Case-Control Studies Using Only MalformedInfants Who Were Prenatally Exposed toDrugs. What Do the Results Mean?

To the Editor:In 1989, Kallen et al. (’89) developed a method to

analyze teratogenic specificity. With one of us(M.L.M.F.) having participated in that study, we be-came concerned about the real meaning of the results.This method has been adopted by the InternationalClearinghouse for Birth Defects Monitoring Systems(ICBDMS) as a specific framework referred to as theMADRE Project (MAlformation DRug Exposure surveil-lance). Although our program—the Spanish Collabora-tive Study of Congenital Malformations, ECEMC—is amember of the ICBDMS, we currently do not partici-pate in the MADRE project because of our concernregarding the interpretation of the results, some ofwhich have already been published (Robert et al., ’94).Now, having had the opportunity to study this meth-odology in greater detail, we believe that it may lead toerroneous interpretations of the data.

The methodology of the MADRE project consists ofcalculating the odds ratio (OR) by using 2 3 2 tablesthat only include the subset of malformed infants whowere prenatally exposed to any type of drugs duringthe first trimester of pregnancy. Thus, for each drugand each malformation group, exposed infants arethose whose mothers used the drug under study (withor without other drugs); nonexposed infants, thosewhose mothers used other drug(s); cases, infants withmalformation(s) under investigation (alone or in com-bination with other malformations); and noncases, in-fants with other malformations. Consequently, all theinfants in the study are malformed, and all of themhave been prenatally exposed to drugs.

We recently showed that the OR estimated with theuse of all other malformed infants as controls is notcomparable to the OR obtained using healthy controls(Prieto and Martınez-Frıas, ’99). This is because theOR obtained when using healthy controls is an approx-imation to the relative risk (RR), while the OR obtainedwhen using other malformed infants as controls is notan approximation to the RR, but only a measure ofspecificity. The OR resulting from methodologies suchas that used by the MADRE project is somewhat anal-ogous to the OR obtained with the method that usesonly malformed infants. However, in interpreting the

results, it is also necessary to consider the fact that allmalformed infants in the study have been prenatallyexposed to drugs, since the stratum of malformed in-fants not exposed to any drug is not considered in the2 3 2 tables. This introduces a new twist that furthercomplicates the interpretation of the results.

To understand the meaning of the MADRE project,and to interpret the results correctly, we must considerthe study in the population. That is, we must includethe malformed infants who were not exposed to anytype of drug, as well as the healthy children (Table 1).

The ORMADRE is:

ORMADRE 5adcb 5 X (A)

But this ORMADRE is not an approximation to the RRof the drug being studied to produce the defect being

Grant sponsor: Instituto de Salud Carlos III, Ministerio de Sanidad yConsumo, Madrid, Spain.

*Correspondence to: M.L. Martınez-Frıas, ECEMC, Facultad de Me-dicina, Universidad Complutense, 28040, Madrid, Spain.E-mail: luisama@eucmos.sim.ucm.es

Received 13 October 1999; Accepted 14 December 1999

TABLE 1. Total population of infants exposed to thedrug(s) being studied, exposed to other drugs, and

nonexposed to drugs

Prenatalexposure toa specific

drug(exposed)

Prenatal exposureto other drugs(nonexposed)

Nonexposureto any drug

Thestudydefect a b f

Rest ofdefects c d g

Healthyinfants h I jTotal N M K

TERATOLOGY 62:5–7 (2000)

© 2000 WILEY-LISS, INC.

studied. Then, what does the ORMADRE indicate? Wecan express the ORMADRE through its value in thepopulation as:

ORMADRE 5~a/N : f/K! : ~c/N : g/K!

~b/M : f/K! : ~d/M : g/K!5 X (B)

where (a/N : f/K) is the RR of the study drug to producethe study malformation (RRSD-SM), (c/N : g/K) is the RRof the study drug to produce defects other than themalformation under study (RRSD-OM), (b/M : f/K) is theRR of the other drugs to produce the study malforma-tion (RROD-SM), and (d/M : g/K) is the RR of the otherdrugs to produce other malformations (RROD-OM).

Because

ORMADRE 5adcd 5

a/cb/d 5

a/c z M/Nb/d z M/N 5

a/N : c/Nb/M : d/M

5~a/N : f/K! : ~c/N : g/K!

~b/M : f/K! : ~d/M : g/K!5

RRSD-SM : RRSD-OM

RROD-SM : RROD-OM5 X

(C)

But, which is the interpretation of the x-value? Con-sidering this equation, the X value means that thequotient between the relative risk of the drug understudy to produce the malformation being studied andthe relative risk of this same drug to produce othermalformations, is X times higher than the quotientbetween the relative risk of the other drugs to producethe malformation being studied and the relative risk ofthe other drugs to produce other malformations. This israther complicated and does not inform as to the risk ofthe drug being studied to produce the malformationbeing studied, or the specificity between the study drugand the study malformation. In fact, different combi-nations of the values of the four RRs of the equationgive different meanings for the same value of theORMADRE.

