All hospital discharges of male veterans with a primary ICD-9-CM diagnosis of AMI (410) recorded in the PTF between January 1, 1994, and September 30, 1995, were eligible. Until 1995, the Department of Veterans Affairs (VA) used the “primary” diagnosis for each discharge. Currently, the VA uses the term “principal” diagnosis. In pilot data, we found that for 85% of AMI cases in the VA, the primary diagnosis was the reason for admission (in other words, was equivalent to the principal diagnosis). We therefore used the primary diagnosis in this study. The PTF contains a patient identifier, patient characteristics, discharge diagnoses, and ICD-9-CM procedure codes. For the purposes of our research, we used a sequentially applied algorithm of exclusions to refine our cohort and to focus on decision making for incident cases of AMI. Exclusions were length of stay greater than 180 days, discharged alive with a length of stay less than 3 days, transfer from a non-VA hospital, AMI that occurred after noncardiac surgery, cardiac procedure coded in the 90 days prior to admission, or AMI coded anytime during the prior year. Cases with a fifth-digit ICD-9-CM code of 2 (indicating AMI in the prior 8 weeks) were excluded because the purpose was to identify the initial admission for AMI.
A random sample of 5,151 patients was generated. Because of the differential rates of cardiac procedure use across Veterans Affairs Medical Centers (VAMCs), we sampled patients stratified by the on-site availability of cardiac procedure technology.23
We used the Cooperative Cardiovascular Project structured review instrument24
and specific criteria to confirm the diagnosis of AMI. Four registered nurses collected data from the medical record including date of birth, race, symptoms on presentation, laboratory values, and electrocardiography findings.
As in other studies of coding, we used three categories of data to evaluate the diagnosis of AMI: patient symptoms, electrocardiographic data, and cardiac enzyme values.16
Patient symptoms included chest pain, discomfort, pressure or heaviness, or epigastric discomfort; angina; discomfort or pain in arms, back, or jaw; nausea; vomiting; diaphoresis; sense of impending doom or anxiety; cardiac or respiratory arrest; sudden death; syncope; shortness of breath; or new-onset pedal edema. Criteria for this category were met if the record indicated at least one of these symptoms. Electrocardiographic criteria were evaluated by review of the admitting electrocardiogram. Reviewers assessed the presence of new Q waves, progressive evolution of T wave changes, or ST elevation or ST depression, plus one of the preceding. Cardiac enzyme criteria were met when at least one of the patient's laboratory cardiac enzyme values (peak creatine phosphokinase [CK], CK-MB band greater than 5%, or peak lactate dehydrogenase level greater than normal with an isoenzyme fraction 1 greater than fraction 2) from the first 48 hours following the onset of symptoms was above normal for the institution. Patients meeting at least two of three categories of clinical criteria were judged to have had an AMI, as were patients who died within 24 hours of admission, met the symptom criterion, but did not meet either the enzyme or electrocardiographic criteria.18
All cardiac procedures documented on the discharge summary sheet or in the progress notes were recorded. Nurses were not blinded to the PTF coding of these procedures.
We considered the medical record abstraction data to be the standard against which the accuracy of the ICD-9-CM codes in the PTF should be assessed. The positive predictive value is the conditional probability that an AMI was present on admission given that it was coded in the PTF. Patient and hospital characteristics associated with the confirmation of a diagnosis of AMI were assessed using χ2tests.
The sensitivity of procedure coding is the proportion of cases with an ICD-9-CM procedure code in the PTF confirmed by chart review. The specificity of the procedure coding is the proportion of all cases not noted to have the procedure coded in the PTF and confirmed by chart review.