Evaluating Sources of Traumatic Spinal Cord Injury Surveillance Data in Colorado

Transcription

1 American Journal of Epidemiology Copyright C 1997 by The Johns Hopkins University School of Hygiene and Public Health All rights reserved Vol. 148, No. 3 Printed In USA. Evaluating Sources of Traumatic Spinal Cord Injury Surveillance Data in Colorado Renee L Johnson, 1 ' 2 Barbara A. Gabella, 2 Kenneth A. Geitiart, 1 Jeannie McCray, 2 Jean C. Menconi, 2 and Gale G. Whiteneck 1 The purpose of this study is to evaluate the sources reporting hospitalized spinal cord injury cases to the statewide, population-based surveillance system in Colorado for the year Three reporting sources were evaluated: clinical contact persons, medical records departments, and a centralized statewide hospital discharge database. Two evaluation strategies were utilized; these include both measures of accuracy and estimates of missed cases. For the latter, capture-recapture techniques were used to estimate the number of hospitalized spinal cord injury cases missed by all three reporting sources. The clinical contact persons reported 84 confirmed cases, missed 80 confirmed cases, and reported 10 cases that were later determined not to have spinal cord injuries, resulting in a sensitivity of Medical records departments and the discharge database reported 143 and 147 cases, respectively, missed 21 and 17 confirmed cases, and reported 118 and 69 cases that were later determined not to be cases of hospitalized injuries of the spinal cord, resulting in sensitivities of 0.87 and Capture-recapture results indicate all three sources combined missed an estimated 1-5 cases, yielding a total annual incidence rate for hospitalized spinal cord injury ranging from 45.1 to 46.3 per million population. Am J Epidemiol 1997; 146: evaluation studies; population surveillance; spinal cord injuries Accurate surveillance is required to quantify the morbidity and mortality associated with injury (1). As with disease prevention programs, the ability to conduct accurate surveillance is also key to developing and sustaining effective injury control programs (1-3). Spinal cord injury has been a model or prototype for injury surveillance (3), because it is a relatively rare condition that is clinically identifiable and generally unequivocally diagnosed. Indeed, it is often severely disabling, affects primarily young people, and is extremely costly in personal, financial, and societal terms. Previous reported annual hospitalized incidence rates of spinal cord injury range from 28.5 to 85.0 per million population (4-10). In 1987 the Council of State and Territorial Epidemiologists recommended that spinal cord injury be the first injury condition to be reported to the National Notifiable Disease System, which, under the auspices of the Centers for Disease Control and Prevention, Received for publication November 8, 1996, and accepted for publication April 7, Abbreviation: ICD-9-CM, International Classification of Diseases, Ninth Revision, Clinical Modification. "* Craig Hospital, Research Department, Englewood, CO. 2 Colorado Department of Public Health and Environment, Injury Epidemiology Program, Denver, CO. Reprint requests to Renee Johnson, Craig Hospital, 3425 S. Clarkson Street, Englewood, CO assembles and analyzes data collected by the individual states (3). Colorado is one of the 34 states currently involved in some form of spinal cord injury surveillance. Along with the other 33 states, Colorado faces several surveillance challenges. With limited resources available, surveillance must strive for efficiency by maximizing both the sensitivity and the positive predictive value of reporting sources (1, 2, 11-14). Each case that surveillance fails to identify not only reduces the accuracy of the estimates, but also diminishes the opportunity for individuals to receive needed services. When specific types of cases or individuals are systematically missed, interventions may be mistargeted, research may be misguided, and efficacy evaluations may be inaccurate. On the other hand, a case reported to the surveillance program that does not meet the system's carefully defined case definition (a "false positive" report) takes staff time to review, verify, and exclude, time that could be devoted elsewhere. The purpose of this study is to describe the Colorado spinal cord injury surveillance system and to evaluate the effectiveness of three distinct, yet overlapping sources of hospitalized spinal cord injury data in Colorado. These sources are 1) key clinicians in designated hospitals, 2) medical records departments, and 3) a centralized statewide hospital discharge database. 266

2 Evaluating Sources of Traumatic Spinal Cord Injury Surveillance Data 267 Effectiveness was evaluated using two approaches. First, the traditional approach of calculating both sensitivity and positive predictive value was used to evaluate the accuracy of each source (1, 14, 15). Second, the number of cases missed by each reporting source was estimated using capture-recapture analysis (16-21). In addition, the annual incidence of hospitalized spinal cord injury in Colorado was estimated. MATERIALS AND METHODS The Colorado Spinal Cord Injury Early Notification System is a statewide, population-based, traumatic spinal cord injury surveillance system. New cases of hospitalized spinal cord injury are included in the surveillance system if they meet all the criteria in the following case definition. 1) The injury results in some degree of spinal cord-related motor, sensory, and bowel and/or bladder impairment, which can be temporary or permanent. 2) The injury is sustained by a Colorado resident, whether or not the injury event occurred within the state. 