinical applica-tion of this knowledge, however, has proved problematic. Sev-eral blue-ribbon panels have published consensus statements on

the use of genetic testing for AD (American College of Med-ical Genetics et al., 1995; Medical and Scientific AdvisoryCommittee and Alzheimer's Disease International, 1995; Na-tional Institute on Aging and Alzheimer's Association Work-ing Group, 1996; Post et al, 1997); although they have ex-

pressed the nearly unanimous opinion that APOE genotypingshould not be used for predicting future disease in asympto-matic individuals, controversy remains about whether it shouldbe used for diagnosis in patients who show signs of dementia.

The National Institutes of Health-Department of Energy(NIH-DOE) Working Group on Ethical, Legal, and Social Im-plications of Human Genome Research created a Task Force

47

48 McCONNELL ET AL.

probability that the test will be negative in people who do nothave the disease), positive predictive value (PV+; the proba-bility that people who have positive test results have or will de-velop the disease), and negative predictive value (PV—; theprobability that people who have negative test results do nothave or will not develop the disease).

(iv) Data that demonstrate clinical utility—the benefits andrisks that accrue from both positive and negative results—mustbe collected.

Have recent clinical studies provided the evidence needed to

support the widespread clinical use of APOE genotyping forthe diagnosis of AD? To answer this question, we assessed crit-ically the available literature based on the four NIH-DOE TaskForce criteria. We also developed a formal framework for eval-uating the usefulness of APOE genotyping using decision-the-oretic principles.

ASSOCIATION OF APOE GENOTYPEWITH AD

Many studies since Strittmatter's seminal report (Strittmat-ter et al, 1993) have demonstrated an association between in-heritance of the e4 alíele of the APOE gene and AD. A recent,large meta-analysis found an approximate two- to three-fold in-crease in risk of AD among Caucasians inheriting one e4 al-íele, and an approximate 12- to 15-fold increase for those in-heriting two eA alíeles (Farrer et al., 1997). However, both thefrequency of the eA alíele and the strength of the associationbetween APOE and AD vary in people from different ethnicand cultural backgrounds (Farrer et al, 1997; Tang et al, 1998).Age (Blacker et al, 1997), and possibly gender (Corder et al,1995; Duara et al, 1996; Payami et al, 1996; Farrer et al,1997; Combarros et al, 1998) also influence the association.These differences have Significant implications for the clinicalapplication of APOE genotyping, and require further investi-gation if genetic test results are to be interpreted accurately inheterogeneous populations.

ANALYTIC SENSITIVITY AND SPECIFICITYOF APOE GENOTYPING

APOE alíeles encode three major isoforms, or protein vari-ants, of human apolipoprotein E. The isoforms differ in amino-

Table 1. Validity of APOE Genotyping inPatients with a Clinical Diagnosis of AD

Duke CERAD(Saunders et al., 1996) (Welsh-Bohmer et al., 1997)

n = 67 n = 162(%) {%)

PrevalenceSensitivitySpecificityPV+PV-

8575

10010042

8683839744

acid sequence at one or both of two positions: residues 112 and158 (Weisgraber et al, 1981). APOE genotyping is generallycarried out in a polymerase chain reaction (PCR)-based processwhereby the appropriate parts of the gene from a patient's DNAsample are amplified, cleaved using a restriction enzyme, andthe fragments separated by gel electrophoresis (Hixson andVernier, 1990). The results of this procedure permit unam-

biguous genotyping based on unique combinations of fragmentsizes, and the test is considered to be 99% accurate (Roses,1996).

CLINICAL VALIDITY OF APOE GENOTYPING

The concept of validity relates to the question, "Are we mea-

suring what we are trying to measure?" (Bourke et al, 1985).The validity of a diagnostic test can be determined only if re-

sults can be gauged against a gold standard for identifying truedisease state. To determine the clinical validity of APOE geno-typing, an investigator must ultimately be able to identify cor-

rectly which patients have AD and which do not. The limits ofcurrently available diagnostic methods are, of course, what pro-vide the incentive to develop more definitive tests. Until suchtests exist, accurate calculations of clinical validity require stud-ies that include genetic information as well as the findings ofneuropathological examination at autopsy, the latter of whichprovide the current gold standard for AD diagnosis.

