SEXUAL HISTORIES GUIDE DIAGNOSTIC sexually transmitted infection (STI) and HIV testing1–3 and counseling efforts. The U.S. Preventive Services Task Force guidelines recommend that clinicians elicit sexual histories with all adolescent and adult patients.1 In addition, the Centers for Disease Control and Prevention (CDC) now advocate an approach to HIV prevention in which interventions of variable intensity are directed to persons based on risk.4 Sexual histories can serve as a starting point in triaging patients into intensified prevention interventions. Information from such histories can be a source of behavioral epidemiologic data to assess population risk and response to STI/HIV prevention interventions. Despite the importance of eliciting a sexual history, no standardized sexual history risk assessment instruments have been delineated5,6 in the United States that could be used by clinics to guide patient management as well as to provide behavioral surveillance data. Many publications encourage clinicians to perform sexual histories; however, few data are available regarding what questions are typically asked by clinicians when they undertake sexual histories in clinical practice.7–18
We set out to collect and describe sexual history forms in use at clinical settings across the United States that provide STI care and HIV counseling and testing services.
Materials and Methods
We focused on cities with populations >200,000 that reported year 2000 gonorrhea, chlamydia, and syphilis cases (n = 65) to assess sexual history documentation practices in reasonably high-volume STI care sites.19 Within each city, a public health sexually transmitted disease clinic or other STI care facility was randomly selected from national directories20 that were maintained for national STI/HIV testing and care telephone hotline referrals and thus were considered to be current and complete at the time. Staff at the selected clinics were asked to send the clinic’s sexual history form(s). We solicited forms in 2002–2003 with up to 3 mailings and 3 phone calls. Clinic operation hours were obtained from web sites. The study received a human subjects certificate of exemption from the University of Washington.
Forms were transcribed into an Excel (Microsoft, Redmond, WA) database and descriptive data of key chart elements were summarized. All data in the database were crosschecked against the original forms for quality assurance. Statistical analysis of the number of sex history items was conducted using STATA 8.0 (STATA Corp., College Station, TX). Sex history-specific items were considered to include questions regarding reason for visit, symptoms, prior STIs; recent sexual behavior, sex partner queries, contraceptive, obstetric, and gynecologic history, HIV risk and testing items, and counseling notation; excluded from this consideration were physical examination, labs, diagnoses, treatment, and drug allergy items.
Forty-eight of 65 clinics participated (74% response), representing 29 states from western, midwestern, northeastern, and southern state regions, Washington, DC, and Puerto Rico. Most (72%) were public STI clinics; others included family planning, college health, gay men’s, and teen clinics. The 17 nonresponding clinics did not differ by geographic distribution from the responding sites. The mean number of hours that responding clinics operated per week was 32 (standard deviation [SD], 12.7), with 34 clinics (71%) open for 40 or more hours.
Sexual History Format
The majority used a single form for both women and men (58%), whereas the remainder used separate forms for males and females. Many used a checklist format with some narrative (58%), whereas 38% used checklists only. Only 2 forms were part of an electronic medical record system, and several others were machine-scannable. Common domains were reason for visit, symptoms, recent sexual behaviors, STI history, HIV risks, obstetric and gynecologic history, and counseling. Other chart elements included examination findings (19% had anatomic pictures of genitalia for charting), diagnosis and treatment, labs, and referrals.
Sexual History Questions
The mean number of “root” questions relevant to sexual history-taking and STI/HIV risk assessment was 28.2 (SD, 14.8; range, 7–71). The number of these items was not associated with the number of clinics hours of operation (P = 0.45 in a linear regression controlling for region of the country, data not shown), with some full-time clinics using fewer items than clinics that saw patients only part-time.
Figure 1 provides a rank order summary of the key questions that are detailed subsequently.
Reason for Visit.
Most asked this (85%), with responses generally including symptoms, contact with STI/HIV or a symptomatic partner, positive screen, and desiring a STI/HIV screen or test.
Symptoms were nearly always noted (96%) using a variety of terms to focus on urethral and vaginal discharge, genital and nongenital lesions, pruritus, dysuria, lymphadenopathy, abdominal pain, and anorectal symptoms, among others. Symptom duration was noted on most of the forms (71%).
Recent Sexual Behaviors.
Most inquired about sex partner gender (male, female, or both) and number of recent partners (83% for each question), and 81% asked date of last sexual exposure.