3) The injury is caused by trauma. Three sources were utilized to identify potential cases of spinal cord injury diagnosed during First, designated clinicians, in key hospitals equipped to treat neuro-trauma survivors, reported new injuries upon admission. Second, a medical records reporting system identified cases postdischarge, using the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM), codes 806 (fracture of vertebral column with spinal cord injury) and 952 (spinal cord injury without evidence of spinal bone injury) (11, 22); a state Board of Health regulation mandates such reporting by all acute care and rehabilitation facilities. Finally, the third source of spinal cord injury cases was the Colorado Hospital Association discharge data set. The Colorado Hospital Association is a voluntary membership organization to which all nonfederal acute care facilities in Colorado report hospital discharge data. These acute care facilities also report their inpatient rehabilitation discharge data to the Colorado Hospital Association. Additionally, four of Colorado's seven freestanding inpatient rehabilitation facilities also report to the hospital association. The 15 discharge diagnosis fields in the Colorado Hospital Association data set were queried again using the ICD-9-CM codes 806 and 952 (11, 22). Both the medical records and hospital association reports were additionally queried for the first half of 1995 to ensure inclusion of persons who had sustained their injuries in 1994 but were not discharged until All spinal cord injury reports, regardless of reporting source, were subjected to a medical records review for validation of spinal cord injury. This validation of the spinal cord injury diagnosis was conducted by reviewing for the following components of the International Standards for Neurological and Functional Classification of Spinal Cord Injury, Revised 1992 (23): absence or impairment of motor and/or sensory functioning, establishment of a neurologic level, and the presence of a clinically documented spinal cord syndrome. These key clinical features had to be noted in the medical records. Motor and sensory testing, neurologic examinations of both spinal and peripheral nerves, and diagnostic imaging reports were reviewed. This records review was conducted by staff specifically trained in the diagnosis and/or care of spinal cord injury and included both a physician and a physical therapist. When a report did not meet the eligibility criteria, reasons for this "false positive" report were recorded. The accuracy of each reporting source was evaluated in terms of the number of the total true positive cases as well as the number of false positive cases that were reported by each source. This information was used to calculate sensitivity, that is, the proportion of cases of a disease or health condition that are detected by each source, and to determine positive predictive value, the proportion of reports by each source that actually have die condition being monitored (14, 15). In calculating sensitivity, we used the "gold standard" of the total number of confirmed cases from all sources as the denominator (the total number of "true positive" cases). Capture-recapture statistical techniques (16-21) were used to estimate the number of cases not ascertained by any of the reporting sources and to estimate overall incidence, using both the number of cases common to sources and the number of cases unique to each source. As described by Hook and Regal (17, 20), the nearly unbiased estimator based on the maximum likelihood estimates from the log linear model, which adjusts for small sample bias, was used in this analysis. Since all three sources provided admission-based data, one might expect these sources to be positively dependent. An extreme example of positive dependence would be when the list of cases from one source for a particular hospital is directly obtained from other sources for the same hospital (18). Positive dependence results in an underestimate of the total injuries in the population (17-20). To estimate the number of missed cases and the total incidence while adjusting for all possible dependencies among sources, we used all three sources simultaneously by pooling two of the three sources and comparing them as a unit with the third source (referred to as "source X" in Results) (17, 19, 20). This method was repeated three times, by

3 268 Johnson et al. changing which two reporting sources were pooled and which source was not. Next, the three sources were used separately, and the potential dependencies were modeled explicitly by testing all two-way interaction terms (19, 21). Given the nature of spinal cord injury and the potential for each individual case to have more than one hospital admission, some cases had more than one opportunity to be reported by each source, particularly those patients who required inpatient rehabilitation in addition to their acute care hospitalization. To examine this difference in reporting opportunity, the reports were stratified by whether or not inpatient rehabilitation was utilized (17). RESULTS The combination of all three sources (clinical contacts, medical records, and the Colorado Hospital Association) identified a total of 311 reports of suspected spinal cord injury diagnosed in Of these, 164 reports were verified as meeting the surveillance case definition (true positive cases), and the remaining 147 reports did not meet the case definition (false positive cases). Persons who had some form of paralysis but who did not meet the case definition or persons who had sustained their spinal cord injury in a previous year accounted for 32.0 percent of the total 147 false positive reports. Persons who had a bone fracture of the vertebral column with no associated spinal cord injury-related