Study population
The HUNT Study regularly invites the total adult population in the northern county of Trøndelag (≥ 20 years) for an extensive health screening (22). Data collected includes information on sex, age, clinical examinations, blood samples and self-reported health-related questionnaires. In HUNT2 (1995–1997), 93898 persons were invited with 65004 (69.2%) agreeing to participate and in HUNT3 (2006–2008), 93860 were invited with 50663 (53.9%) agreeing to participate.
The albuminuria sub-study.
HUNT2 and HUNT3 participants were invited to a sub study where morning urine samples were collected for ACR analysis. Participants were selected according to predefined selection criteria: i) a 5% random selection of the HUNT participants, ii) those with self-reported diabetes (yes/no), iii) those with self-reported hypertension and/or self-reported antihypertensive treatment (yes/no) (HUNT2 only), and; iv) in HUNT3, the HUNT2 albuminuria study participants were re-invited (23, 24). In this paper, participants from selection group iv) were only included once (HUNT2 participation date). The study participants were asked to provide urine samples over three consecutive days and answered questionnaires related to the urinary samples including history of a urinary tract infection during the last week, persistent haematuria over the last year, and whether women were pregnant or menstruating at collection time (23). Of those invited to the albuminuria study 84.1% and 63.3% participated in HUNT 2 and HUNT3, respectively.
Clinical Examination
Trained nurses undertook standardised clinical examinations at baseline. The examinations included measurements of the participants height, waist circumference and weight (all without shoes and wearing light clothing, to the nearest cm or kg) (22). Body mass index was calculated as kg/m2. We used the average of second and third systolic blood pressure (mmHg) and pulse (heart rate/minute) measurements, recorded by an automatic oscillometric method (Dinamap 845XT; Criticon, Tampa, Florida, USA) after > 5 minutes resting in a sitting position (22).
Laboratory Data
Blood sampling was performed in a non-fasting state at the time of clinical examination. Serum total and high-density lipoprotein (HDL) cholesterol were analysed using enzymatic colorimetric methods (Boeheringer Mannheim, Germany) (22). Urine samples were returned by the HUNT participants using prepaid envelopes and standardised receptacles. Fresh blood and urine samples were analysed at an accredited laboratory (ISO-9001 certified and ISO/IEC-17025) at Levanger Hospital (Norway). For the analysis, HUNT2 used a Hitachi 911 autoanalyzer (Hitachi, Mito, Japan) with reagents from Boehringer Mannheim (Mannheim, Germany) and HUNT3 used an Architect ci8200 autoanalyzer (Abbot Diagnostic, Longford, Ireland) with reagents from Mulitigent (Abbot Laboratories, USA) (22).
HUNT used the Jaffe ́ method to measure serum concentration of creatinine, calibrated to isotope-dilution mass-spectroscopy. Creatinine was thereafter used to calculate the estimated glomerulus filtration rate (GFR, ml/min), using the Chronic Kidney Disease Epidemiology Collaboration equation (CKD-EPI) .
Immunoturbidimetric methods were applied to determine urine albumin using antihuman serum albumin, and ACR was calculated in mg/mmol. In HUNT2, the supplier was DakoAS, Glostrup, Denmark (25) and in HUNT3, Abbot Laboratories (22). All information on quality control and calibration methods is previously published (23).
Self-report measures
Participants self-reported anxiety and depression symptoms experienced over the past week using the Hospital Anxiety and Depression rating scale (HADS). The HADS includes 14 questions, 7 mirroring non-somatic depression symptoms from the ICD depression criteria (26), such as anhedonia and psychomotor retardation, and 7 anxiety questions with one item mirroring panic disorder and the other six general anxiety – including worrying and rumination (27). Each question is answered according to symptom severity using a scale from 0 (no symptoms) to 3 (high symptom load), i.e. each subscale ranges 0–21 points.
Use of antihypertensive medication and diabetes was self-reported (yes/no). Smoking status was reported as never, previously or current. Self-reported diabetes was categorised as yes or no, and antihypertensive medications use was categorised as yes (current or previous) or no (never) (22). Physical activity was self-reported as hours of intensive physical activity per week, defined as physical activity involving sweating or feelings of breathlessness. Physical activity duration was categorised as 0 hours (none), < 1 hour, 1–3 hours and > 3 hours.