As seen in Figure 2, the number of past sex partners was ascertained for varying contact periods. The most commonly used timeframe was the past 30 days (38%); 46% used more than 1 timeframe. Fewer than half (40%) asked about type of partner such as new (35%), regular or main and casual (19%), and anonymous (4%).
Few forms (17%) included questions assessing risks for men who have sex with men (MSM) separately from heterosexuals or allowed clinicians to record information separately for male and female partners of persons with partners of both genders. Only 2 clinic forms (4%) were designed to record information about sexual behaviors in relation to partner HIV status.
Most forms (88%) recorded general questions about condom use rather than documenting condom use specifically with anal or vaginal sex acts (13%) or type of sex partner (19%). Of those with condom use questions, one fourth used categorical responses (always, sometimes, never) and one third sought yes/no responses. None asked about characteristics of sex partners with whom condoms were or were not used. Only 5 sites asked about condom use correctness or problems with condom use such as putting the condom on backwards or after genital contact begins.
Sexually Transmitted Infection History.
Almost all (46 of 48 sites) noted previous STI history; 65% noted date of last episode.
Many forms (71%) included at least 1 HIV risk question21,22 such as sex with a bisexual man for females or sex with an HIV-positive partner. For those forms listing an HIV risk section, there was not consistency in which risk factors were queried, although common items can be seen in Figure 3. Several sites (8%) included perinatal HIV exposure as a patient risk factor. Only 5 sites (10%) consistently asked ever and last date of exposure for a range of HIV risk questions.
More than half of the forms documented HIV testing history. Six sites recorded whether the patient wanted to be tested for HIV during the visit (13%).
Obstetric and Gynecologic History.
Most forms (85%) recorded female patients’ pregnancy history; 3 (6%) asked about pregnancy intent. Almost all (44 of 48 sites) asked about contraception, and 48% asked this of both men and women. Contraceptives queried were barrier (92%), hormonal (83%), surgical (65%), and other methods (69%).
Eighty-one percent recorded didactic information such as “health teaching.” Many also noted “counseling” (71%), although whether this involved a client-centered approach23 was difficult to determine. Few (25%) listed specific risk reduction plans such as “maintain monogamy” or “reduce number of sex partners.” Approximately 1 in 3 (29%) documented that informational materials such as videos or pamphlets were presented.
Partner notification parameters, i.e., discussion of ways to contact sex partners of STI/HIV-positive patients about their infection risk, were not consistently present on the sex history forms. Half of the forms recorded referrals to a disease investigation specialist or other referral for partner follow up; 15% of the forms listed contact from the health department for partner exposure as a reason for the patient visit.
Substance use other than injecting drug use (IDU) was assessed approximately half the time (46%); few (8%) noted drug use frequency. Substances commonly assessed included alcohol (25%) and stimulants such as cocaine and amphetamines (25%). One third of the forms (33%) assessed history of sexual assault.
STIs, including HIV, are among the dominant health problems in the world.24 Sexual histories are the starting point for clinical STI/HIV care, counseling, and prevention, and provide the most readily accessible source of behavioral data related to sexual risk. Despite the critical importance of sexual histories, our study demonstrates that even in clinics primarily devoted to STI care and HIV testing, there is little standardization of what constitutes a sexual history. In particular, we identified several areas where omissions may adversely affect clinical or preventive care and may undermine epidemiologic surveillance.
First, questions involving periods of time during which recent sexual exposures occurred (contact periods) are inconsistent. Although short contact periods of 2 weeks or 1 month may provide the most accurate information in terms of recall,25 and encompass incubation periods for many bacterial pathogens as well as for genital herpes, they do not encompass the total infectivity periods for most viral and bacterial pathogens.26 On the one hand, the infectivity for many STIs, including HIV, is thought to peak during the acute phase of infection, and then probably declines as the infection subsides. Furthermore, the person with onset of acute STI symptoms probably was exposed to a source contact during the typical incubation period. Thus, matching the period of inquiry to the incubation period as a probable period of peak infectivity for specific STIs will yield the highest rate of infection in partners brought to examination or offered treatment. For purposes of partner notification, the CDC currently advocates asking about sexual contacts during the past 60 days for chlamydia and gonorrhea, reflecting studies demonstrating a high prevalence of infection in sexual partners encountered during this timeframe.3,27,28 However, 2 months may not be an optimal contact period for assessing HIV risk, in which a 12-month timeframe appears to be considerably more sensitive.29,30 The optimal contact period for sexual histories depends on which STI is of concern, and asking about 2 periods may be best. We believe sexual histories could be standardized to focus on 2 months and 12 months.