paralysis generated 28.6 percent of the false reports. Nineteen percent of the persons excluded had unilateral symptoms at one neurologic level that TABLE were most likely due to a radicular or peripheral nerve deficit. Finally, 17.7 percent of the false reports appeared to be inexplicable ICD-9-CM coding errors. These persons with miscoded records had a wide variety of diagnoses, including neck and back pain, outpatient radiologic and laboratory procedures, an upper extremity amputation, a brachial plexus injury, and a contusion of the coccyx. Table 1 displays the numbers of confirmed hospitalized spinal cord injury cases (true positive cases) identified by each source. Among the true positive cases, 21 (13 percent) were identified by only one source: five were reported by a clinical contact only, nine were reported solely by a medical records department, and seven were reported only by the hospital association. A total of 76 cases (46 percent) were identified by only two sources, and the remaining 67 cases (41 percent) were reported by all three sources. Similarly, table 1 also displays the numbers of false positive cases identified by each source. It can be seen in table 2 that clinical contact persons reported 84 (51 percent) of the confirmed cases, medical records reported 143 (87 percent), and the Colorado Hospital Association reported 147 (90 percent) of the confirmed cases. The sensitivity and positive predictive value of each of the reporting sources are presented in table 2. Clinical contact persons were the least sensitive. Conversely, the hospital association data set was the most sensitive. Among the three sources, the clinical contact persons had the highest positive predictive value. Also displayed in table 2 is the fact that, when any two of the reporting sources are combined, the resulting sensitivity was 0.95 or higher. Total number of true and false positive hospitalized spinal cord injury (SCI) cases by reporting sources, Colorado, Confirmed cases of hospitalized SCI cases (no 164) False positive reports of hospitalized SCI (n-147) Reported only by dwcal contacts (CC) Reported only by medfcaj records (MR) Reported only by Colorado Hospital Association (CHA) All reports stratified by case inclusion status Reported Reported Reported Reported by both by both by both bycc, CCandMR MR and CHA CC and CHA MR, and CHA Case inclusions stiatified by utilization ofinpatient rehabilitation services Hospitalized cases that reported inpatient rehabilitation (n-102) Hospitalized cases that did not require inpatient rehabilitation (n-62)

4 Evaluating Sources of Traumatic Spinal Cord Injury Surveillance Data 269 TABLE 2. Senaitlvfty and positive predictive value of hospitalized spinal cord injury (SCI)* by reporting sources, Colorado, 1994 Reporting sources Clinical contacts (CC) Medical records (MR) Colorado Hospital Association (CHA) True positives Cnje( +)) False posllves (lateen)) Raise mgativas (lalseh) SenslMtyt (tnie(+v [true{+) + false(-)d Positive predictive vakiet (true< +V [tme(+) + tatoe(+)d CC and MR combined CC and CHA combined MR and CHA combined * Total confirmed cases (true(+) + false(-)): n 164 for all three sources (table 1). t Klaucke DN. Evaluating public health surveillance. In: Teutsch SM, Churchhill RE, eds. Principles and practice of public health surveillance. Oxford: Oxford University Press, 1994: Based on the capture-recapture method, table 3 demonstrates that the estimates of 1994 spinal cord injury incidence in Colorado range from to Regardless of which single source ("source X") was evaluated against the combination of the other two sources ("pooled source"), the resulting annual spinal cord injury rates varied little, from 45.1 to 46.3 per million population. Using the three sources separately with an interaction term for medical records and the hospital association, the model resulted in an estimate of 4.8 missing cases or an annual total incidence of cases. Medical records and the Colorado Hospital Association were positively dependent (p < 0.01, G 2 = 3.45, df = 2), while the evaluation of the other two-way interactions indicated that they were not dependent (19, 21). Cases who received inpatient rehabilitation had a minimum of two separate hospital admissions and, therefore, a greater probability of being reported than cases who did not. Table 1 examines these differences in reporting among the two types of cases. Clinical contact persons were able to identify 75 percent of the cases that went to inpatient rehabilitation, while they identified only 13 percent of the cases that did not. On the other hand, medical records and the Colorado Hospital Association were equally good at identifying both rehabilitation and nonrehabilitation cases. When the capture-recapture estimates of missed cases from each stratum are summed, the total estimate of missed cases is 19, based on the three-source model with a hospital association-medical record interaction. DISCUSSION This study examined three sources of spinal cord injury surveillance data: two that have been used by Colorado regularly since 1989 as a part of its comprehensive surveillance and a third that was identified and tapped particularly for the research reported here. The first two sources, clinical contact persons based in key acute care hospitals and trauma centers and retrospective ICD-9-CM code-based reporting by all of Colorado's hospital medical records departments, reported 157 confirmed cases. They also accounted for another 127 falsely identified cases, giving Colorado's regular surveillance system a sensitivity of 0.96 and a positive TABLE 3. Estimates of number of cases missed and total incidence of hospitalized spinal cord injury (SCI) based on the capture-recapture method, Colorado, 1994 Source X Clinical contacts Colorado Hospital Association Medical records In source Xandlnlhe pooled source* (a) Unique to source X Not In source X but hi the pooled source (c) Total In source X Estimate of missed Estimated total Incidence [(a+ 6+1) (a+1)) (a + 1)] Estimated annual Incidence rate/ rrtdtont * Total confirmed cases (n «164 for all three sources). t The Colorado population figure of 3,653,600 for 1994 is a 1993-based projection from the Demographic Section, Colorado Department of Local Affairs

