We sought to determine the incidence of idiopathic deep venous thrombosis and secondary thromboembolism among the four major ethnic groups in California-white persons, African Americans, Hispanic persons, and Asians and Pacific Islanders-by using a large, linked, hospital discharge data set. We hypothesized that the incidence of idiopathic deep venous thrombosis and the incidence of secondary venous thromboembolism are lower among Asians and Pacific Islanders than among white persons residing in California.

Methods

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Data Set and Study Sample

Our study was approved by the Human Subjects Committees at the University of California, Davis, School of Medicine and the California Health and Welfare Agency. We used the linked California Patient Discharge Data Set to determine the incidence of idiopathic and secondary deep venous thrombosis [5]. All nonfederal licensed hospitals are required by the state of California to submit information-including admission date, discharge date, birth date, race (White, Black, Hispanic, Native American/Eskimo, Asian/Pacific Islander, Other, or Unknown), and sex-for each inpatient after hospital discharge. The source of information used to code a patient's race is either the physician's note at admission or the admission clerk's assessment, which is based on interview or review of previous records. An Asian/Pacific Islander is defined as "A person having origins in any of ... the original oriental peoples of the Far East, Southeast Asia, Indian subcontinent, or Pacific Islands." The data also include a principal diagnosis (the factor prompting admission), up to 24 additional diagnoses, 1 principal procedure, and up to 20 additional procedures (with dates). All diagnoses and procedures are coded by using the International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM).

We analyzed patients 18 years of age or older who were hospitalized between 1 January 1991 and 30 June 1994, allowing 6 months of follow-up after hospitalization. A patient with idiopathic deep venous thrombosis was defined as a person with a principal diagnosis of venous thrombosis (ICD-9-CM codes 451.11, 451.19, 451.2, 451.81, 451.9, 453.1, 453.2, 453.8, and 453.9) and hospitalization for at least 3 days without cancer (ICD-9-CM codes 141 through 172 and 174 through 208) or hospitalization in the previous 6 months. These criteria were used to include patients likely to have been treated for thromboembolism and to exclude patients with secondary venous thrombosis associated with "temporary" risk factors, such as surgery, trauma, or another medical event [6]. Our analysis was restricted to the first hospitalization for venous thrombosis on each patient's linked record.

Secondary thromboembolism, including pulmonary embolism [ICD-9-CM code 415.1], was analyzed after 1) anterior, lateral, or anterolateral myocardial infarction without bypass surgery or revascularization [n = 45 062]; 2) partial or total colectomy in the absence of cancer [n = 21 793]; 3) partial or total colectomy in the presence of a malignant neoplasm of the colon, rectum, or sigmoid colon [n = 21 590]; 4) open reduction and internal fixation of the femur after hip fracture [n = 53 970]; 5) partial, total, or revision hip arthroplasty [n = 77 629]; 6) acute cerebral thrombosis with paralysis without carotid endarterectomy [n = 56 255]; and 7) spontaneous vaginal or cesarean delivery (n = 1 466 928). The ICD-9-CM codes are available from the authors on request.

Outcomes

We calculated the age- and sex-adjusted standardized annual incidence of idiopathic deep venous thrombosis per 1 000 000 persons 18 years of age or older by using the direct method and 1990 U.S. Census figures [7]. We also determined the cumulative incidence of recurrent thromboembolism, which was defined as rehospitalization for pulmonary embolism or venous thrombosis within 3 months of the index event.

Secondary venous thromboembolism was defined as venous thrombosis or pulmonary embolism that occurred during the index hospitalization or that required hospitalization within 91 days (3 months) of the day of surgery or medical admission. For patients after childbirth, we also used the ICD-9-CM codes for postpartum thromboembolism (671.40, 671.42, and 671.44).

Validation of Coding

As part of a different study, we validated the coding of the principal diagnosis of venous thrombosis among 198 patients who were hospitalized for at least 3 days at four hospitals in the Sacramento, California, area.

Statistical Analysis

The CIs for incidence rate ratios for idiopathic deep venous thrombosis were calculated by using the approximate method [8]. The age-adjusted (in 10-year increments) and sex-adjusted relative risk for secondary or recurrent venous thromboembolism after each type of operation or medical condition, by ethnicity, was calculated by using Cox proportional-hazards regression analysis (proc phreg, SAS Institute, Cary, North Carolina); white persons were the reference group. We tested the proportionality assumption by using log-minus-log survival plots [9]. Cases were censored at the time of death or intervening major surgery. Death was determined by using the coded disposition and a linked state death registry. For pregnant women, we analyzed all deliveries and adjusted for the method of delivery (vaginal or cesarean) and the presence of complications, according to the assigned diagnosis-related groups.