Second, sexual histories tend to be undifferentiated by type of patient. Fewer than 1 in 5 clinics asked questions designed to elicit risk histories from MSM specifically. It is worth noting that there had not been specific government agency or national organization recommendations about STI screening among MSM until publication of the 2002 CDC STD Treatment Guidelines.3 Relatively little information was collected about sex partners or behavior with specific types of partners. Recent epidemiologic studies have begun identifying partner characteristics associated with STI/HIV risk. Having older partners has been associated with herpes simplex virus type 2 among heterosexuals in the United States and HIV among MSM in the United States31 and heterosexuals in Africa.32 Among MSM sexually transmitted disease clinic patients, new HIV diagnosis is associated with anonymous sex partners and unprotected anal sex with HIV-positive partners or partners of unknown HIV status.33 To the extent that individuals vary their condom use based on perceived riskiness of the sexual partner and type of relationship,34 interpretation of an aggregate condom use question is problematic and difficult to use in focusing counseling plans. Although the behaviors of one’s partners undoubtedly influences one’s risk of STI/HIV, the accuracy of one’s perception of partner behaviors may be low.35,36 Whether partner-based sexual history questions will prove to have clinical use remains to be seen. Nonetheless, existing data are sufficient to justify incorporating questions about condom use and partner HIV status into routine sexual histories for MSM. An example of this question that is in clinical use can be obtained from the authors upon request.
Problems with condom use are rarely assessed, although user failure is common37 and has been associated with HIV seroconversion among MSM.38,39 Because both condom use consistency and correctness affect STI/HIV risk,40 redressing condom use problems may be an important part of preventive care.
Third, chart organization suggests that if STI/HIV risk reduction plans are negotiated, they are not documented. Defining a specific risk reduction plan has been shown in randomized clinical trials in STI clinics to be effective in reducing incident STI.41–43 Specifying a risk reduction plan may improve documentation and provider adherence to counseling standards,44–46 including counseling of HIV-positive individuals.47
Finally, clinical charts do not consistently record plans for partner notification and treatment. In some clinics, such plans are recorded in records maintained by disease intervention specialists, public health personnel who assist persons in ensuring that their sex partners are treated. However, STI clinics in the United States provide partner notification services to fewer than one third of all persons newly diagnosed with HIV and 20% of persons with bacterial STI.48 It is uncertain the extent to which the failure to record partner notification plans reflects a wider failure to consistently address the issue of partner treatment.
This study is limited by having data based not on observations of actual or standardized patient interviews, but rather on review of forms used to document such encounters. Forms may not capture all existing components of a given patient–clinician interaction. Nonetheless, they do remain the medicolegal document of record regarding the clinical encounter.
We believe that the absence of any defined standard for sexual history-taking is an impediment to clinical care and public health surveillance. However, the barriers to developing a standardized group of questions related to sexual history are formidable. The optimal elements of a general sexual history have not been defined in the United States, and the process of identifying such elements is likely to be complex. As diagrammed in Figure 4, sexual and medical history information collected in STI clinics serve many purposes, and the intended use(s) of data strongly influences what data need to be collected. This reality complicates the goal of standardizing sexual history forms. Identifying a core set of questions that can be used across subpopulations could be based on the association of patient responses with STI/HIV risk and the definition of key modifiable behaviors to be targeted in risk reduction plans and monitored for surveillance purposes. Recently developed guidelines for risk screening of HIV-positive individuals includes some examples of the types of data that might be collected.49 In addition, the CDC has begun to define standardized behavioral surveillance questions.50 Disagreement surrounding the questions selected for this surveillance51 illustrate the difficulties inherent in attaining expert consensus on core sexual health and STI questions.