We constructed three categories for “education and work” based on self-reported education for HUNT2 participants and Classification of Occupation from Statistics Norway for HUNT3 participants; <10 years of school or unskilled worker, 10–12 years of school or intermediate worker, > 12 years of education or belonging to the salariat class (22).
Follow-up and outcome AMI ascertainment
Of the 15 421 participants who delivered at least one urinary sample in either HUNT2 or HUNT3, we excluded 1058 (6.9%) with previous AMI, 145 ( 0.9%) with an ACR indicating overt kidney disease (ACR ≥ 30 mg/mmol), 245 (1.6%) with missing data on necessary covariates and participants who did not self-report AMI, but had hospital records indicating an AMI at a date prior to baseline (n = 34 (0.2% ) in HUNT2 and n = 11 (0.1%) in HUNT3). Figure 1 shows the flow of participants in this study. A total of 2 950 (19.1% ) participants had observations from both HUNT2 and HUNT3 and these were followed-up from their first assessment in HUNT2 (Fig. 1). HUNT3 participants were followed from HUNT3 participation date. Together 11014 participants were followed-up from their HUNT participation to either a first AMI or untill the end of the follow-up period (31 December 2016). AMI was diagnosed according to the European Society of Cardiology/ American College of Cardiology consensus guidelines (28, 29). Criteria for AMI included: (i) specific clinical symptoms according to case history information, (ii) changes in blood levels of cardiac enzymes, and (iii) specified ECG changes. The national cause of death registry was another data source, providing the ICD codes for AMI that never reached hospitals using code 410 in the 9th revision and codes I21 and I22 in the10th revision in order to confirm AMI (11).
Table 1
Baseline characteristics for participants (n = 11,014) and overview of missing data
Variables with complete data n = 11, 014 | | Missing n (% ) |
1Women (n, %) | 6234 (56.6) | NA |
1Blood pressure medication, never (n, %) | 5204 (47.3) | NA |
1Diabetes, yes (n,%) | 1553 (14.1) | NA |
1Age, years (mean, SD) | 57.6 (15.7) | NA |
1Systolic blood pressure, mmHg (mean, SD) | 146.6 (23.8) | NA |
1Total Cholesterol, mmol/L (mean, SD) | 6.2 (1.3) | NA |
1HDL cholesterol, mmol/L (mean, SD) | 1.3 (0.4) | NA |
1Antihypertensive treatment (yes (n,%) | 5810 (52.6) | NA |
Estimated glomerulus filtration rate, ml/min (mean, SD) | 89.3 (19.5) | NA |
Body mass index, kg /m2 (mean, SD) | 27.9 (4.6) | NA |
Waist circumference, cm (mean, SD) | 90.8 (12.3) | NA |
Pulse, beats/minute (mean, SD) | 72.8 (13.4) | NA |
Variables with incomplete data | | Missing n (% ) |
Albumine Creatinine Ratio (ACR), mg/mmol (mean, SD) | 2.7 (SD 9.8) | 1004 (9.1) |
ACR, no (ACR < 3 mg/mmol) (n, %) | 8773 (79.7) | |
ACR, yes (ACR 3–30 mg/mmol) (n, %) | 1237 (11.2) | |
HADS Depression Scale (HADS-D, range 0–21) (mean, SD) | 3.8 (3.1) | 506 (4.6) |
Depression Symptoms, no (HADS-D < 8) (n, %) | 9159 (83.2) | |
Depression Symptoms, yes (HADS-D 8–21 (n, %) | 1349 (14.6) | |
HADS Anxiety Subscale (HADS-A, range 0–21) (mean, SD) | 4.2 (3.4) | 741 (6.7) |
Anxiety Symptoms, no (HADS-A < 8) (n, %) | 8673 (84.4) | |
Anxiety Symptoms, yes (HADS-A 8–21) (n, %) | 1600 (15.6) | |
Anxiety or Depression symptoms, no (n, %) | 8360 (78.3) | 486 (4.4) |
Anxiety or Depression Symptoms, yes (n, %) | 2313 (21.7) | |
1Smoking status (n, %) | | 226 (2.1) |
Never | 4973 (46.1) | |
Previously | 3401 (31.5) | |
Current | 2414 (22.4) | |
Education (HUNT2) or work status (HUNT3) (n, %) | | 846 (7.7) |
10 years of school or less or unskilled worker | 4899 (48.2) | |
10–12 years of school or intermediate working class | 3892 (38.3) | |
>12 years of School or salariat working class | 1377 (13.5) | |
Intensive exercise (n %) | | 4355 (39.5) |
None | 2970 (44.6) | |
< 1hour per week | 1518 (22.8) | |
1–2 hours per week | 1368 (20.6) | |
3 hours or more per week | 803 (12.0) | |
1 Framingham cardiovascular risk factors |
Statistical analysis
Participants who provided only one or two urine samples, answered less than 5 items on each HADS subscale, self-reported pregnancy, urinary tract infection, haematuria or menstruation, or had urinary samples with deviations in ACR measurements were set to missing and thereafter imputed. The ACR values were dichotomised as ACR (no/yes) and cut off for normal values (ACR, no) was set at 3 mg/mmol.