Table 1 summarizes two such studies. These studies evalu-ated the sensitivity, specificity, PV+, and PV— of APOE geno-typing in patients who had a clinical diagnosis of probable AD,and for whom autopsy and genetic data were available.

The first study, of 67 patients who were seen at the DukeMemory Disorders Clinic (Saunders et al, 1996), included no

specific information about the age, gender, or ethnic back-ground of the subjects. Based on the specificity and PV+ as-

sociated with the eA alíele for AD in this study, the authors con-

cluded, "From a practical point of view, determination of theeAleA, e3/eA, and e2/e4 genotypes in clinically diagnosed ADpatients increases the accuracy of diagnosis." They also founda low PV—, concluding that "absence of an eA alíele does not

preclude the presence of AD."The second study involved a clinical series of 162 AD pa-

tients enrolled in a multicenter study of the Consortium to Es-tablish a Registry for Alzheimer's Disease (CERAD) (Welsh-Bohmer et al, 1997). The patient sample was predominantlywhite (95%), and included slightly more men (57%) thanwomen. Among the authors' conclusions were that "once theclinical diagnosis of AD is established, the presence of an eAalíele reliably predicts the ultimate CERAD neuropathologicaldiagnosis of AD" and, again, due to the low PV —, genotypingprovides "no useful information for diagnosis of AD when theeA alíele is not present."

Note that high PV+ results are attributable in part to APOEgenotyping sequentially following a clinical work up for AD.The predictive value of any test is determined not only by itssensitivity and specificity, but also by the prevalence of the dis-ease in the population in which it is carried out. We can in-crease the PV+ either by increasing the specificity of the test

(by changing the criteria by which results are deemed "posi-tive"), or by increasing the prevalence of disease in the popu-

DIAGNOSTIC USE OF APOE GENOTYPING

Table 2. Comparison of Single and Sequential Diagnostic Tests for AD

AClinical

diagnosisalone

n = 2,188(%)

BAPOE

genotypingalone

n = 2,188(%)

APOE genotypingplus clinical criteria

("net")n = 2,188

<%)

DAPOE genotypingin patients meeting

clinical criterian = 1,833

(%)PrevalenceSensitivitySpecificityPV+PV-

8193559064

8165689031

8161849434

9065659418

Note: Columns A, B, and C are extracted from Mayeux et al (1998); column D is calculated on the basis of data from thesame study.

lation (by targeting the testing program to individuals at highrisk). Thus, PV+ will be higher when APOE genotyping is per-formed in a group of patients already clinically diagnosed as

having probable AD.Regarding diagnostic accuracy, if we begin with a popula-

tion of patients who have a clinical diagnosis of probable AD,a second diagnostic test will not identify additional cases thatwere missed by the first (Mayeux, 1998). One common reason

for administering a second test, only among those who test pos-itive on the first, is to identify patients who do not have the dis-ease. APOE genotyping does not serve this purpose: PV— cal-culations tell us that APOE status cannot be used to rule outAD when applied to patients who have a clinical diagnosis ofthe disease. These studies did not evaluate the test's perfor-mance in patients who did not have a clinical diagnosis of AD.

A recent study by Mayeux and colleagues, however, pooleddata from 26 Alzheimer's Disease Centers in the United Statesfor 2,188 patients who had autopsy-confirmed dementia of var-

ious kinds (Mayeux et al, 1998). Thus, investigators were ableto compare the validity of clinical diagnosis alone, APOE geno-typing alone, and clinical diagnosis followed by APOE geno-typing. Of their subjects, 97% were white, and 51% were

women.

Table 2, column A, shows the results for clinical diagnosisalone. The results for APOE genotyping alone, shown in Table2, column B, confirm that the sensitivity and specificity ofAPOE genotyping alone is poor in patients who do not have a

clinical diagnosis of AD. Even when the definition of a posi-tive test result was restricted to the presence of two eA alíeles,specificity increased to 94%, but sensitivity fell to 14%(Mayeux et al, 1998).

