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the Findings were unchanged when the analysis was restricted to participants with complete data. Laboratory Analysis The cervicovaginal lavage specimens were refrigerated within

1 hour of collection and transported to a central laboratory for processing. A 1-mL aliquot was removed and frozen (at -70 °C and kept on dry ice during shipment) until preparation for HPV determination by polymerase chain reaction (PCR)-based analysis, which was performed without knowledge of medical history, cytologic diagnosis, or other data. Of the 1-mL aliquot,

50 (iL was prepared for PCR analysis (13). Negative controls containing only human DNA (from the K562 cell line;

the American Type Culture Collection, Rockville, Md.) were interspersed to monitor potential con- tamination during sample preparation. Gene amplification methods including PCR primers, buffer, reaction components, and cycling parameters have been previously described in detail (14). Duplicate aliquots of

5 u.L (0.05% of the original 10-mL lavage) were tested using a consensus primer PCR method targeting the LI gene region of HPV (15,16). The amplification of a human B-globin gene fragment was used as an internal control for sample integrity. The PCR products were analyzed using dot blot hybridization as previously described (14), with minor modifications including the use of a nonradioactive detection method (17). Blots were individually hybridized with the generic probe, designed to detect most known HPV types, as well as probes specific for HPV types 6711, 16, 18, 26, 31, 33, 35, 39, 40, 42, 45, 51, 52, 53, 54, 55, 57, and 59. In addition, we probed for preliminarily identified types known as PAP88, PAP155, PAP238A, PAP251, PAP291, and W13B. Recombinant plasmids were provided by Dr. G. Orth (Institut Pasteur, Paris, France) for HPVs 33, 39, 42, 54, and 55;

Dr. K. Shah (the Johns Hopkins University, Baltimore, Md.) for HPV 45;

Dr. S. Silverstein (Columbia University, New York, N.Y.) for HPV 51;

Dr. W. Lancaster (Wayne State Medical School, Detroit, Mich.) for HPV 52;

and Dr. R. Ostrow (University of Minnesota, Minneapolis) for HPV 26. On the basis of previous work (18), HPV types were combined into three groups differing in their association with invasive cervical cancer: high-risk HPV types (16 or 18), intermediate-risk types (31, 33, 35, 39, 45, 51, or 52), and low-risk types (6/11, 42, or unknown and all other types). Multiple infections, which accounted for 35% of infections, were assigned hierarchically to the highest risk category applicable. Statistical Analysis Unconditional logistic regression analysis was used to compute odds ratios as estimates of the relative risk (RR) of CIN, with 95% confidence intervals (CIs). CIs excluding 1.0 indicated statistically significant association. We estimated the association of each epidemiologic factor with and without adjustment for HPV test results. If a strong association (indicated by an elevated RR) disappeared with adjustment for HPV (RR approached 1.0), we inferred that HPV infection mediated the association between that epidemiologic factor and the risk of CIN (19). We calculated the attributable proportion, the fraction of disease attributable to HPV, by the following formula: attributable proportion = % HPV positivity in case subjects X (1 - [1/RR]). Because of the age difference between case subjects (median age,

28 years) and control subjects (median age,

34 years), all RR estimates were adjusted for age in sextiles (16-24, 25-28, 29-34, 35-39, 40-46, and 47+ years) or tertiles (16-28, 29-39, and 40+ years) in analyses with few subjects. Cytology Review and Case Definition The original cytologic diagnoses were used to define case and control subjects in the main analysis, but the results of possible misclassification of case-contro........