5 270 Johnson et a). predictive value of These findings compare favorably with those reported by spinal cord injury surveillance programs in both Utah and Oklahoma. Utah has reported a sensitivity of 0.89 and a positive predictive value of 0.61 for a hospital discharge databased system (7). Oklahoma reports an overall spinal cord injury surveillance system sensitivity of 0.77 but reports no information on cases that are reported and then later excluded (8). The third data source, the Colorado Hospital Association's hospital discharge data set, is one that has tended to become available much later after discharge than the other two and, thus, up until the time of this research, it has not been a useful means of timely spinal cord injury surveillance in Colorado. Nonetheless, in yielding a sensitivity of 0.90 and a positive predictive value of 0.68, its data were slightly better than clinical and medical records reporting in Colorado and at least comparable to the values reported by Oklahoma and Utah. In terms of any one single source of case identification, the total number identified by either the hospital association (147 cases) or medical records (143 cases) provided 87.0 percent or 84.6 percent, respectively, of the highest capture-recapture estimate. We feel that either of these two sources alone, followed by thorough medical records review, may provide adequate case identification for the purpose of basic surveillance. As expected, medical records departments and the Colorado Hospital Association were found to be positively dependent, meaning that being in one source influences the probability of being captured by the other source. This positive dependence explains why the incidence estimates generated when using only two sources were so similar to estimates generated by using all three sources (with an interaction term for medical records and the hospital association). Both medical records and the hospital association reported a similar number of true positive cases. In fact, 131 of the 164 confirmed cases (80 percent) were common to both data sources. These two sources also had similar values for sensitivity and a similar (though low) positive predictive value. The absolute number of false positive reports from both sources was large, accounting for 138 (94 percent) of all 147 false positive reports. The high number of false positive cases reported by both the hospital association and medical records causes one to consider excluding the subset of 806 and 952 coded cases that have either a fourth or fifth digit that indicates an "unspecified spinal cord injury." To address this issue, we have inspected die hospital association data set. All reports of admissions were considered; the 216 cases identified by the hospital association resulted in 427 admissions reported. When the hospital association reports with unspecified codes are grouped and compared with the reports from the hospital association without unspecified codes, we find that 28 percent of those reports that were later excluded had an unspecified code while 33 percent of those reports later included had unspecified codes. This difference is nonsignificant (chi-square = 1.151, df = 1, p = 0.28); thus, excluding the "unspecified" coded records would not be a useful refinement to the surveillance system. Perhaps most importantly, the large number of false positive reports from medical records departments or the hospital association was costly, in terms of the staff time needed to review medical records and to verify case inclusion. Yet these reviews could not be eliminated given the low positive predictive value of either source. Overall, both medical records departments and the hospital association contributed a similar number and type of cases to surveillance. The only major difference in reporting between medical records and the hospital association that impacts surveillance activities was the current lack of specific personal identifiers available from the hospital association data. This lack of an identifier unique to the person, such as a name, makes identifying the same case from multiple sources difficult and time consuming. Clinical contact persons reported only 84 (51 percent) of the confirmed spinal cord injury cases in 1994, similar to the number in previous years (4). Clinical contact persons, however, performed better with respect to persons who went to inpatient rehabilitation. Also, clinical contact persons reported few false positive cases, thereby reducing the resources wasted by reviewing the medical records of false positive reports. In fact, 84 (89 percent) of the 94 reports by clinical contact persons truly had a spinal cord injury. Neither medical records nor the hospital association was as accurate (68 percent and 55 percent, respectively). The accuracy of each individual reporting source is impacted by the accuracy of the chosen "gold standard." The fact that the total number of confirmed spinal cord injury cases (164 cases) included in the "gold standard" was so close to the lowest estimate generated by capture-recapture (164.8 cases) indicates the validity of the calculations presented in table 2. In fact, when the highest estimate generated by capturerecapture (169.0 cases) was used to calculate the measures of accuracy, the sensitivity for all sources drops only slightly: clinical contacts from 0.51 to 0.50, medical records from 0.87 to 0.85, and the Colorado Hospital Association from 0.90 to The positive pre-

