238
Letters
yanani VL, Krebs C, eds. Applications of noninvasive vascular techniques. Philadelphia: WB Saunders, 1988:29-52. 2. Risberg J, Smith P. Prediction of hemispheric blood flow from carotid velocity measurements. Stroke 1980; 11 :399402.
Epidemiology of preeclampsia and eclampsia To the Editors: We read the article by Saftlas et al. (Saftlas AF, Olson DR, Franks AL, Atrash HK, Pokras R. Epidemiology of preeclampsia and eclampsia in the United States, 1979-1986. AM J OBSTET GV!\"ECOL 1990;163:460-5) with great interest and share the concern about the apparent steady trend of increasing rates between 1983 and 1986. Whereas diagnosis-related groups may indeed have prompted more accurate reporting and even overdiagnosing, we have found that reliance on discharge diagnoses at our institution with the use of International Classification of Diseases, Injuries, and Causes of Death, 9th revision (ICD 9) categories, as was done in this study, is enormously problematic for studies of hypertensive disorders in pregnancy. I First, fully 25% of the cases generated by ICD 9 codes at our institution proved to be incorrect diagnoses and were excluded after review of the hospital records. Second, review of all hospital records documented that 53.3% of all true cases had been missed by the coders at our insitution. Given this, we wonder what Saftlas et al. may have done to verify the diagnoses in the National Hospital Discharge Survey data base and to check on the completeness of ascertainment. Clearly this presents a formidable task, but it is nevertheless crucial. Our data suggest that the national rates of preeclamspia and eclampsia may well be at least twice that reported in this article. Kathy L. Ales, MD, and Mary E. Charlson, MD Department of Internal Medicine, Clinical Epidemiology Unit, The New York Hospital-Cornell University Medical College, 515 E. 71st St., 5-900, New York, NY 10021
REFERENCE 1. Ales KL, Charlson ME. In search of the true inception cohort . .J Chron Dis 1987;40:881-5.
July 1991 Am J Obstct Gynecol
perfect. We were unable, however, to review the medical records included in our study; to preserve the confidentiality of the participating hospitals, the National Hospital Discharge Survey does not permit outside reviews. We have, however, had the opportunity to verify discharge diagnoses of preeclampsia from a population-based study we are conducting among Navajo Indian women. When we reviewed medical records from the five hospitals in the study, we found that we had underestimated the incidence by 25% when we had used hospital discharge diagnoses as our data source. In the validation study by Ales and Charlson, I the accuracy of all hypertension complicating pregnancy was assessed, not just preeclampsia. Clearly, the validity of hospital discharge diagnoses can be expected to vary depending on the disease conditions under study.' Our review of their article indicates that at their institution, the rate of all hypertension complicating pregnancy based on hospital discharge diagnoses (7.5%) was underestimated by 57%. Thus it is not clear to us why Drs. Ales and Charlson believe that the national rate of preeclampsia was underestimated at least 100% when their data do not indicate this. Even if the national rate of preeclampsia had been underestimated by as much as 57%, the adjusted rate would be only 4.1 %, a rate much lower than those reported by most hospitalbased studies. The rates of preeclampsia and eclampsia estimated from the National Hospital Discharge Survey are the only nationally representative rates published to date. Although these may underestimate the true incidence, we do not believe that the underestimate is as high as 100%. These data can be used to track trends over time and compare rates across geographic areas, but because they are population-based they should not be expected to compare to rates derived from large referral medical centers. Audrey F. Saftlas, PhD, MPH, and Hani K. Atrash, MD, MPH Division of Reproductive Health, Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control, 1600 Clifton Road, NE, Atlanta, GA 30333
David R. Olson, PhD Reply
To the Editors: We thank Drs. Ales and Charlson for their interest in our article. We are not suprised that the national rate of preeclampsia (2.6%) is lower than that found at individual hospitals such as The New York Hospital because such hospitals serve as major referral centers for high-risk pregnancies. Moreover, because of the demographic makeup of the obstetric population at The New York Hospital, the rates of obstetric conditions in this population almost certainly differ from the rates observed in the general obstetric population of the United States. We agree with Drs. Ales and Charlson that the sensitivity and specificity of discharge diagnoses are not
National Institute of Occupational Safety and Health, Centers for Disease Control, 1600 Clifton Road, NE, Atlanta, GA 30333
Adele L. Franks, MD Office of Surveillance and Analysis, Center for Chronic Disease Prevention and Health Promotion, Centers faT Disease ContTol, 1600 Clifton Road, NE, Atlanta, GA 30333
Robert Pokras, MS National Center for Health Statistics, Hyattsville, MD 20782
REFERENCES 1. Ales KL, Charlson ME. In search of the true inception cohort. J Chron Dis 1987;40:881-5. 2. Institute of Medicine. Reliability of National Hospital Discharge Survey data. Washington DC: National Academy of Sciences. 19RO.
