Introduction 1 2 3 4 5 6 Methods Data sources 7 8 9 9 10 13 6 Cohort enrolment from the hospital register  7 Cohort enrolment from linked registers  After linkage with the population register using linkage variables ‘date of birth’, ‘gender’ and ‘numerical part of postal code’, 25,142 and 22,470 admissions came from patients with a unique combination of linkage variables in the population register (88% and 87%, respectively). Thus, each remaining admission linked to only one unique individual in the population register (one unique individual in the Netherlands). Admissions linking with more than one person (e.g., administrative twins; two persons with the same date of birth, gender and numeric part of postal code registered in the population register) or with no person at all (e.g., non-residents or administrative errors) in the population register were excluded. Selection of the first admission per person of all subsequent admissions of a person occurring during 1995 and 2000 yielded 23,172 patients in 1995 and 20,414 patients in 2000. Thus, 1,970 double counts had occurred in 1995 (8%) and 2,056 in 2000 (9%). Information on admissions in previous years of the patients in 1995 was obtained by selecting all hospital admissions registered in the hospital register with principal diagnosis AMI in the period 1991–1995. These admissions were linked to the cohort of 23,172 patients with linkage variables ‘date of birth’, ‘gender’ and one or both of the variables ‘numerical part of postal code’ and ‘hospital-specific patient identification code’. Linkage with the population register was not possible, since this register started in October 1994. Subjects who linked in this process were patients with previous hospital admissions for AMI (recurrent AMI patients) and were excluded (1,607 patients (7%)). This resulted in the final cohort of 1995 consisting of 21,565 patients. Information on hospital admissions in previous years of the patients in 2000 was obtained by linking of the hospital register of 1995–2000 to the population register with linkage variables ‘date of birth’, ‘gender’ and ‘numerical part of postal code’. All uniquely linked admissions with principal diagnosis AMI were selected and linked to the cohort of 20,414 patients. Patients with previous hospital admissions for AMI (recurrent AMI patients) were excluded (1,356 patients (7%)). This resulted in the final cohort of 2000 consisting of 19,058 patients. Thus the linked register comprises information for only part of the Dutch population (i.e., those that were unique on date of birth, gender, and postal code), and does not include double counts, and recurrent AMI admissions. Data analysis 14 15 Results In both 1995 and 2000, the gender and age distribution of the cohort based on the hospital register was comparable to the cohort based on linked registers. In 1995 and 2000, two-thirds comprised men. The mean age in 1995 was 63.7 years in men and 71.5 years in women based on the hospital register. This was 64.3 and 71.9 years, respectively, based on linked registers. In 2000, the mean age based on the hospital register was 63.6 years in men and 70.9 years in women. Based on linked registers, this was 64.2 and 71.6 years, respectively. 