To facilitate the calculation, we can develop the pre-vious equation (C) as

ORMADRE 5RRSD-SM : RRSD-OM

RROD-SM : RROD-OM5

RRSD-SM z RROD-OM

RROD-SM z RRSD-OM

5 X (D)

If, for instance, the ORMADRE is equal to 4, this valuedoes not necessarily mean that the drug being studiedis specifically associated to the defect(s) being studied,since this value can come from different values of thefour RRs, with different interpretations. That is:

ORMADRE 54 z 11 z 1 5

2 z 21 z 1

51 z 41 z 1 5

1 z 10.5 z 0.5 5

0.33 z 3.030.5 z 0.5 5 4 (E)

In the same way, a protective ORMADRE, let us say of0.25, could be obtained for a study drug that is notrelated with the study defect:

ORMADRE 51 z 14 z 1 5 0.25 (F)

Similarly, we can obtain an ORMADRE of .1, beingthe study drug protective for the study malformation.For instance:

ORMADRE 50.5 z 51 z 1 5 2.5 (G)

Thus, it is not easy to interpret an ORMADRE, becauseit could result from four RRs that have different values,and each one suggests that:

The study drug increases the risk of the study defect.The study drug decreases the risk of the study defect.The study drug decreases the risk of other defects.The study drug increases the risk of other defects.Other drugs increase the risk of the study defect.Other drugs decrease the risk of the study defect.Other drugs decrease the risk of other defects.Other drugs increase the risk of other defects.

In each situation, combinations of two or more of theanterior possibilities can occur. Therefore, if we ob-tained an ORMADRE .1 or ,1, we cannot identify whichof the possible situations outlined above occurred, aswe cannot know the value of the four RRs. Only in thesituation in which the RRSD-SM is .1 or ,1 and theother three RRs are 1, would the value of the ORMADREindicate that the study drug increases or decreases therisk of the study defect. And although in general, onemight assume that the RROD-OM and the RRSD-OM havevalues near the unity because they include all the otherdrugs and all the other malformations together, this isnot highly probable. In fact, if the drug being studied isa teratogen, it would not be surprising for it to produceother malformations. In that case, the RRSD-OM wouldhave a magnitude over one. On the other hand, it is lessplausible for the RROD-SM to be 1, since some of theother drugs may also be related (by increasing or de-creasing the risk) with the study malformation. Allthese situations decrease the probability that all theRRs, but the RRSD-SM have a value equal to 1.

When performing a case-control study, it is essentialto keep in mind that we are calculating an approxima-tion to the RR, which can only be obtained in prospec-tive cohort studies. For this reason, if we develop a newtechnique to study risk factors based on a case-controlmethodology, we should analyze it in terms of the RRsin the population. As we have previously shown (Prietoand Martınez-Frıas, ’99), when we perform a case con-trol study using only malformed infants, the resultingOR is a measure of the times the RR of the exposure forthe defect being studied is higher than the RR for therest of the observed defects, and not an approximationto the RR of the exposure to produce the study defect in

6 LETTERS TO THE EDITOR

the population. Now, if the analysis not only uses mal-formed infants but is also restricted to the subset ofmalformed infants exposed to drugs (as in the MADREproject), the interpretation of the OR is further compli-cated, as we have shown here. Thus, since the analysisof the MADRE project takes into consideration severalpossible relationships between different drugs and dif-ferent malformations, we feel that this methodology isnot clearly discriminating as to allow for the study ofhuman teratogenicity or teratogenic specificity.

A clear methodology and an adequate interpretationof the results in epidemiological studies are crucial forto the correct assessment of the risk, without over orunderestimating it. By contrast, the use of an unclearmethodology to conclude that a relationship may existbetween a particular drug and a defect may lead toerroneous interpretations of the data and inappropri-ate clinical decisions.

ACKNOWLEDGMENTS

The authors thank Dr. Jaime Frıas from Tampa,Florida, who corrected the manuscript.

LUIS PRIETODepartamento de BioestadısticaFacultad de MedicinaUniversidad ComplutenseMadrid, Spain

MARIA LUISA MARTINEZ-FRIAS*

ECEMC/Departamento de FarmacologıaFacultad de MedicinaUniversidad ComplutenseMadrid, Spain

LITERATURE CITED

Kallen B, Robert E, Mastroiacovo P, Martınez-Frıas ML, Castilla EE,Cocchi G. 1989. Anticonvulsant drugs and malformations: is there adrug specificity? Eur J Epidemiol 5:31–36.

Prieto L, Martınez-Frıas ML. 1999. Case-control studies using onlymalformed infants: are we interpreting the results correctly? [let-ter] Teratology 60:1–2.

Robert E, Vollset SE, Botto L, Lancaster PAL, Merlob P, MastroiacovoP, Cocchi G, Ashizawa M, Sakamoto S, Orioli I. 1994. Malformationsurveillance and maternal drug exposure: the MADRE project. IntJ Risk Safety Med 6:75–-11.

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