Regarding sequential use of the tests, Mayeux and associ-ates measured the combined, or net, validity of clinical workup followed by APOE genotyping (Table 2, column C). Theydescribe this procedure as follows:

For the sequential testing procedure, the 1,833 patientsmeeting clinical criteria for Alzheimer's disease were firstidentified and then the results of the APOE genotype were

applied. The presence of one or more APOE eA alíeles de-creased the sensitivity to 61 percent, but increased thespecificity to 84 percent (Mayeux et al, 1998).

On the basis of these findings, the authors concluded that,when it is used in combination with clinical criteria, APOEgenotyping improves the specificity of the diagnosis (Mayeuxet al, 1998). However, the footnote accompanying their tabu-lar presentation of the supporting data is crucial to our under-standing of how to interpret the results. Despite their narrativereference to 1,833 clinically diagnosed patients, they calculatedthe 84% specificity level using all 2,188 patients. They re-

stricted the definition of a positive test result to a positive clin-ical diagnosis and an e4+ genotype, whereas they defined a

negative test result as any other combination of clinical and ge-netic results (positive clinical diagnosis and eA— genotype, neg-ative clinical diagnosis and e4+ genotype, or negative clinicaldiagnosis and eA— genotype). Grouping the patients in this wayincreases the number of true-negative results, but expands thepopulation examined to include all subjects, rather than justthose with a clinical diagnosis of AD. As we mentioned previ-ously, narrowing the definition of a positive test result—in thiscase, relative to the definition of a negative test result—in-creases the measured specificity of any test.

We can calculate the value of using these tests sequentiallyanother way, which is to assess APOE genotyping in the con-

text of only the 1,833 patients first clinically diagnosed withAD. These results are shown in Table 2, column D. This methodallows comparison of this new and larger study with the pre-vious Duke and CERAD studies, and may reflect more accu-

rately Mayeux and colleagues' conclusion that "APOE geno-typing might be reserved for patients who meet the clinicalcriteria for Alzheimer disease" (Mayeux et al, 1998).

The concept of conditional dependence gives us another wayof demonstrating Mayeux's finding that the sensitivity andspecificity of APOE genotyping depend on whether the popu-lation tested already has a clinical diagnosis of AD. Extractingthe data from their study, we constructed a tree illustrating theproportions of positive and negative clinical examination andgenetic test results found in that population (Fig. 1). When eval-uating a combination of diagnostic tests, clinicians must gen-erally assume conditional independence, meaning that, amongpatients who have the disease, the frequency of a result on thesecond test is the same regardless of the result on the first test(Sox et al, 1988c). Bayes' theorem, a common means of cal-culating the probability of disease after new information is ac-

50 McCONNELL ET AL.

True Disease State Results of Clinical Results of APOEby Autopsy Diagnosis Genotyping

E4+ (p=0.65)AD (p=0.93)

Demented patientsn = 2,188

n = 1643

Not AD (p=0.07)

n = 1076

E4- (p=0.35)

n = 127

AD (p=0.45)

:567

E4+ (p=0.52)n = 66

E4- (p=0.48)n = 61

E4+ (p=0.35)

Not AD

n = 190

NotAD(p=0.55)

n = 66

E4- (p=0.65)

n = 228

n= 124

E4+ (p=0 29)n = 67

E4-(p=0 71)

FIG. 1. Proportions of positive and negative clinical and ge-netic test results found by Mayeux et al (1998).

quired, relies on this fundamental assumption. However, Fig-ure 1 shows that APOE genotyping is conditionally dependenton a clinical diagnosis of AD. The probability of finding an eAalíele in patients who have a positive clinical result is not thesame as the probability of finding an eA alíele in patients whohave a negative clinical result.

In summary, recent studies that include genetic informationas well as neuropathological examination indicate that APOEgenotyping alone has poor sensitivity and specificity for the di-agnosis of AD. Finding the eA alíele in a patient who alreadyhas a clinical diagnosis of AD can increase confidence in thatdiagnosis. In the Mayeux study, for example, among primarilywhite patients referred to AD specialty centers, the PV+ in-creased from 90% to 94%. The absence of the eA alíele in pa-tients who have a clinical diagnosis of AD does not rule out thedisease. The subjects for these studies probably are not repre-sentative of patients seen in other clinical environments. Onepotential use of APOE genotyping is to assist primary-careproviders who may have less clinical experience in making an

AD diagnosis. Patients seen in such settings will come from a

variety of ethnic backgrounds, making interpretation of resultsdifficult. In addition, the prevalence of AD in those populationswill be lower, and thus so will be the PV+ of APOE geno-typing.

