Blood vs. Interstitial Glucose: How Pet CGMs Work (and Why It Matters)

Your pet’s CGM measures glucose in interstitial fluid via a tiny dermal filament using glucose oxidase, which generates a current proportional to ISF glucose—not blood glucose. Because glucose must cross capillaries and diffuse through tissue, ISF trails blood during rapid swings, with phase delay and damped amplitude. Low perfusion, edema, inflammation, temperature, motion, and placement further affect lag and accuracy. Use trend arrows and rates-of-change to time feeding or insulin. Calibrate well and choose sites wisely to make safer decisions.

What CGMs Actually Measure in Dogs and Cats

Continuous glucose monitors in dogs and cats don’t sample blood; they measure glucose in interstitial fluid (ISF)—the fluid surrounding cells in the subcutaneous tissue. You place a filament into the dermis, where sensor technology detects glucose via enzymatic electrochemistry (typically glucose oxidase). The enzyme converts glucose to gluconolactone, generating hydrogen peroxide, which the electrode oxidizes to produce a current proportional to ISF glucose. The transmitter converts that signal to calibrated glucose monitoring data.

You’re measuring a compartment influenced by capillary filtration, cellular uptake, and lymphatic clearance. Warmth, perfusion, and local inflammation can alter sensor microenvironment and signal quality. Adhesion, placement depth, and site choice modulate noise and drift. Factory calibration maps current to glucose units, while onboard algorithms filter artifacts, reject compression lows, and stabilize trend accuracy.

Why Interstitial Glucose Lags Behind Blood Glucose

Although ISF glucose tracks blood glucose closely at steady state, it lags during rapid change because glucose must traverse capillary endothelium and diffuse through interstitial spaces before reaching the sensor. You’re measuring interstitial fluid, not blood, so the signal reflects transport kinetics. First, glucose exits capillaries via endothelial pores or transcellular routes driven by concentration gradients. Then it disperses through extracellular matrix, where diffusion distance and tissue tortuosity slow flux. Local uptake by cells further buffers the gradient, transiently lowering interstitial glucose relative to blood glucose during rises, and delaying recovery during falls. The result is a physiologic phase delay plus mild amplitude damping. Understanding this compartmental transfer helps you interpret CGM traces mechanistically and leverage algorithms that model diffusion to reconstruct near-real-time blood dynamics in pets.

Factors That Affect Sensor Accuracy and Lag

Because CGMs sample interstitial fluid rather than blood, multiple biological and technical variables shape both accuracy and lag: tissue perfusion (regional blood flow, capillary permeability), diffusion distance (sensor depth, edema, scar), local metabolism (cellular uptake, inflammation), sensor chemistry (enzyme kinetics, oxygen dependence, temperature sensitivity), calibration quality (timing, reference meter bias), and motion or compression artifacts.

You’ll see greater lag when perfusion is low (cold skin, vasoconstriction) and when diffusion distance increases from edema or fibrotic tissue. Inflammation near the filament alters glucose consumption and local oxygen, shifting amperometric output. Enzyme-based sensors exhibit sensor limitations when oxygen tension drops or temperature drifts; these environmental influences change proportionality between current and glucose. Poor calibration timing, hematocrit bias in reference meters, and delayed capillary sampling propagate error. Motion, pressure, or grooming compress tissue, transiently lowering readings.

How to Read Trends, Arrows, and Patterns Safely

Even when spot values look stable, trend arrows and rate-of-change data tell you where glucose is heading and how fast, which should drive your next decision. You’re not managing a number; you’re managing momentum. Use trend analysis to quantify velocity (mg/dL/min) and direction, then match your intervention’s onset to the projected curve. Arrow interpretation refines timing: steeper arrows imply faster interstitial changes and a higher likelihood that blood glucose is already further along.

1. Quantify rate-of-change: ±1–2 mg/dL/min suggests mild drift; ≥3–4 mg/dL/min signals clinically meaningful movement requiring proactive action.
2. Align action kinetics: choose carbs or insulin with onset that meets the projected nadir or peak, minimizing overshoot.
3. Validate patterns: repeated postprandial spikes or nocturnal dips indicate modifiable regimen elements (dose timing, macronutrient mix).

Calibration, Placement, and Practical Tips for Better Decisions

Trend arrows only help if the sensor feeds you accurate, timely data, so you need to control the variables that affect CGM performance: calibration strategy, anatomical placement, and daily handling. Use a structured sensor calibration plan: verify against a lab-quality capillary meter during euglycemia, not during rapid change; average two readings if discordant; recalibrate only when stable. For placement tips, target low-motion, well-perfused subcutaneous sites with consistent fat depth; avoid scars, tumors, edema, and pressure points. Insert along hair grain; clip, don’t shave, to reduce microtrauma. Anchor with flexible adhesive; minimize shear by routing the transmitter away from collars or harnesses. Warm-up fully before dosing decisions. Replace sensors on schedule; retire early if lag exceeds 20 minutes or MARD drifts upward.

Conclusion

You’ve learned that pet CGMs read interstitial—not blood—glucose, so numbers can trail during rapid swings. You’ll interpret trends over single points, watch rate-of-change arrows, and factor in perfusion, temperature, hydration, and compression artifacts. Calibrate when indicated, place sensors over well-perfused, low-motion sites, and confirm hypoglycemia with a meter. When push comes to shove, treat the patient, not the gadget: use CGM to guide dosing, timing, and safety—especially around exercise, meals, illness, and insulin adjustments.