1 Table 1 Incidence (per 100,000 persons per year) of hospitalized acute myocardial infarction in 1995 and 2000 based on the national hospital discharge register and based on linked registers   1995 2000  Hospital register Linked registers   Hospital register Linked registers   Age Incidence Incidence 1 2 Incidence Incidence 1 2 Men <30 2 1 1 (0 to 1) 1.56 (0.99 to 2.48) 2 1 0 (0 to 1) 1.33 (0.88 to 2.01) 30–39 41 32 9 (4 to 14) 1.28 (1.12 to 1.47) 40 32 7 (3 to 12) 1.23 (1.08 to 1.41) 40–49 207 173 34 (22 to 45) 1.20 (1.12 to 1.27) 180 143 36 (26 to 47) 1.25 (1.17 to 1.34) 50–59 509 409 100 (79 to 121) 1.24 (1.19 to 1.30) 407 332 76 (58 to 93) 1.23 (1.17 to 1.29) 60–69 887 733 154 (122 to 186) 1.21 (1.16 to 1.26) 675 551 124 (97 to 151) 1.23 (1.17 to 1.28) 70–79 1,226 1,009 217 (169 to 265) 1.21 (1.16 to 1.27) 1,008 824 184 (143 to 225) 1.22 (1.17 to 1.28) 80–89 1,254 1,045 209 (126 to 291) 1.20 (1.12 to 1.29) 1,144 925 220 (144 to 295) 1.24 (1.15 to 1.33) ≥90 843 660 184 (−20 to 387) 1.28 (0.97 to 1.68) 568 464 104 (−59 to 267) 1.22 (0.89 to 1.68) Total Crude 255 221 34 (29 to 39) 1.15 (1.13 to 1.18) 222 190 33 (28 to 37) 1.17 (1.15 to 1.20) 3   47 (42 to 52) 1.22 (1.19 to 1.25)   43 (38 to 47) 1.23 (1.20 to 1.26) Women <30 1 0 0 (0 to 1) 3.59 (1.21 to 10.67) 1 1 0 (0 to 1) 1.31 (0.65 to 2.63) 30–39 9 8 1 (−1 to 4) 1.17 (0.88 to 1.56) 11 8 3 (0 to 5) 1.33 (1.02 to 1.73) 40–49 38 32 6 (0 to 11) 1.17 (1.01 to 1.36) 46 38 8 (2 to 13) 1.21 (1.06 to 1.38) 50–59 113 95 19 (9 to 29) 1.20 (1.09 to 1.32) 95 80 16 (7 to 24) 1.20 (1.08 to 1.32) 60–69 306 262 44 (26 to 62) 1.17 (1.10 to 1.25) 243 196 47 (31 to 63) 1.24 (1.15 to 1.33) 70–79 581 486 95 (67 to 123) 1.20 (1.13 to 1.26) 472 396 76 (51 to 100) 1.19 (1.13 to 1.26) 80–89 750 634 116 (73 to 160) 1.18 (1.11 to 1.26) 643 540 103 (64 to 143) 1.19 (1.11 to 1.27) ≥90 491 436 55 (−32 to 142) 1.13 (0.93 to 1.36) 450 394 56 (−21 to 133) 1.14 (0.95 to 1.37) Total Crude 118 106 13 (9 to 16) 1.12 (1.09 to 1.16) 104 91 13 (10 to 16) 1.14 (1.11 to 1.18) 3   19 (16 to 22) 1.18 (1.15 to 1.22)   18 (15 to 21) 1.20 (1.16 to 1.24) 1 2 3  2 Table 2 Hospital mortality (%) of acute myocardial infarction in 1995 and in 2000 based on the national hospital discharge register and based on linked registers   1995 2000  Hospital register Linked registers   Hospital register Linked registers   Age Mortality  Mortality 1 2 Mortality Mortality 1 2 Men <30 1.9 3.6 −1.6 (−9.5 to 6.2) 0.54 (0.03 to 8.29) 6.9 10.8 −3.9 (−15.9 to 8.0) 0.64 (0.17 to 2.40) 30–39 3.2 2.4 0.9 (−1.4 to 3.1) 1.36 (0.59 to 3.11) 2.8 3.1 −0.3 (−2.6 to 2.0) 0.90 (0.42 to 1.95) 40–49 3.3 3.7 −0.4 (−1.5 to 0.7) 0.89 (0.65 to 1.22) 3.2 2.6 0.7 (−0.4 to 1.8) 1.26 (0.85 to 1.86) 50–59 4.4 4.0 0.4 (−0.5 to 1.3) 1.10 (0.88 to 1.37) 3.7 3.5 0.2 (−0.6 to 1.1) 1.07 (0.84 to 1.37) 60–69 8.4 8.2 0.2 (−0.9 to 1.3) 1.02 (0.89 to 1.16) 7.2 7.1 0.1 (−1.0 to 1.3) 1.02 (0.87 to 1.20) 70–79 17.1 17.0 0.0 (−1.6 to 1.6) 1.00 (0.91 to 1.10) 15.4 15.3 0.1 (−1.6 to 1.7) 1.00 (0.90 to 1.12) 80–89 28.1 28.5 −0.3 (−3.6 to 2.9) 0.99 (0.88 to 1.11) 24.4 25.5 −1.1 (−4.3 to 2.1) 0.96 (0.84 to 1.09) ≥90 34.7 32.6 2.1 (−10.7 to 15.0) 1.07 (0.73 to 1.56) 47.1 50.7 −3.7 (−19.5 to −2.2) 0.93 (0.67 to 1.28) Total Crude 10.7 10.9 −0.3 (−0.9 to 0.4) 0.98 (0.92 to 1.04) 9.5 9.8 −0.3 (−1.0 to 0.4) 0.97 (0.91 to 1.04) 3   0.1 (−0.6 to 0.7) 1.01 (0.95 to 1.07)   0.0 (−0.6 to 0.7) 1.00 (0.94 to 1.07) Women <30 5.9 25.0 −19.1 (−63.0 to 24.8) 0.24 (0.02 to 3.01) 5.0 7.7 −2.7 (−20.0 to 14.7) 0.65 (0.04 to 9.50) 30–39 6.9 6.1 0.8 (−6.1 to 7.7) 1.13 (0.38 to 3.33) 3.4 3.3 0.1 (−4.6 to 4.9) 1.03 (0.25 to 4.22) 40–49 4.8 5.1 −0.3 (−3.4 to 2.9) 0.95 (0.50 to 1.79) 6.2 6.7 −0.5 (−3.8 to 2.7) 0.92 (0.56 to 1.53) 50–59 4.9 5.3 −0.4 (−2.6 to 1.8) 0.92 (0.60 to 1.41) 4.9 5.1 −0.1 (−2.3 to 30.) 