BENEFITS AND RISKS OF APOEGENOTYPING

It is not possible to assign an arbitrary level of clinical va-

lidity (e.g., sensitivity of 80%) that makes a diagnostic test use-

ful. Instead, the utility of the resulting information must be eval-uated based on whether it changes what is done for the patient(Sox et al, 1988b). Theoretically, additional diagnostic infor-mation could lead to a more definitive prognosis, affect treat-ment decisions, or influence decisions related to the patient'spersonal life. In the context of AD, does increasing the proba-bility that a patient with dementia has AD have a meaningfuleffect in these realms?

PrognosisMost studies to date suggest that the eA alíele is associated

with a lower age at onset of AD symptoms (Blacker et al.,1997), but does not affect rate of progression of the disease(Gomez-Isla et al, 1996; Growdon et al, 1996; Hyman et al,1997). Therefore, for patients already clinically diagnosed withAD, APOE genotyping currently will not render additionalprognostic information.

Treatment decisionsSeveral researchers suggest that APOE genotyping may have

a future prominent role in treatment planning (Poirier et al,1995; Roses, 1996). We can evaluate the use of diagnostic teststo inform treatment decisions by using a decision-analyticframework (Pauker and Kassirer, 1980; Sox et al, 1988a). Be-cause clinicians can seldom make a diagnosis with absolute cer-

tainty, they typically must decide to treat or withhold treatmentbased on the probability of disease. The treatment thresholdprobability is the probability of a disease above which treat-ment is preferred and below which withholding treatment ispreferred. The treatment threshold probability is usually near

1.0 if the treatment has the potential to cause substantial harmin patients who do not have the disease, and has limited to mod-erate benefit in patients who have the disease (e.g., chemother-apy for cancer). The treatment threshold probability is usuallynear 0.0 if the treatment is relatively benign in patients who donot have the disease, and has substantial benefit in patients whohave the disease (e.g., antibiotic therapy for bacterial infec-tions). There are analytic methods to determine an appropriatetreatment threshold; these require that probabilities, risks, andutilities be assigned to cover the range of treatment decisions,test results, patient disease states, and subsequent outcomes(Pauker and Kassirer, 1980; Sox et al, 1988b).

In general, a diagnostic test should be performed only if itcan cause the probability of disease to cross the treatmentthreshold, and thereby change the management of the patient.Although there are other reasons that diagnostic tests may beused, for instance, concern about malpractice, or for reassur-

ance, from the viewpoint of medical management, a test thatdoes not have the possibility of changing the probability of dis-ease enough to cross a treatment threshold provides no usefulinformation.

Although derivation of the treatment threshold for AD is be-yond the scope of this paper, it is still possible to calculate fromFig. 1 the probability of a patient having AD, given variouscombinations of test results (Table 3, column A), and plot thisinformation in graphical form (Fig. 2). The shaded areas in Fig.2 illustrate where the treatment threshold for AD would haveto fall for APOE genotyping to change the treatment-no treat-ment decision for a patient.

For example, after a patient in the Mayeux study received a

positive clinical diagnosis, the probability of that patient hav-ing AD was 0.896. Performing APOE genotyping on this pa-tient would change his or her probability of AD to either 0.821(negative eA result) or 0.942 (positive eA result). If the recom-

mended treatment of a patient who has a 0.821 or 0.942 prob-ability of AD differs from that of a patient with a 0.896 prob-ability of AD, then APOE results could affect the patient'smanagement. However, if changes of this magnitude do not af-

DIAGNOSTIC USE OF APOE GENOTYPING 51

Table 3. Probability of AD asTest Results Become Known

Test results* B

[l]P (AD) 0.809 0.670[2] P (AD I pos C) 0.896 0.806[3] P (AD I pos C and then neg E4) 0.821 0.687[4] P (AD I pos C and then pos E4) 0.942 0.886[5] P (AD I neg C) 0.358 0.211[6] P (AD I neg C and then neg E4) 0.275 0.154[7] P (AD I neg C and then pos E4) 0.496 0.321