6 Evaluating Sources of Traumatic Spinal Cord Injury Surveillance Data 271 dictive value is not affected by the change in the number of false negative (unidentified) cases. With regard to persons who did not receive inpatient rehabilitation, medical records departments and the hospital association were better sources for reporting than were the clinical contact persons. This trend is evidenced by Colorado's own spinal cord injury surveillance history. The spinal cord injury surveillance system began in 1986 with clinical contact persons as the only reporting source. Based on this source, the number of cases ranged from 86 in 1986 to 80 in Correspondingly, annual spinal cord injury rates ranged from 26 to 24 per million. In 1989, hospitals became subject to a Board of Health regulation requiring them to report hospitalized spinal cord injury to the state health department. With the addition of this second source, incidence increased to 121 in 1989, with an annual rate of 36.8 per million population (4). As the incidence rate rose, it was clearly due to the inclusion of persons with less severely injured spinal cords. The three reporting sources varied greatly with regard to timeliness. Clinical contacts were the most timely, generally reporting cases within the first few days of injury. This not only allows for timely surveillance but also for provision of services to the newly injured person. Bom medical records and the hospital association had a lengthy delay in reporting, and they both reported after discharge, making early contact with survivors and provision of services impossible. Medical records departments respond to regular mailings and requests for cases identified out of their database. In Colorado these requests are made either every other month or every 6 months. Hospitals participating in the clinical contact system are queried every 6 months. Both request patterns allow for data collection and medical records review to be an ongoing process throughout the calendar year. A full year of data on discharges in the prior calendar year was available from the Colorado Hospital Association 6 months after the close of the prior calendar year. Since the current method of surveillance uses two data sources and a third source became available, we were interested in seeing how the addition of or a change to the third source would impact the overall surveillance results. A pooled approach was used to adjust for possible dependencies between the data sources. Based on goodness-of-fit statistics (G 2, the Akaike Information Criterion, and the Bayesian Information Criterion), more than one model provided a good fit for these data. Statisticians do not agree on which goodness-of-fit statistic to use (19). We chose the model with only one interaction (hospital association and medical records) based on both the G 2 statistic and on the fact that this interaction was the only one that was statistically significant. This model is equivalent to pooling hospital association and medical records sources, except that it provides the usual maximum-likelihood estimates and, therefore, does not adjust for small samples. Additionally, the positive dependency seen between the hospital association and medical records is plausible given what we know about the data sources. Both the medical records and hospital association sources are based on the medical record. In contrast, the clinical contacts are a variety of health care professionals located throughout the hospital who do not necessarily impact the diagnostic coding that appears on the medical record. We think that the chosen model gives a reasonable estimate of overall incidence. The estimate of 19 missed cases, based on stratifying by use of inpatient rehabilitation, provides an outside range. This stratification, however, introduces error related to the hospital association's inability to identify all rehabilitation admissions and the clinical contacts' decreased likelihood of needing to provide care and services to the less severely injured. One limitation of capture-recapture analysis is that it can only estimate the cases that are systematically missed by all three sources if the case was in the pool of cases to be captured. Therefore, the number of Colorado residents injured out of state and who do not return to Colorado for inpatient services cannot be estimated by the capture-recapture method. In addition to this group of potentially missed cases, there is also a very small group of patients that had a reduced chance of being reported. The Colorado Hospital Association, a voluntary membership organization, does not capture patients who were seen at Colorado's federal facilities, although they are covered by the medical records reporting requirement. In summary, this study measured sensitivity and positive predictive value and described the timeliness of three reporting sources of spinal cord injury in Colorado. Evaluation of these commonly assessed attributes of surveillance systems (14, 15) has resulted in the following conclusions/recommendations: 1. Careful review of all suspected cases of spinal cord injury is necessary because of the substantial number of false positive cases identified. 2. Both medical records reports and the hospital discharge data set were very complete and may be best for case identification of all types and severities of injury. 3. Clinical contacts provided the most timely reporting of cases that were transferred to inpatient rehabilitation; however, they were less likely to identify less severely injured cases.