Efforts are underway in Australia52,53 and the United Kingdom54 to define standardized sexual histories. Initiating a similar process in the United States is likely to be contentious and time-consuming, but would ultimately be valuable, allowing for national pooling of data and comparison across sites of common data elements relevant to STI management, epidemiology, and prevention. These recommended elements, if definable, would be a starting point from which clinics could add or retain locally relevant risk factors, counseling and referral detail, historical data elements, and other items as seen fit by each clinic. Like with clinical guideline promulgation, a consensus-building process would be needed to determine an initial set of data elements and then to update those elements as new pathogens and risk factors become established.
The utility of developing a standardized sexual history is substantially influenced by what can be done with the data generated. For many purposes such as to assess clinic productivity or establish STI screening thresholds, data usability is enhanced by, or even predicated on, having electronic databases. Most forms reviewed in this study appeared to require human data entry into electronic information systems. An analysis of clinic technology needs concluded 7 years ago that computer information systems would be crucial for sexually transmitted disease clinics to increase efficiencies in the face of reduced staffing and increased service demands,55 but it remains unclear how many such clinics today have sufficient computer infrastructure. Optimally, plans to develop a standardized sexual history would include an effort to upgrade electronic information systems to use the data generated to improve clinical services and public health surveillance.
Regardless of chart form used, clinician training to assure consistent and appropriate administration and documentation of sexual histories remains an ongoing need. Provider training infrastructure through the National Network of STD/HIV Prevention Training Centers and the AIDS Education Training Centers is valuable and should be expanded with additional funding and use of distance education technologies, as some have begun doing. Medical, nursing, physician assistant, and allied health schools should maintain or initiate clinical affiliation with STI care facilities56 and incorporate sexual history-taking into curricula focused on the art of clinical history-taking.
STI/HIV risk assessment, clinical management, including counseling, surveillance, and epidemiologic research, would be facilitated by development and adoption of a more standardized sexual history. Despite the complexities of doing so, we believe that discussion for such a process could be conducted under the leadership of the CDC and clinical provider associations and should be explored.
1. US Preventive Services Task Force. The Guide to Clinical Preventive Services: Report of the United States Preventive Services Task Force, 2nd ed. NJ: Reiter’s Scientific and Professional Books, 1997.
2. Peipert JF. Clinical practice. Genital chlamydial infections. N Engl J Med 2003; 349:2424–2430.
3. Centers for Disease Control and Prevention. Sexually transmitted diseases treatment guidelines 2002. MMWR Recomm Rep 2002; 51:1–78.
4. Janssen RS, Holtgrave DR, Valdiserri RO, Shepherd M, Gayle HD, De Cock KM. The serostatus approach to fighting the HIV epidemic: Prevention strategies for infected individuals. Am J Public Health 2001; 91:1019–1024.
5. Curtis R, Holmes K. Individual-level risk assessment for STD/HIV infections. In: Holmes KK, Sparling PF, Mardh P, et al., eds. Sexually Transmitted Diseases, 3rd ed. New York: McGraw-Hill, 1999:669–683.
6. Handsfield HH. STD risk assessment and chlamydia screening: What’s missing? Am J Prev Med 2000; 18:183–185.
7. Temple-Smith MJ, Mulvey G, Keogh L. Attitudes to taking a sexual history in general practice in Victoria, Australia. Sex Transm Infect 1999; 75:41–44.
8. Jolley S. Taking a sexual history: The role of the nurse. Nurs Times 2002; 98:39–41.
9. Tao G, Irwin KL, Kassler WJ. Missed opportunities to assess sexually transmitted diseases in US adults during routine medical checkups. Am J Prev Med 2000; 18:109–114.
10. Bull SS, Rietmeijer C, Fortenberry JD, et al. Practice patterns for the elicitation of sexual history, education, and counseling among providers of STD services: Results from the gonorrhea community action project (GCAP). Sex Transm Dis 1999; 26:584–589.
11. Hurst C, MacDonald J, Say J, Read J. Routine questioning about non-consenting sex: A survey of practice in Australasian sexual health clinics. Int J STD AIDS 2003; 14:329–333.
12. Maheux B, Haley N, Rivard M, Gervais A. Do physicians assess lifestyle health risks during general medical examinations? A survey of general practitioners and obstetrician–gynecologists in Quebec. CMAJ 1999; 160:1830–1834.
13. Haley N, Maheux B, Rivard M, Gervais A. Sexual health risk assessment and counseling in primary care: How involved are general practitioners and obstetrician–gynecologists? Am J Public Health 1999; 89:899–902.