Note: Column A is calculated based on data from Mayeuxet al (1998); column B applies the same proportions to a

hypothetical population that has a 67% prevalence of AD.al. The probability (or prevalence) of AD in the population

of interest.2. The probability of AD, given a positive clinical result.3. The probability of AD, given a positive clinical result

and then a negative E4 result.4. The probability of AD, given a positive clinical result

and then a positive E4 result.5. The probability of AD, given a negative clinical result.6. The probability of AD, given a negative clinical result

and then a negative E4 result.7. The probability of AD, given a negative clinical result

and then a positive E4 result.

feet what is done for the patient, the value of APOE testing inthis population is reduced in terms of informing treatment de-cisions. The fact that other diagnostic tests for dementia havenot been evaluated with this rigor (National Institute on Agingand Alzheimer's Association Working Group, 1996) does not

negate the opportunity these data provide to make a critical as-

sessment of the value of APOE genotyping.The prevalence of AD in the Mayeux study is noteworthy—

81%, as compared to the approximately two-thirds of demen-tia cases for which AD generally accounts in the United States

(Small et al, 1997). This difference probably is due to selec-tion bias created by the screening that takes place before a pa-tient is referred to an AD center or specialty clinic. Table 3,column B demonstrates the probability of AD, using 67% as

the prior probability and assuming the same proportions of clin-ical examination and genetic test results as found by Mayeux'sgroup. This latter assumption is subject to other biases; the pa-tients in the Mayeux study probably differ in other importantways from broader populations of AD patients.

Other decisions that APOE genotyping could potentially af-fect are those regarding the type of treatment. Currently, phar-macological approaches to AD have limited benefit, are effec-tive in only some patients, and can cause significant side effects(Small et al, 1997). Research stratified by genotype couldsomeday help to predict who is likely to respond to particulardrugs (Farlow et al, 1998). For example, preliminary studiesindicate that response to the cholinesterase inhibitor tacrine mayvary based on genotype and gender (Farlow, 1997). However,APOE genotyping will change patient management only if re-

search indicates that available drugs are completely ineffectivein patients who have specific genotypes or that the adverse af-fects outweigh any benefit. Patient management would alsochange if therapies are developed that offer an alternative forthose patients who are expected to have a poor response to cur-

rent treatments.Another decision related to treatment is whether to under-

take additional diagnostic tests. There may be a role for APOEgenotyping as a gateway to more expensive tests, such as neu-

roimaging studies, with the goal of economic savings (Roses,1996). Under this scenario, those patients who meet certain clin-ical criteria and have eA alíeles generally would be presumedto have AD, whereas those without eA alíeles might be referredfor more exhaustive testing. For the many patients who do nothave eA alíeles yet who do have AD, this strategy could po-tentially detract from the original, accurate diagnosis. To de-termine whether APOE can obviate the need for other diag-nostic tests, we must identify the treatment threshold for AD,

LU O"LU LU

O

<

üO)o>c

D<

O

< <<

O O

< <

0.0 0.1 0.2

FIG. 2. Probability of AD as test results become known. This bar indicates the change in probability of AD for the patientpopulation as test results become known. The black square represents the prior probability of AD before any test has been done.The solid arrows indicate how this probability of AD changes with the results of clinical testing (either positive or negative).The dashed arrows show how a subsequent positive or negative APOE test affects the probability of AD. The shaded areas in-dicate the range of probability of AD within which a change in the patient's treatment would have to occur for the APOE testto have clinical value.

52 McCONNELL ET AL.

and analyze the validity of current diagnostic tools to assess theincremental contribution of each. At this time, a point prior tothe performance of more expensive tests may or may not befound at which APOE genotyping affects the decision to treat.

Personal decision makingThere are currently no treatment options for AD other than

the palliation of existing symptoms, and palliative treatment can

be initiated independent of the final diagnosis (van Gool, 1996).Nevertheless, an increase in diagnostic certainty may be im-portant for reasons related to the patient's and family's attitudestoward the risks and benefits of the procedure. For instance, fora family struggling with the probable nature of the diagnosis,increased confidence resulting from a positive eA finding couldbring psychological relief. Such relief may enable the family tomove forward with financial and health-care arrangements forthe patient and, if it is engendered sufficiently early, could al-low the patient to participate in the planning. In addition, health-care providers have a strong preference for a high degree ofcertainty when they must convey a devastating diagnosis. Theseclaims warrant further inquiry, however, before they are usedto justify the widespread diagnostic use of APOE genotyping.