Per protocol, we replaced 1–2 missing items on the HADS depression and HADS anxiety subscale with 6/7 and 5/7 of the values provided (11) and defined these as our complete case numbers (see statistical section for more details regarding managing missing data). Dichotomisation of depression symptoms (no/yes) and anxiety symptoms (no/yes) were according to score < 8 (no) or 8–21 (yes) on respective HADS sub-scales as in a previous HUNT study related to additive risk factors (30). Due to the coexistence of anxiety or depression, a variable “anxiety or depression symptoms” (no/yes) was created, where “yes” was defined as 8–21 in either of the HADS sub-scales (30). .
In survival analyses, individuals were followed from baseline measurements (participation in HUNT2 or HUNT3) to date of AMI, death or end of follow-up (31 December 2016), whichever came first. Attained age was used as the underlying timescale. We used multivariable Cox regression models to assess the relative risk and associated 95% confidence interval (95% CI) for incident AMI against three exposures: i) ACR and depression symptoms (n = 5 599 for Complete Case (CC)); ii) ACR and anxiety symptoms (n = 5 607 for CC) and; iii) ACR and anxiety or depression symptoms (n = 5 607 for CC). All exposures (i-iii) were adjusted for confounders between exposures and endpoint in four different models, successively adding confounders to investigate how the HRs of interest changed. We based our statistical models on modern methodology where a-priori knowledge regarding confounders of the association between the exposures and outcome guided the selection and the models were built stepwise to tease out the mechanisms between the exposure and outcome (31). In Model 1, we adjusted for sex by stratification as this variable otherwise violated proportional hazard assumption (p < 0.001). Model 2 included in addition to age and sex the other cardiovascular risk factors from the Framingham Heart Study; diabetes, antihypertensive medication, systolic blood pressure, smoking, total cholesterol and HDL cholesterol. Model 3 further included estimated GFR related to the study selection criteria and body mass index, waist circumference, intensive physical activity and education and work status, which can all be viewed as lifestyle-related confounders. Model 4 included a statistical interaction-term between the exposures of interest; i) ACR and depression; ii) ACR and anxiety; iii) ACR and anxiety or depression. A Likelihood ratio (LR) test to determine interaction were used between model 3 and 4. As the results in model 4 were substantially different in the complete case dataset compared to multiple imputation dataset we ran missing and imputation diagnostics to explore the differences further (Supplementary Table 2). Further, we ran multicollinearity diagnostics using the estat vce, corr option in Stata.
To explore additivity (absolute risk), we defined four mutually exclusive exposure categories based on presence of none, one (either ACR or one of the HADS measures) and both exposure categories. The unexposed to both the first and the second risk factor were defined as reference category. Thus, by defining three indicator variables, three absolute risk coefficients were estimated from a Cox regression model, and the corresponding covariance matrix were used for calculation of confidence intervals. These were plotted in the excel sheet developed by Anderson (32) to calculate the Relative Excess Risk due to Interaction (RERI), Attributable Proportion (AP) of events due to interaction and Synergy Index (S). The latter measure is interpreted as the excess risk from both exposures when interaction is present relative to the risk from exposure when interaction is absent (17). RERI and AP are statistically significant if the 95% CI do not cross 0, and S is significant if it does not cross 1. Multiple imputation by fully conditional specification were used when imputing missing data, generating a total of 10 complete datasets. Both the “mi chained”-routine and the user-written command smcfcs (33) were used for this purpose.
We used Stata version16.1 for all analysis (StataCorp. 2019. Stata Statistical Software: Release 16. College Station, TX: StataCorp LLC).