Letters
yanani VL, Krebs C, eds. Applications of noninvasive vascular techniques. Philadelphia: WB Saunders, 1988:29-52. 2. Risberg J, Smith P. Prediction of hemispheric blood flow from carotid velocity measurements. Stroke 1980; 11 :399402.
Epidemiology of preeclampsia and eclampsia To the Editors: We read the article by Saftlas et al. (Saftlas AF, Olson DR, Franks AL, Atrash HK, Pokras R. Epidemiology of preeclampsia and eclampsia in the United States, 1979-1986. AM J OBSTET GV!\"ECOL 1990;163:460-5) with great interest and share the concern about the apparent steady trend of increasing rates between 1983 and 1986. Whereas diagnosis-related groups may indeed have prompted more accurate reporting and even overdiagnosing, we have found that reliance on discharge diagnoses at our institution with the use of International Classification of Diseases, Injuries, and Causes of Death, 9th revision (ICD 9) categories, as was done in this study, is enormously problematic for studies of hypertensive disorders in pregnancy. I First, fully 25% of the cases generated by ICD 9 codes at our institution proved to be incorrect diagnoses and were excluded after review of the hospital records. Second, review of all hospital records documented that 53.3% of all true cases had been missed by the coders at our insitution. Given this, we wonder what Saftlas et al. may have done to verify the diagnoses in the National Hospital Discharge Survey data base and to check on the completeness of ascertainment. Clearly this presents a formidable task, but it is nevertheless crucial. Our data suggest that the national rates of preeclamspia and eclampsia may well be at least twice that reported in this article. Kathy L. Ales, MD, and Mary E. Charlson, MD Department of Internal Medicine, Clinical Epidemiology Unit, The New York Hospital-Cornell University Medical College, 515 E. 71st St., 5-900, New York, NY 10021
REFERENCE 1. Ales KL, Charlson ME. In search of the true inception cohort . .J Chron Dis 1987;40:881-5.
July 1991 Am J Obstct Gynecol
perfect. We were unable, however, to review the medical records included in our study; to preserve the confidentiality of the participating hospitals, the National Hospital Discharge Survey does not permit outside reviews. We have, however, had the opportunity to verify discharge diagnoses of preeclampsia from a population-based study we are conducting among Navajo Indian women. When we reviewed medical records from the five hospitals in the study, we found that we had underestimated the incidence by 25% when we had used hospital discharge diagnoses as our data source. In the validation study by Ales and Charlson, I the accuracy of all hypertension complicating pregnancy was assessed, not just preeclampsia. Clearly, the validity of hospital discharge diagnoses can be expected to vary depending on the disease conditions under study.' Our review of their article indicates that at their institution, the rate of all hypertension complicating pregnancy based on hospital discharge diagnoses (7.5%) was underestimated by 57%. Thus it is not clear to us why Drs. Ales and Charlson believe that the national rate of preeclampsia was underestimated at least 100% when their data do not indicate this. Even if the national rate of preeclampsia had been underestimated by as much as 57%, the adjusted rate would be only 4.1 %, a rate much lower than those reported by most hospitalbased studies. The rates of preeclampsia and eclampsia estimated from the National Hospital Discharge Survey are the only nationally representative rates published to date. Although these may underestimate the true incidence, we do not believe that the underestimate is as high as 100%. These data can be used to track trends over time and compare rates across geographic areas, but because they are population-based they should not be expected to compare to rates derived from large referral medical centers. Audrey F. Saftlas, PhD, MPH, and Hani K. Atrash, MD, MPH Division of Reproductive Health, Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control, 1600 Clifton Road, NE, Atlanta, GA 30333
David R. Olson, PhD Reply
To the Editors: We thank Drs. Ales and Charlson for their interest in our article. We are not suprised that the national rate of preeclampsia (2.6%) is lower than that found at individual hospitals such as The New York Hospital because such hospitals serve as major referral centers for high-risk pregnancies. Moreover, because of the demographic makeup of the obstetric population at The New York Hospital, the rates of obstetric conditions in this population almost certainly differ from the rates observed in the general obstetric population of the United States. We agree with Drs. Ales and Charlson that the sensitivity and specificity of discharge diagnoses are not
National Institute of Occupational Safety and Health, Centers for Disease Control, 1600 Clifton Road, NE, Atlanta, GA 30333
Adele L. Franks, MD Office of Surveillance and Analysis, Center for Chronic Disease Prevention and Health Promotion, Centers faT Disease ContTol, 1600 Clifton Road, NE, Atlanta, GA 30333
Robert Pokras, MS National Center for Health Statistics, Hyattsville, MD 20782
REFERENCES 1. Ales KL, Charlson ME. In search of the true inception cohort. J Chron Dis 1987;40:881-5. 2. Institute of Medicine. Reliability of National Hospital Discharge Survey data. Washington DC: National Academy of Sciences. 19RO.