0.97 (0.63 to 1.49) 60–69 9.6 8.9 0.7 (−1.2 to 2.5) 1.07 (0.88 to 1.31) 8.8 8.1 0.7 (−1.3 to 2.8) 1.09 (0.86 to 1.39) 70–79 18.7 18.3 0.4 (−1.6 to 2.5) 1.02 (0.92 to 1.14) 16.6 16.4 0.2 (−1.9 to 2.3) 1.01 (0.89 to 1.15) 80–89 30.4 31.8 −1.4 (−4.3 to 1.5) 0.96 (0.87 to 1.05) 28.4 29.3 −0.9 (−4.0 to 2.1) 0.97 (0.87 to 1.08) ≥90 40.5 40.6 −0.1 (−9.3 to 9.2) 1.00 (0.79 to 1.25) 31.5 34.4 −2.9 (−11.5 to 5.6) 0.91 (0.71 to 1.19) Total Crude 17.7 18.1 −0.4 (−1.6 to 0.8) 0.98 (0.92 to 1.05) 15.9 16.5 −0.5 (−1.8 to 0.7) 0.97 (0.90 to 1.04) 3   −0.1 (−1.3 to 1.0) 0.99 (0.93 to 1.06)   −0.2 (−1.3 to 1.0) 0.99 (0.92 to 1.06) 1 2 3  3 Table 3 Trends in the incidence (per 100,000 persons per year) and hospital mortality (%) of hospitalized acute myocardial infarction from 1995 to 2000 based on the national hospital discharge register and based on linked registers   Trends in incidence  Trends in hospital mortality   Risk difference  Relative risk  Risk difference Relative risk  Age Hospital register Linked registers Hospital register Linked registers Hospital register Linked registers Hospital register Linked registers Men <30 0 (0 to 1) 0 (0 to 1) 1.16 (0.80 to 1.68) 1.36 (0.83 to 2.22) 5.0 (−2.5 to 12.5) 7.2 (−4.9 to 19.4) 3.59 (0.41 to 31.07) 3.03 (0.36 to 25.62) 30–39 −1 (−6 to 4) 1 (−4 to 5) 0.98 (0.87 to 1.10) 1.02 (0.88 to 1.18) −0.4 (−2.5 to 1.6)  0.7 (−1.7 to 3.1) 0.87 (0.44 to 1.72) 1.30 (0.53 to 3.20) 40–49 −28 (−39 to −17) −30 (−41 to −19) 0.87 (0.82 to 0.92) 0.83 (0.77 to 0.89) −0.1 (−1.1 to 0.9) −1.2 (−2.4 to 0.0) 0.97 (0.71 to 1.33) 0.69 (0.46 to 1.02) 50–59 −102 (−122 to −83) −78 (−97 to −59) 0.80 (0.77 to 0.83) 0.81 (0.77 to 0.85) −0.7 (−1.5 to 0.1) −0.6 (−1.5 to 0.4) 0.84 (0.68 to 1.04) 0.86 (0.67 to 1.12) 60–69 −211 (−242 to −181) −181 (−210 to −152) 0.76 (0.73 to 0.79) 0.75 (0.72 to 0.79) −1.2 (−2.3 to −0.2) −1.2 (−2.4 to 0.0) 0.86 (0.75 to 0.98) 0.86 (0.73 to 1.00) 70–79 −218 (−265 to −172) −186 (−229 to −143) 0.82 (0.79 to 0.86) 0.82 (0.78 to 0.86) −1.7 (−3.2 to −0.2) −1.7 (−3.5 to 0.0) 0.90 (0.82 to 0.99) 0.90 (0.81 to 1.00) 80–89 −110 (−192 to −28) −121 (−196 to −45) 0.91 (0.85 to 0.98) 0.88 (0.82 to 0.95) −3.8 (−6.8 to −0.8) −3.0 (−6.4 to 0.4) 0.87 (0.77 to 0.97) 0.89 (0.79 to 1.01) ≥90 −276 (−470 to −81) −196 (−370 to −23) 0.67 (0.51 to 0.89) 0.70 (0.51 to 0.96) 12.3 (−1.3 to 26.0) 18.1 (2.9 to 33.3) 1.35 (0.97 to 1.89) 1.56 (1.07 to 2.26) Total Crude −32 (−37 to −27) −31 (−36 to −27) 0.87 (0.86 to 0.89) 0.86 (0.84 to 0.88) −1.1 (−1.8 to −0.5) −1.1 (−1.8 to −0.4) 0.89 (0.84 to 0.95) 0.90 (0.84 to 0.96) 1 −48 (−53 to −43) −46 (−51 to −41) 0.82 (0.80 to 0.83) 0.80 (0.78 to 0.82) −1.2 (−1.8 to −0.6) −1.2 (−1.9 to −0.5) 0.89 (0.84 to 0.94) 0.89 (0.83 to 0.96) Women <30 0 (0 to 1) 0 (0 to 1) 1.21 (0.64 to 2.32) 3.34 (1.09 to 10.23) −0.9 (−15.6 to 13.8) −17.3 (−62.1 to 28) 0.85 (0.06 to 12.59) 