14. Ramsey PG, Curtis JR, Paauw DS, Carline JD, Wenrich MD. History-taking and preventive medicine skills among primary care physicians: An assessment using standardized patients. Am J Med 1998; 104:152–158.
15. Wenrich MD, Curtis JR, Carline JD, Paauw DS, Ramsey PG. HIV risk screening in the primary care setting. Assessment of physicians skills. J Gen Intern Med 1997; 12:107–113.
16. Wenrich MD, Carline JD, Curtis JR, Paauw DS, Ramsey PG. Patient report of HIV risk screening by primary care physicians. Am J Prev Med 1996; 12:116–122.
17. Maheux B, Haley N, Rivard M, Gervais A. STD risk assessment and risk-reduction counseling by recently trained family physicians. Acad Med 1995; 70:726–728.
18. Rabin DL, Boekeloo BO, Marx ES, Bowman MA, Russell NK, Willis AG. Improving office-based physician’s prevention practices for sexually transmitted diseases. Ann Intern Med 1994; 121:513–519.
20. STD Clinic Locator. Accessed May 20, 2002 at www.unspeakable.com
(web site no longer available); Am Social Health Association HIV/STI referral database. Accessed December 19, 2002.
21. Revision of the case definition of acquired immunodeficiency syndrome for national reporting—United States. MMWR Morb Mortal Wkly Rep 1985; 34:373–375.
22. Rietmeijer CA, Lansky A, Anderson JE, Fichtner RR. Developing standards in behavioral surveillance for HIV/STD prevention. AIDS Educ Prev 2001; 13:268–278.
23. Revised guidelines for HIV counseling, testing, and referral. MMWR Recomm Rep 2001; 50:1–57.
24. Hogg R, Cahn P, Katabira ET, et al. Time to act: global apathy towards HIV/AIDS is a crime against humanity. Lancet 2002; 360:1710–1711.
25. Catania JA, Gibson DR, Chitwood DD, Coates TJ. Methodological problems in AIDS behavioral research: Influences on measurement error and participation bias in studies of sexual behavior. Psychol Bull 1990; 108:339–362.
26. Rowley J, Berkely S. Sexually transmitted diseases. In: Murray CJL, Lopez AD, eds. Health Dimensions of Sex and Reproduction. Boston: Harvard University Press for WHO/World Bank, 1998.
27. Potterat JJ, Zimmerman-Rogers H, Muth SQ, et al. Chlamydia transmission: Concurrency, reproduction number, and the epidemic trajectory. Am J Epidemiol 1999; 150:1331–1339.
28. Potterat JJ, Woodhouse DE, Pratts CI, Markewich GS, Fogle JS. Women contacts of men with gonorrhea: Case-finding yields. Sex Transm Dis 1983; 10:29–32.
29. Colfax GN, Buchbinder SP, Cornelisse PG, Vittinghoff E, Mayer K, Celum C. Sexual risk behaviors and implications for secondary HIV transmission during and after HIV seroconversion. AIDS 2002; 16:1529–1535.
30. MacKellar DA, Valleroy LA, Secura GM, et al. Repeat HIV testing, risk behaviors, and HIV seroconversion among young men who have sex with men: A call to monitor and improve the practice of prevention. J Acquir Immun Defic Syndr 2002; 29:76–85.
31. Morris M, Zavisca J, Dean L. Social and sexual networks: Their role in the spread of HIV/AIDS among young gay men. AIDS Educ Prev 1995; 7:24–35.
32. Gregson S, Nyamukapa CA, Garnett GP, et al. Sexual mixing patterns and sex-differentials in teenage exposure to HIV infection in rural Zimbabwe. Lancet 2002; 359:1896–1903.
33. Vu MQ, Steketee RW, Valleroy L, Weinstock H, Karon J, Janssen R. HIV incidence in the United States, 1978–1999. J Acquir Immun Defic Syndr 2002; 31:188–201.
34. Peterman TA, Lin LS, Newman DR, et al. Does measured behavior reflect STD risk? An analysis of data from a randomized controlled behavioral intervention study Project RESPECT Study Group. Sex Transm Dis 2000; 27:446–451.
35. Stoner BP, Whittington WL, Aral SO, Hughes JP, Handsfield HH, Holmes KK. Avoiding risky sex partners: perception of partners’ risks v partners’ self reported risks. Sex Transm Infect 2003; 79:197–201.
