Glomerular Filtration Rate (GFR)

 

Glomerular Filtration Rate (GFR) is the primary test used to check how well the kidneys are functioning. It measures the amount of blood that passes through the glomeruli—the tiny filters in the kidneys that remove waste—every minute.

GFR is a key measure of how well your kidneys are filtering blood. It estimates how much blood (in millilitres) the kidneys filter per minute to remove waste and excess fluid.

Why GFR is Important

Your kidneys act as the body's filtration system. A high GFR means your kidneys are efficiently clearing waste, while a low GFR indicates that waste products (like creatinine) are building up in your blood because the filters are damaged or sluggish.

Standard Unit: mL/min/1.73m^2 (millilitres per minute per 1.73 square meters of body surface area).

Normal Range: Roughly 90–120mL/min for healthy adults.

• Detects early kidney disease

• Monitors progression of chronic kidney disease (CKD)

• Helps guide drug dosing

• Assesses overall renal health, especially in people with diabetes or hypertension

eGFR vs. mGFR

In a clinical setting, you will likely see eGFR (estimated GFR) on your lab results rather than a direct measurement.

Measured GFR (mGFR): The "gold standard" involves injecting a substance, such as inulin or Iohexol, into the blood and measuring how quickly the kidneys clear it. This is a complex process and is rarely done outside of research.

Estimated GFR (eGFR): This is a calculated estimate using a mathematical formula. It typically requires:

Serum Creatinine: A waste product from muscle breakdown.

Age and Sex: These factors account for natural differences in muscle mass.

Common Formulas: The CKD-EPI (recommended standard) or the MDRD formula

Normal & Abnormal GFR Values (Adults)

GFR (mL/min/1.73 m²)	Kidney Function CKD	D Stage
≥ 90	Normal (if no protein in urine)	Stage 1
60 – 89	Mild decrease	Stage 2
45 – 59	Mild–moderate decrease	Stage 3a
30 – 44	Moderate–severe decrease	Stage 3b
15 – 29	Severe decrease	Stage 4
< 15	Kidney failure	Stage 5

A low GFR for more than 3 months indicates chronic kidney disease.

How GFR is Calculated

GFR is usually estimated (eGFR) using:

• Serum creatinine

• Age

• Sex

• Body size

• Sometimes cystatin C (more accurate in some cases)

Common formulas:

• CKD-EPI (preferred)

• MDRD (older)

Causes of Low GFR

• Diabetes (most common)

• High blood pressure

• Glomerulonephritis

• Long-term painkiller use (NSAIDs)

• Urinary obstruction

• Dehydration (temporary drop)

Given your earlier discussions around high HbA1c (12), monitoring GFR is very important, as uncontrolled diabetes can silently reduce kidney function.

Symptoms of Low GFR (Often Late)

• Swelling of feet/face

• Fatigue

• Reduced urine output

• Foamy urine (protein)

• Nausea, itching

• Shortness of breath

How to Protect or Improve GFR

(Depends on cause; damage may be slowed, not always reversed)

Medical

• Tight blood sugar control

• Control blood pressure (ACE inhibitors/ARBs often used)

• Avoid unnecessary painkillers

• Treat infections promptly

Lifestyle

• Reduce salt intake

• Moderate protein intake

• Stay well hydrated

• Quit smoking

• Maintain a healthy weight

Alternative Support (Adjunct, not replacement)

• Siddha/Ayurveda: Punarnava, Varuna, Neermulli (only under expert guidance)

• Yoga & breathing to reduce BP and stress

When to See a Doctor Urgently

• Rapid fall in GFR

• GFR < 60 with protein in urine

• Swelling + reduced urine

• Diabetic patient with rising creatinine

Understanding how medications affect GFR requires looking at the "pipes" leading into and out of the kidney's filters (the glomeruli).

To keep the filtration rate steady, the kidney needs a certain amount of internal pressure. It maintains this pressure by adjusting two main vessels: the Afferent Arteriole (the "inlet" pipe) and the Efferent Arteriole (the "outlet" pipe).

1. ACE Inhibitors: Opening the "Outlet"

ACE Inhibitors (like Lisinopril or Enalapril) and ARBs (like Losartan) block Angiotensin II, a hormone that normally constricts the outlet pipe (efferent arteriole).

• The Effect: These drugs cause the outlet pipe to widen (vasodilation).

• Impact on GFR: Because the "exit" is now wide open, the pressure inside the filter drops. This usually causes an initial, small decrease in eGFR (a rise in creatinine).

• Why doctors use them anyway: Even though GFR drops slightly at first, lowering that internal pressure protects the filters from long-term wear and tear. It’s like lowering the water pressure in a delicate pipe to prevent it from bursting over time.

2. NSAIDs: Narrowing the "Inlet"

NSAIDs (like Ibuprofen, Naproxen, or Celebrex) block Prostaglandins, which are chemicals that normally keep the inlet pipe (afferent arteriole) open.

• The Effect: Without prostaglandins, the inlet pipe narrows (vasoconstriction).

• Impact on GFR: Less blood can get into the filter, which reduces the pressure and lowers the GFR.

• The Risk: In healthy people, the body can usually compensate. However, if you are dehydrated or already have kidney issues, NSAIDs can "starve" the kidney of blood, leading to a sudden drop in function.

3. The "Triple Whammy"

The most dangerous scenario for GFR occurs when a patient takes three types of medications together:

1. ACE Inhibitor/ARB: Opens the outlet (lowers pressure).

2. NSAID: Closes the inlet (reduces flow).

3. Diuretic (Water pill): Reduces overall blood volume.

When all three are used, the kidney loses its ability to regulate its own internal pressure. This can lead to Acute Kidney Injury (AKI) because the GFR plummets dangerously low.

Comparison Table: Hemodynamic Effects

Medication	Primary Action	Effect on Vessel	Resulting GFR
ACE Inhibitors	Blocks Angiotensin II	Opens the Outlet (Efferent)	Initial Slight Decrease
NSAIDs	Blocks Prostaglandins	Closes the Inlet (Afferent)	Decrease (risk of AKI)
Normal State	Balance of both	Both vessels adjusted	Stable GFR