Lett~~~~ersto te Eior
Letters to the Editor are welcomed and willbepublished, iffoundsuitable, as space penits. Submission of a Letter to the Editor constitutes pennission for its publication in the JoumaL Letters should not duplicate similar material being submitted orpublished elsewhere. Letters refermng to a recent Journal arficle should be received within three months of the article'spublication. The editors reserve the righ to edit and abridge letters, topublish replies, and to solicit responses from authors and others. Letters should be submitted in duplicate, double-spaced (including references), and should not exceed 400 words.
Maternal Transfers and Hospital Perinatal Mortality Rates Mayfield et al. examine perinatal mortality rates in relation to two indicators of medical care, hospital obstetrical volume and nursery level.1 When comparing rates among hospitals or groups of hospitals, a problem arises when patients are transferred: should the outcome be attnbuted to the sending or to the receiving facility? Like other researchers before,3 Mayfield attributes infant transfers to the sending hospital but maternal transfers while the fetus is in utero to the receiving hospital. Failure to assign maternal transfers to the sending hospital can result in bias of perinatal mortality rates against primary facilities and in favor of secondary and tertiary facilities. This bias arises because maternal transfers are carried out only when the fetus is still alive. Delivery typically occurs soon after arrival at a secondary or tertiary center, which makes fetal death very unlikely. In contrast, most fetal
August 1991, Vol. 81, No. 8
deaths happen prior to presentation at a primary hospital; this provides no opportunity for transfer. Maternal transfer is therefore highly associated with pregnancy outcome and is a potential source of selection bias. To illustrate how this bias arises, consider the extreme situation where a tertiary hospital only accepts transfer patients whereas a primary hospital transfers all women with fetuses that are alive but in trouble. The primary hospital would keep virtually all of its fetal deaths because most occur prior to presentation, whereas the tertiary hospital would have virtually none. Fetal mortality rates and thus perinatal mortality rates would indicate better medical care at the tertiary facility even if the capabilities and performances were identical at the primary facility. This is an extreme example, yet there are settings where pregnancies undergoing very premature labor are routinely transferred unless the fetus has already died. Thus, this bias is particularly serious in low-birthweight categones. For one of our hospitals that routinely transfers women with high-risk pregnancies, the percentile ranking among all California hospitals, based on annual age/sex/race/birthweight-adjusted perinatal mortality rates,4 increased by 31 percentage points after we reassigned to it its 52 maternal transfers. By attributing a maternal transfer to the primary care hospital, as is done for an infant transfer, bias can be avoided. Ifdata on maternal transfer are of poor quality or missing, researchers should focus on neonatal rather than perinatal mortality. The separation of fetal and neonatal mortality has been avoided because of concerns about misclassifying some neonatal as fetal deaths. Such misclassification, however, may bias mortality statistics less
than inappropriate attnbution of maternal transfers. O Bnwse H. Firman, MA Marilyn K Go(dher, MPH The authors are with the Division of Research, Northern California Kaiser Permanente Medical Care Program, 3451 Piedmont Avenue, Oakland, CA 94611.
References 1. Mayfield JA, Rosenblatt RA, Baldwin LM, Chu J, Logerfo JP. The relation of obstetrical volume and nursery level to perinatal mortality.AmJPublcHealh 1990;80:819823. 2. Williams RL, Chen PM. Identifying the sources of the recent decline in perinatal mortality rates in California. NEnglJMeL 1982;306:207-214. 3. Bowes WA. A review of perinatal mortality in Colorado, 1971 to 1978, and its relationship to the regionalization of perinatal services. Am J Obstet GynecoL 1982;141:10451050. 4. Williams RL, Rust FP, Chen PM, Wailer S. 1982-1986 Maternal and Child Health Data Base: Descriptive Narrative. Santa Barbara: University of California, Community and Organization Research Institute: 1990.
Mayfield Responds The letter from Fireman and Goldhaber regarding maternal transfers and hospital perinatal mortality rates raises two important issues of methods used to evaluate perinatal care. We are happy to have the opportunity to discuss these concerns of methods as they apply to our study.'
