CKD Heat Maps: Definition, Interpretation, and Patient Stratification
What Are CKD Heat Maps?
CKD heat maps are visual risk stratification tools that combine two key measurements—estimated glomerular filtration rate (eGFR) and albumin-to-creatinine ratio (ACR)—into a color-coded grid system that predicts risk of CKD progression, cardiovascular disease, and mortality. 1
The heat map was developed by KDIGO (Kidney Disease: Improving Global Outcomes) to guide clinicians in risk assessment and clinical decision-making throughout CKD management. 1 This represents the first comprehensive attempt in guidelines to integrate both kidney function and kidney damage markers into a unified risk classification system. 1
Structure of the Heat Map
The heat map is organized as a grid with:
- Vertical axis (G categories): eGFR stages from G1 (≥90 mL/min/1.73 m²) through G5 (<15 mL/min/1.73 m²), with G3 subdivided into G3a (45-59) and G3b (30-44). 2
- Horizontal axis (A categories): Albuminuria levels classified as A1 (<30 mg/g), A2 (30-300 mg/g), and A3 (>300 mg/g). 2
- Color coding: Green (low risk), yellow (moderately increased risk), orange (high risk), and red (very high risk). 1
How to Interpret the Heat Map
Risk Categories and Their Meanings
Green zones indicate low risk if no other markers of kidney disease are present—in fact, these patients may not have CKD at all. 1 This applies to individuals with eGFR ≥60 mL/min/1.73 m² and ACR <30 mg/g. 1
Yellow zones represent moderately increased risk and include patients with eGFR 45-59 mL/min/1.73 m² (G3a) with minimal albuminuria, or those with eGFR ≥60 but ACR 30-300 mg/g (A2). 1 These patients require annual monitoring and cardiovascular risk reduction. 1
Orange zones signify high risk, encompassing patients with eGFR 30-44 mL/min/1.73 m² (G3b) or those with higher eGFR but ACR >300 mg/g (A3). 1 These patients need more intensive monitoring every 6 months and nephrology referral consideration. 3
Red zones indicate very high risk, including patients with eGFR <30 mL/min/1.73 m² (G4-G5) or those with both reduced eGFR and severe albuminuria. 1 These patients require immediate nephrology referral and preparation for renal replacement therapy. 3
Key Principle: Additive Risk
The critical concept is that cardiovascular and renal risks increase with lower eGFR AND higher albuminuria, and these risks are at least additive—not simply one or the other. 1, 4 A patient with eGFR 50 mL/min/1.73 m² and ACR 200 mg/g carries substantially higher risk than someone with eGFR 50 and ACR 10 mg/g. 4
Research demonstrates that albuminuria independently predicts CKD progression, cardiovascular events, and mortality at any eGFR level. 1, 5 Even patients with normalized eGFR but persistent albuminuria remain at elevated risk compared to those without kidney damage markers. 5
How to Stratify Patients Using the Heat Map
Step 1: Obtain Both Measurements
Measure eGFR using the CKD-EPI creatinine equation (preferred over MDRD) and obtain a spot urine sample for albumin-to-creatinine ratio. 2, 1 Do not rely on dipstick alone—it has poor sensitivity (43.6%) and high false-discovery rates for detecting ACR ≥30 mg/g. 6
Step 2: Plot on the Grid
Locate the patient's position on the heat map by finding the intersection of their eGFR category (vertical) and ACR category (horizontal). 1
Example stratifications:
- eGFR 55 mL/min/1.73 m² + ACR 15 mg/g = G3a + A1 = Yellow (moderately increased risk) 1
- eGFR 35 mL/min/1.73 m² + ACR 250 mg/g = G3b + A3 = Red (very high risk) 1
- eGFR 70 mL/min/1.73 m² + ACR 400 mg/g = G2 + A3 = Orange (high risk) 1
Step 3: Apply Risk-Based Management
For green zones (low risk): Annual monitoring of eGFR and ACR is sufficient if no other CKD markers exist. 1 Focus on primary prevention of CKD through blood pressure and glycemic control. 1
For yellow zones (moderately increased risk): Monitor eGFR and ACR annually, initiate ACE inhibitor or ARB if albuminuria is present, target blood pressure ≤130/80 mmHg, and implement cardiovascular risk reduction strategies including statin therapy. 1, 3
For orange zones (high risk): Increase monitoring frequency to every 6 months, ensure RAAS blockade is optimized, screen for CKD complications (anemia, bone disease, metabolic acidosis), and consider nephrology referral especially if eGFR <45 mL/min/1.73 m². 1, 3
For red zones (very high risk): Immediate nephrology referral is mandatory, monitor every 3 months or more frequently, aggressively manage complications, and begin discussions about renal replacement therapy options. 3 Hospitalization rates in this category exceed 500 per 1,000 person-years. 7
Clinical Trajectories Differ by Disease
Diabetic nephropathy and nephrosclerosis follow markedly different trajectories on the heat map. 8 Diabetic nephropathy typically shows concomitant macroalbuminuria when eGFR falls below 60 mL/min/1.73 m², with faster rates of eGFR decline. 8 In contrast, nephrosclerosis patients often maintain normoalbuminuria or microalbuminuria even when eGFR drops below 45 mL/min/1.73 m². 8 This underscores why both dimensions are essential for accurate risk stratification.
Common Pitfalls to Avoid
Do not assume patients with eGFR 45-59 mL/min/1.73 m² and no albuminuria are low risk. 1 They remain in the yellow zone with moderately increased risk and require annual monitoring. 1
Do not delay nephrology referral until eGFR <30 mL/min/1.73 m². 3 Referral should occur at eGFR <45 mL/min/1.73 m² (crossing into G3b), as late referral is associated with increased mortality after dialysis initiation. 3
Do not use urine dipstick alone for albuminuria screening. 6 It misclassifies 10.4% of the population into different risk categories compared to ACR-based assessment. 6
Do not ignore the red zone's heterogeneity. 1 While KDIGO groups all very high-risk patients together, those with the lowest eGFR and highest proteinuria carry quantitatively different risks than those at the borders and warrant even more intensive management. 1
Confirm chronicity before finalizing CKD diagnosis. 2 Abnormalities in eGFR or ACR must persist for >3 months to distinguish CKD from acute kidney injury or transient changes. 2, 1