36. Drumright LN, Gorbach PM, Holmes KK. Do people really know their sex partners? Concurrency, knowledge of partner behavior, and sexually transmitted infections within partnerships. Sex Transm Dis 2004; 31:437–442.
37. Warner L, Clay-Warner J, Boles J, Williamson J. Assessing condom use practices. Implications for evaluating method and user effectiveness. Sex Transm Dis 1998; 25:273–277.
38. Calzavara L, Burchell AN, Remis RS, et al. Delayed application of condoms is a risk factor for human immunodeficiency virus infection among homosexual and bisexual men. Am J Epidemiol 2003; 157:210–217.
39. Stone E, Heagerty P, Vittinghoff E, et al. Correlates of condom failure in a sexually active cohort of men who have sex with men. J Acquir Immun Defic Syndr Hum Retrovirol 1999; 20:495–501.
40. Fishbein M, Pequegnat W. Evaluating AIDS prevention interventions using behavioral and biological outcome measures. Sex Transm Dis 2000; 27:101–110.
41. Fortenberry JD. Clinic-based service programs for increasing responsible sexual behavior. J Sex Res 2002; 39:63–66.
42. Kamb ML, Fishbein M, Douglas JM Jr, et al. Efficacy of risk-reduction counseling to prevent human immunodeficiency virus and sexually transmitted diseases: A randomized controlled trial. Project RESPECT Study Group. JAMA 1998; 280:1161–1167.
43. Is HIV/Sexually Transmitted Disease Prevention Counseling Effective Among Vulnerable Populations? A subset analysis of Data collected for a randomized, controlled trial evaluating counseling efficacy (Project RESPECT). Sex Transm Dis 2004; 31:469–474.
44. Castrucci BC, Kamb ML, Hunt K. Assessing the Center for Disease Control and Prevention’s 1994 HIV counseling, testing, and referral: Standards and guidelines: How closely does practice conform to existing recommendations? Sex Transm Dis 2002; 29:417–421.
45. Erbelding EJ, Chung SE, Kamb ML, Irwin KL, Rompalo AM. New sexually transmitted diseases in HIV-infected patients: Markers for ongoing HIV transmission behavior. J Acquir Immun Defic Syndr 2003; 33:247–252.
46. Silvestre AJ, Gehl MB, Encandela J, Schelzel G. A participant observation study using actors at 30 publicly funded HIV counseling and testing sites in Pennsylvania. Am J Public Health 2000; 90:1096–1099.
47. Incorporating HIV prevention into the medical care of persons living with HIV. Recommendations of CDC, the Health Resources and Services Administration, the National Institutes of Health, and the HIV Medicine Association of the Infectious Diseases Society of America. MMWR Recomm Rep 2003; 52:1–24.
48. Golden MR, Hogben M, Handsfield HH, St Lawrence JS, Potterat JJ, Holmes KK. Partner notification for HIV and STD in the United States: Low coverage for gonorrhea, chlamydial infection, and HIV. Sex Transm Dis 2003; 30:490–496.
49. Recommendations for incorporating human immunodeficiency virus (HIV) prevention into the medical care of persons living with HIV. Clin Infect Dis 2004; 38:104–121.
50. Rietmeijer CA, Lansky A, Anderson JE, Fichtner RR. The current good should precede the future best: A response to a response. AIDS Educ Prev 2002; 14:348–350.
51. Catania JA, Dolcini MM, Laumann EO, Osmond D, Bolan G, Canchola J. A response to ‘developing standards in behavioral surveillance for HIV/STD prevention.’ AIDS Educ Prev 2002; 14:343–347.
52. NSW Health Sexual Health Minimum Data Set: Data Dictionary, version 2.0. February 2002.
53. Tideman RL, Pitts MK, Fairley CK. Current practice of sexual history taking by sexual health physicians in Australia and New Zealand. Sex Transm Infect 2004; 80:389–391.
54. Robinson A. Modernisation in GUM/HIV Services. London: RCP Joint Specialty Committee for GU Medicine, 2004.
55. Conlon RT. Introducing technology into the public STD clinic. Health Educ Behav 1997; 24:12–19.
56. Stamm WE KS, Holmes KK. Clinical training in venereology in the United States and Canada. JAMA 1982; 248:2020–2024.