Results

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In our validation study, 183 of 198 patients (93%) who met our case definition for idiopathic deep venous thrombosis had objectively documented thrombosis and received heparin and warfarin.

Between 1 January 1991 and 30 June 1994, 17 991 patients (8598 men and 9196 women) met our criteria for idiopathic venous thrombosis. The Table 1 shows the directly standardized age- and sex-adjusted annual incidences of idiopathic deep venous thrombosis and incidence rate ratios, using white persons as the reference group. The 3-month cumulative incidence of recurrent thromboembolism after hospitalization for idiopathic venous thrombosis was 5.6% (95% CI, 5.2% to 5.9%). The relative risk for acute recurrent venous thromboembolism was significantly lower among Asians and Pacific Islanders (Table 1).

Relative risks for thromboembolism after specific illnesses or surgical procedures are shown in the (Figure 1). For each medical or surgical diagnosis, the relative risk for thromboembolism was significantly lower for Asians and Pacific Islanders (range of relative risk, 0.22 to 0.61).

View larger version (22K): [in this window] [in a new window]   Figure 1. Age- and sex-adjusted relative risk for developing venous thromboembolism within 3 months of a specific medical or surgical event among Asians and Pacific Islanders, African Americans, and Hispanic persons, with white persons as the reference group. Black bars represent 95% CIs.

Discussion

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Our most striking findings were a 74% lower incidence of idiopathic deep venous thrombosis, a 35% lower relative risk for recurrent venous thrombosis, and a uniformly lower relative risk for secondary venous thromboembolism among Asians and Pacific Islanders compared with white persons. These findings parallel the observation that the known prevalence of the factor V Leiden mutation, the most common genetic condition predisposing to venous thrombosis, is low in Asian populations [11-13]. This suggests that a lower prevalence of the factor V Leiden mutation (and possibly other genetic disorders) may explain the lower incidence of venous thrombosis in this ethnic group.

However, we found that the incidence of idiopathic venous thrombosis was almost 30% higher in African Americans than in white persons. Similarly, after surgical or medical diagnoses, African Americans had a higher relative risk for thromboembolism than white persons. Because the prevalence of the factor V Leiden mutation also seems to be significantly lower in African Americans than in white persons [11], the higher incidence of thromboembolism in this ethnic group must be explained by other factors.

One or more sociocultural factors affecting patients' clinical symptoms their thresholds for seeking medical attention, or the medical care that they receive may explain the lower rate of idiopathic and secondary venous thrombosis that we found in Asians and Pacific Islanders. However, this is unlikely for several reasons. First, many Asians and Pacific Islanders living in California have been in the state for more than one generation; this would tend to reduce major cultural differences. Second, this group is culturally heterogeneous, composed of persons of Japanese, Korean, Chinese, Vietnamese, Laotian. Indian, Filipino, and Polynesian ancestry who have widely varying cultural behaviors. Third, most studies from Asia that have reported the results of screening venography after different surgical procedures [2, 3, 14-17] have noted a much lower incidence of asymptomatic thrombosis than have studies from Europe and the United States.

California has remarkable ethnic diversity. The 1990 U.S. Census showed that among adults older than 18 years of age in California, 2 040 000 were Asian and Pacific Islander, 1 537 000 were African American, 4 860 800 were Hispanic, and 15 780 000 were white [10]. However, the coding of race or ethnicity is known to be problematic and to have several potential sources of error [18], including confusion about coding persons of mixed race and persons with Latino surnames. Moreover, there is considerable ethnic diversity within each major grouping.

In a 1988 reabstraction study of 2579 medical-surgical hospital admissions in which the original medical record was used as the criterion standard, there was agreement on race in 94% of cases [5]. Of 117 Asians and Pacific Islanders, 28 (24%) were originally coded as white or Hispanic. The predictive value of a code for Asian or Pacific Islander was 89%, whereas that of a code for African American was 98%. Thus, only a small percentage of cases coded as Asians and Pacific Islanders might actually have been from other ethnic groups. Given the results of our analysis and the findings of this reabstraction study, the true incidence of idiopathic or secondary venous thromboembolism among Asians and Pacific Islanders is probably slightly lower than we report here.

In conclusion, our findings suggest that in California, the incidence of idiopathic venous thrombosis and secondary venous thromboembolism is significantly lower among Asians and Pacific Islanders than among white persons. Further studies are needed to confirm these findings because of their implications about the need for prophylaxis in different ethnic groups.