The potential for bias when maternal transfer occurs has been discussed in the literature.2 Maternal transfers are generally conducted for women with high-risk conditions deemed stable enough to complete the transfer before delivering; maternal transfers are not conducted for women whose problems are so acute orunexpected that they do not have sufficient time for transport.3 In addition, transports are not usually recommended
American Journal of Public Health 1075
Letters to the Editor are welcomed and willbepublished, iffoundsuitable, as space penits. Submission of a Letter to the Editor constitutes pennission for its publication in the JoumaL Letters should not duplicate similar material being submitted orpublished elsewhere. Letters refermng to a recent Journal arficle should be received within three months of the article'spublication. The editors reserve the righ to edit and abridge letters, topublish replies, and to solicit responses from authors and others. Letters should be submitted in duplicate, double-spaced (including references), and should not exceed 400 words.
Maternal Transfers and Hospital Perinatal Mortality Rates Mayfield et al. examine perinatal mortality rates in relation to two indicators of medical care, hospital obstetrical volume and nursery level.1 When comparing rates among hospitals or groups of hospitals, a problem arises when patients are transferred: should the outcome be attnbuted to the sending or to the receiving facility? Like other researchers before,3 Mayfield attributes infant transfers to the sending hospital but maternal transfers while the fetus is in utero to the receiving hospital. Failure to assign maternal transfers to the sending hospital can result in bias of perinatal mortality rates against primary facilities and in favor of secondary and tertiary facilities. This bias arises because maternal transfers are carried out only when the fetus is still alive. Delivery typically occurs soon after arrival at a secondary or tertiary center, which makes fetal death very unlikely. In contrast, most fetal
August 1991, Vol. 81, No. 8
deaths happen prior to presentation at a primary hospital; this provides no opportunity for transfer. Maternal transfer is therefore highly associated with pregnancy outcome and is a potential source of selection bias. To illustrate how this bias arises, consider the extreme situation where a tertiary hospital only accepts transfer patients whereas a primary hospital transfers all women with fetuses that are alive but in trouble. The primary hospital would keep virtually all of its fetal deaths because most occur prior to presentation, whereas the tertiary hospital would have virtually none. Fetal mortality rates and thus perinatal mortality rates would indicate better medical care at the tertiary facility even if the capabilities and performances were identical at the primary facility. This is an extreme example, yet there are settings where pregnancies undergoing very premature labor are routinely transferred unless the fetus has already died. Thus, this bias is particularly serious in low-birthweight categones. For one of our hospitals that routinely transfers women with high-risk pregnancies, the percentile ranking among all California hospitals, based on annual age/sex/race/birthweight-adjusted perinatal mortality rates,4 increased by 31 percentage points after we reassigned to it its 52 maternal transfers. By attributing a maternal transfer to the primary care hospital, as is done for an infant transfer, bias can be avoided. Ifdata on maternal transfer are of poor quality or missing, researchers should focus on neonatal rather than perinatal mortality. The separation of fetal and neonatal mortality has been avoided because of concerns about misclassifying some neonatal as fetal deaths. Such misclassification, however, may bias mortality statistics less
than inappropriate attnbution of maternal transfers. O Bnwse H. Firman, MA Marilyn K Go(dher, MPH The authors are with the Division of Research, Northern California Kaiser Permanente Medical Care Program, 3451 Piedmont Avenue, Oakland, CA 94611.
References 1. Mayfield JA, Rosenblatt RA, Baldwin LM, Chu J, Logerfo JP. The relation of obstetrical volume and nursery level to perinatal mortality.AmJPublcHealh 1990;80:819823. 2. Williams RL, Chen PM. Identifying the sources of the recent decline in perinatal mortality rates in California. NEnglJMeL 1982;306:207-214. 3. Bowes WA. A review of perinatal mortality in Colorado, 1971 to 1978, and its relationship to the regionalization of perinatal services. Am J Obstet GynecoL 1982;141:10451050. 4. Williams RL, Rust FP, Chen PM, Wailer S. 1982-1986 Maternal and Child Health Data Base: Descriptive Narrative. Santa Barbara: University of California, Community and Organization Research Institute: 1990.
Mayfield Responds The letter from Fireman and Goldhaber regarding maternal transfers and hospital perinatal mortality rates raises two important issues of methods used to evaluate perinatal care. We are happy to have the opportunity to discuss these concerns of methods as they apply to our study.'
The potential for bias when maternal transfer occurs has been discussed in the literature.2 Maternal transfers are generally conducted for women with high-risk conditions deemed stable enough to complete the transfer before delivering; maternal transfers are not conducted for women whose problems are so acute orunexpected that they do not have sufficient time for transport.3 In addition, transports are not usually recommended
American Journal of Public Health 1075