For example, to what extent do current levels of diagnosticuncertainty produce psychological distress in patients and fam-ilies, separate from the specter of AD itself? To what degree isthat distress an obstacle to acceptance of the diagnosis, and doesit impede medical and financial planning? It is perhaps of most

practical importance for clinicians and policy makers to knowwhether the increase in confidence provided by a positive eAresult makes a meaningful difference to patients and families.This question has not been addressed through careful research.

The benefit to health-care providers of greater certainty whencommunicating a diagnosis of AD raises the same question:Does the degree of increased confidence provided by a positiveeA result make a meaningful difference? An informal survey ofmembers of the neurology department at a large academic cen-

ter suggested that the minimum level of certainty for discussinga diagnosis of AD with patients and families varies from 75%to 90%, a range largely compatible with the average 80% to85% accuracy achieved by clinical diagnosis alone (van Gool,1996). A formal study of this question is of particular interestif APOE genotyping eventually becomes a gateway to more ex-

pensive tests. If physicians, patients, and families simply wantall possible certainty, they may be unwilling to forego more ex-

pensive tests to permit economic savings.The potential benefits to families of a positive eA result are

tempered by several potential harms. Testing a patient for di-agnostic purposes unavoidably results in information about thegenotypes of the patient's siblings and children. For example,children of a heterozygous AD patient will learn that they havea 50% probability of having at least one eA alíele, and childrenof a homozygous AD patient will learn that they certainly haveat least one eA alíele. Such information becomes, in essence, a

predictive result for them and is ripe for misinterpretation. Ifthe patient's genetic test results are revealed to family mem-

bers at all, they should be released only be in the context of ap-propriate counseling (Greely, 1999). Unfortunately, the num-

ber of health-care professionals trained in genetics and geneticcounseling has not kept pace with the rate of genetic discovery

and the potential demand for genetic tests (Holtzman and Wat-son, 1997). This deficit should be a significant consideration inany decision to offer APOE genotyping beyond AD researchcenters, where availability of and access to genetic counselingcan be assured. Even if they understand the predictive risk im-plications of the patient's test results, relatives may experiencethe psychological burdens and social costs of predictive genetictesting, including anxiety, stigmatization, and insurance andemployment discrimination (Kapp, 1996).

CONCLUSIONS

The results of the three studies reviewed here suggest that,for patients clinically diagnosed with AD, a positive test for an

eA alíele helps to predict with increased confidence the ultimateneuropathological confirmation of AD. A negative eA resultcould be viewed as adding no useful diagnostic information, or

as stimulating a continued search for other causes of demen-tia—perhaps inappropriately. Thus, we draw these conclusions:

• Neither the presence nor absence of an eA alíele provides di-agnostic certainty, and the proper interpretation of either re-

sult in heterogeneous populations requires further investiga-tion.

• The appropriate role of APOE genotyping among elementsof a traditional assessment for AD has not been determined.

• Whether APOE genotyping provides sufficient informationto change management decisions has not been determined.

• APOE genotyping presents foreseeable, significant psy-chosocial consequences for family members that must beweighed against any hypothetical psychosocial benefits as-

sociated with a modest increase in diagnostic certainty.

These conclusions do not mean that APOE genotyping shouldnot proceed in research settings. Consistent study results show-ing that APOE genotype has an affect on the course of the dis-ease or response to treatment, or that other controllable factorsmodify the effect of APOE, would shift the balance of thisanalysis. Given the strength of the association between APOEand AD, it is essential that this type of research continue. How-ever, the widespread clinical use of APOE genotyping for thediagnosis of AD is premature until such testing is shown to beof practical value. We must also address ethical and legal is-sues at a societal level and in genetic counseling of individu-als and families, to ensure that they do not undermine any valueof APOE genotyping demonstrated in the future.

ACKNOWLEDGMENTS

The authors are grateful to Neil A. Holtzman, M.D., M.P.H.,and Lyn Duprè for their review of this manuscript.

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