7 272 Johnson et al. 4. The purpose for surveillance data is key to choosing a case identification method. The type and timeliness of the surveillance information required may influence the method chosen. The results of this study indicate the potential benefits and limitations of similar reporting sources in other states. This study reinforces the need to evaluate surveillance in other states as surveillance priorities are both established and changed. ACKNOWLEDGMENTS Supported in part by Disability Prevention Cooperative Agreement U59/CCU from the Centers for Disease Control and by grant H133N5OOO1 from the National Institute on Disability and Rehabilitation Research. The authors especially thank Russell Rickard for his statistical expertise. REFERENCES 1. Sniezek JE, Finklea JF, Graitcer PL. Injury coding and hospital discharge data. JAMA 1989;262: Graitcer PL. The development of state and local injury surveillance systems. J Safety Res 1987;18: Harrison CL, Dijkers M. Spinal cord injury surveillance in the United States: an overview. Paraplegia 1991;29: Colorado Department of Public Health and Environment annual report of the spinal cord injury early notification system. Denver, CO: Colorado Department of Transportation Printing Office, Fine PR, Kuhlemier KV, Devivo MJ, et al. Spinal cord injury: an epidemiologic perspective. Paraplegia 1979;17: Kraus JF, Franti CE, Riggins RS, et al. Incidence of traumatic spinal cord lesions. J Chronic Dis 1975 ;28: Thurman DJ, Burnett CL, Jeppson L, et al. Surveillance of spinal cord injuries in Utah, USA. Paraplegia 1994;32: Price C, Makintubee S, Herndon W, et al. Epidemiology of traumatic spinal cord injury and acute hospitalization and rehabilitation charges for spinal cord injuries in Oklahoma, Am J Epidemiol 1994; 139: Acton PA, Farley T, Freni LW, et al. Traumatic spinal cord injury in Arkansas, 1980 to Arch Phys Med Rehabil 1993;74: Warren S, Moore M, Johnson MS. Traumatic head and spinal cord injuries in Alaska ( ). Alaska Med 1995;37: Thurman DJ, Sniezek JE, Johnson D, et al. Guidelines for surveillance of central nervous system injury. Atlanta: Centers for Disease Control and Prevention, Goldberg J, Gelfan HM, Levy PS. Registry evaluation methods: a review and case study. Epidemiol Rev 1980;2: Hanrahan LP, Moll MB. Injury surveillance. Am J Pubic Health 1989;79(suppl): Klaucke DN. Evaluating public health surveillance. In: Teutsch SM, Churchhill RE, eds. Principles and practice of public health surveillance. New York: Oxford University Press, 1994: Klaucke DN, Buehler JW, Thacker SB, et al. Guidelines for evaluating surveillance systems. MMWR Morb Mortal Wkly Rep 1988;37(suppl5):l Fienberg SE. The multiple recapture census for closed populations and incomplete 2k contingency tables. Biometrika 1972;59: Hook EB, Regal RR. Effect of variation in probability of ascertainment by sources ("variable catchability") upon "capture-recapture" estimates of prevalence. Am J Epidemiol 1993;137: Wittes JT, Colton T, Sidel VW. Capture-recapture methods for assessing the completeness of case ascertainment when using multiple information sources. J Chronic Dis 1974;27: Hook EB, Regal RR. Capture-recapture methods in epidemiology: methods and limitations. Epidemiol Rev 1995; 17: Hook EB, Regal RR. The value of capture-recapture methods even for apparent exhaustive surveys. Am J Epidemiol 1992; 135: SAS Institute, Inc. The CATMOD procedure. In: SAS/STAT user's guide, release 6.03 ed. Cary, NC: SAS Institute, Inc., 1988: US Health Care Financing Administration. International classification of diseases. Ninth Revision, clinical modification. Vol 1. Washington, DC: US GPO, (DHHS publication no. (PHS) ). 23. American Spinal Injury Association and International Medical Society of Paraplegia. Internationa] standards for neurological and functional classification of spinal cord injury, revised Chicago: American Spinal Injury Association, 1994.