Our study had some methodologic problems and limitations. First, besides the potential for interobserver variation, we used a 3-item question to score the last variable of the Wells rule (“presence of an alternative diagnosis at least as likely as that of DVT”) instead of a 1-item question as used by Wells and colleagues. Although our measurement seems more precise, it could have resulted in a different performance of the Wells rule in our sample. Our percentage of patients with an alternative diagnosis is similar to those of other studies but could not be compared with that of the original study by Wells and colleagues because they did not provide this information (8 - 9). Second, we used a different reference standard than did Wells and colleagues. They used a single leg ultrasonography and a 3-month follow-up visit (documentation of symptomatic DVT within 3 months) as the reference standard in the lowest-risk group, whereas we used repeated ultrasonography without follow-up. Hence, it could be argued that in the study by Wells and colleagues, the lowest-risk group included patients with a negative initial ultrasonogram and negative follow-up who actually had DVT that would have been detected during repeated ultrasonography. Alternatively, we did not have 3-month follow-up data on our patients and may have missed some cases of DVT. However, if we had such follow-up data, the number of missed cases of DVT in the lowest-risk group would probably be higher than in the present results (Table 2), worsening the performance of the Wells rule in primary care. Third, the limited performance of the categorized Wells rule could be due to the inclusion of patients with previous DVT; these patients were excluded in the study that led to development of the categorized Wells rule (5). To enhance comparability, we therefore validated the categorized Wells rule in patients with a first DVT only (987 patients). The prevalence of DVT in each risk group did not change: 12.2% in the low-risk group, 16.3% in the medium-risk group, and 39.6% in the high-risk group.