0.31 (0.02 to 3.88) 30–39 2 (−1 to 4) 0 (−2 to 3) 1.20 (0.94 to 1.53) 1.05 (0.78 to 1.42) −3.5 (−8.9 to 2.0) −2.8 (−9.1 to 3.6) 0.50 (0.17 to 1.49) 0.55 (0.13 to 2.22) 40–49 8 (3 to 13) 6 (0 to 11) 1.22 (1.07 to 1.38) 1.18 (1.01 to 1.37) 1.3 (−1.5 to 4.2) 1.6 (−1.9 to 5.1) 1.27 (0.75 to 2.17) 1.31 (0.71 to 2.40) 50–59 −18 (−27 to −8) −15 (−24 to −6) 0.84 (0.77 to 0.92) 0.84 (0.76 to 0.94) 0.0 (−1.9 to 2.0)  −0.2 (−2.6 to 2.1) 1.01 (0.68 to 1.04) 0.96 (0.61 to 1.51) 60–69 −64 (−81 to −46) −66 (−83 to −50) 0.79 (0.74 to 0.84) 0.75 (0.69 to 0.80) −0.8 (−2.6 to 1.1) −0.9 (−2.9 to 1.2) 0.92 (0.75 to 0.98) 0.90 (0.71 to 1.15) 70–79 −109 (−136 to −83) −90 (−116 to −64) 0.81 (0.77 to 0.85) 0.82 (0.77 to 0.86) −2.1 (−4.0 to −0.2) −1.9 (−4.1 to 0.3) 0.89 (0.82 to 0.99) 0.90 (0.79 to 1.02) 80–89 −107 (−149 to −64) −94 (−134 to −54) 0.86 (0.81 to 0.91) 0.85 (0.80 to 0.91) −2.0 (−4.8 to 0.7) −2.5 (−5.7 to 0.6) 0.93 (0.77 to 0.97) 0.92 (0.83 to 1.02) ≥90 −40 (−125 to 44) −41 (−121 to 39) 0.92 (0.77 to 1.10) 0.90 (0.75 to 1.10) −9.0 (−17.6 to −0.5) −6.2 (−15.5 to 3.1) 0.78 (0.97 to 1.89) 0.85 (0.66 to 1.09) Total Crude −15 (−18 to −12) −15 (−18 to −12) 0.87 (0.85 to 0.90) 0.86 (0.84 to 0.88) −1.8 (−2.9 to −0.7) −1.6 (−2.9 to −0.3) 0.90 (0.84 to 0.95) 0.91 (0.85 to 0.98) 1 −18 (−22 to −15) −18 (−21 to −14) 0.85 (0.82 to 0.87) 0.83 (0.81 to 0.86) −1.6 (−2.7 to −0.5) −1.6 (−2.9 to −0.4) 0.91 (0.85 to 0.97) 0.91 (0.85 to 0.98) 1  3 Discussion We combined data from the national hospital discharge register with data from the population register to determine the (trends in) incidence and hospital mortality of first hospitalized AMI (double counts and recurrent AMI cases excluded) and compared the outcomes with the incidence and hospital mortality based on routinely collected data in the hospital register (double counts and recurrent AMI cases included). The incidence based on the hospital register was considerably and significantly higher than the incidence based on linked registers, whereas hospital mortality and trends in incidence and hospital mortality were identical using either approach. 6 9 4 16 16 16 4 5 17 21 Overall, our results based on deterministic linkages of sources using gender, date of birth and postal code, are in line with earlier reports from other studies, where linkage was performed using unique identification numbers. In conclusion, our study shows that the incidence based on routinely collected data in the national hospital discharge register overestimates the actual incidence of first AMI based on linked national registers by least 22% in men and 18% in women. Yet, the hospital mortality based on the hospital register accurately reflects the actual hospital mortality of first AMI. Furthermore, trends in incidence and hospital mortality based on the hospital register are not changed when double counts and recurrent cases were excluded. Since trends in incidence and hospital mortality of AMI are often based on national routinely collected data, it is reassuring that our results indicate that findings from such studies are indeed valid and not biased because of recurrent events and double counts. Electronic supplementary material Below is the link to the electronic supplementary material.
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