of 834 patients enrolled in the study. Patients were followed by the study coordinators whenever they presented for routine clinical care to their nephrology clinics. The institutional review boards at all participant sites approved the study, and all patients provided written informed consent. Details of the study design have been published previously \cite{10}.

\section{Study Variables}
At enrollment and follow-up visits, data on demographic, anthropometric, cause of CKD, comorbidities, laboratory variables, medications, and outcomes (ESRD, death, and cardiovascular events/procedures) were collected. Of 834 patients, 820 had $S_k$ values available at study entry and were included in the analyses. $S_k$ values that were obtained from baseline and subsequent clinic visits were classified into the following categories: $\leq 4.0$, 4.0 to 5.5, and $\geq 5.5$ mmol/L.

\section{Statistical Analysis}
Linear regression models were used to assess predictors of $S_k$ at baseline, with the adjusted $R^2$ used to compare models in an all-subsets regression analysis. The distributions of $S_k$ values was illustrated using box plots by eGFR, gender, angiotensin-converting enzyme inhibitor (ACEI) use, and quartiles of serum carbon dioxide (CO$_2$), using all available $S_k$ values during follow-up time. The SAS mixed procedure (SAS Institute, Cary, NC) was used to test for differences in $S_k$ values between groups. To predict $S_k$ during follow-up time, we used mixed linear regression models with a spatial anisotropic exponential covariance structure (to account for correlation between follow-up visits of differing intervals). Separate Cox models evaluated the associations between $S_k$ (as a continuous or as a categorical covariate) and time to death, time to ESRD, the composite outcome of death or ESRD, the composite of death or any cardiovascular event (defined as prespecified coronary disease, cerebrovascular disease, or peripheral vascular disease).

Table 1. Patient demographics, comorbidities, medication use, and laboratory variables by serum potassium categories at enrollment

\begin{table}[h]
\centering
\begin{tabular}{lccccc}
\hline
Variable & Overall (n = 820) & Hypokalemia (n = 122) & Eukalemia (n = 633) & Hyperkalemia (n = 65) & P \\
\hline
\textbf{Demographics} \\
age (years; mean $\pm$ SD) & 60.5 $\pm$ 15.4 & 60.6 $\pm$ 16.1 & 60.7 $\pm$ 15.4 & 58.4 $\pm$ 14.0 & 0.499 \\
male (n [\%]) & 448 (55) & 52 (43) & 346 (55) & 50 (77) & $<0.0001$ \\
black (n [\%]) & 167 (20) & 37 (30) & 117 (19) & 13 (20) & 0.012 \\

\textbf{Vital signs (mean $\pm$ SD)} \\
BMI (kg/m$^2$) & 29.4 $\pm$ 6.9 & 29.8 $\pm$ 6.9 & 29.3 $\pm$ 6.9 & 29.4 $\pm$ 7.7 & 0.847 \\
SBP (mmHg) & 139.7 $\pm$ 21.7 & 142.8 $\pm$ 23.9 & 139.4 $\pm$ 21.6 & 137.8 $\pm$ 18.8 & 0.211 \\
eGFR (ml/min per 1.73 m$^2$) & 25.4 $\pm$ 10.6 & 27.0 $\pm$ 13.7 & 25.3 $\pm$ 9.9 & 23.2 $\pm$ 10.1 & 0.059 \\

\textbf{Cause of CKD (n [\%])} \\
diabetes & 265 (32) & 39 (32) & 199 (32) & 27 (42) & 0.262 \\
hypertension & 402 (49) & 60 (49) & 310 (49) & 32 (49) & 0.99 \\

\textbf{Medication use (yes/no; n [\%])} \\
ACEIs & 332 (41) & 34 (28) & 260 (41) & 43 (66) & $<0.0001$ \\
ARBs & 144 (17) & 35 (29) & 144 (23) & 7 (11) & 0.021 \\
aspirin & 304 (37) & 41 (34) & 236 (37) & 27 (42) & 0.549 \\
$\beta$ blockers & 392 (47) & 62 (51) & 295 (47) & 35 (54) & 0.414 \\
calcium channel blocker & 353 (43) & 64 (52) & 294 (46) & 23 (35) & 0.083 \\
diuretics & 409 (50) & 75 (61) & 317 (50) & 34 (52) & 0.069 \\
erythropoietin & 199 (24) & 30 (25) & 150 (24) & 19 (29) & 0.609 \\
statins & 352 (42) & 39 (32) & 278 (44) & 34 (52) & 0.014 \\

\textbf{Laboratory (mean $\pm$ SD)} \\
serum albumin (g/dl) & 3.8 $\pm$ 0.5 & 3.7 $\pm$ 0.5 & 3.8 $\pm$ 0.5 & 3.7 $\pm$ 0.5 & 0.098 \\
serum CO$_2$ (mEq/L) & 24.2 $\pm$ 3.9 & 26.2 $\pm$ 3.7 & 24.1 $\pm$ 3.8 & 21.2 $\pm$ 3.5 & $<0.0001$ \\
serum potassium (mmol/L) & 4.6 $\pm$ 0.9 & 3.7 $\pm$ 0.3 & 4.7 $\pm$ 0.4 & 5.8 $\pm$ 0.3 & $<0.0001$ \\
serum phosphorus (mg/dl) & 4.2 $\pm$ 1.1 & 4.3 $\pm$ 1.5 & 4.1 $\pm$ 1.0 & 4.5 $\pm$ 1.1 & 0.031 \\

\textbf{Patient outcomes (n [\%])} \\
death & 86 (10) & 16 (13) & 64 (10) & 6 (9) & 0.576 \\
ESRD & 303 (36) & 57 (36) & 213 (35) & 33 (48) & 0.145 \\
coronary artery disease & 70 & 11 (9) & 50 (8) & 9 (14) & 0.258 \\
cardiovascular disease & 50 & 8 (7) & 38 (6) & 4 (6) & 0.977 \\
peripheral arterial disease & 70 & 8 (7) & 55 (8) & 7 (11) & 0.581 \\
any cardiovascular event & 190 & 32 (27) & 139 (22) & 19 (29) & 0.249 \\
\hline
\end{tabular}
\label{table_1}
\end{table}

Hypokalemia: $S_k \leq 4.0$ mmol/L; eukalemia: $S_k > 4.0$ and $< 5.5$ mmol/L; hyperkalemia: $S_k \geq 5.5$ mmol/L. ARB, angiotensin II receptor blocker; BMI, body mass index; SBP, systolic BP.