GFR of a diabetic person is 90 ml/min. Blood glucose is 200 mg/dl. Calculate urinary glucose excretion rate if the Tm for glucose is 150 mg/min
## **Core Concept**
The renal glucose handling and excretion can be understood through the concept of the renal threshold for glucose and the tubular maximum (Tm) for glucose reabsorption. Normally, glucose is almost completely reabsorbed in the proximal convoluted tubule. The Tm for glucose represents the maximum rate at which glucose can be reabsorbed by the renal tubules.
## **Why the Correct Answer is Right**
To calculate the urinary glucose excretion rate, we first need to understand the filtered load of glucose and then subtract the amount reabsorbed by the tubules. The glomerular filtration rate (GFR) is given as 90 ml/min, and blood glucose is 200 mg/dl.
First, convert blood glucose to mg/ml: 200 mg/dl = 20 mg/ml (since 1 dl = 100 ml).
The filtered load of glucose = GFR * Plasma glucose concentration = 90 ml/min * 20 mg/ml = 1800 mg/min.
The Tm for glucose reabsorption is given as 150 mg/min.
Since the filtered load of glucose (1800 mg/min) exceeds the Tm for glucose reabsorption (150 mg/min), the excess glucose will be excreted in the urine.
Urinary glucose excretion rate = Filtered load - Tm = 1800 mg/min - 150 mg/min = 1650 mg/min.
## **Why Each Wrong Option is Incorrect**
- **Option A:** This option would be incorrect if the calculation or understanding of Tm and filtered load was incorrect, leading to a wrong subtraction or not accounting for exceeding Tm.
- **Option B:** Similarly, this would not align with correct calculations based on given parameters.
- **Option C:** This option suggests a direct relation or different calculation method not aligned with Tm and filtered load concepts.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the Tm for glucose reabsorption is typically around 250 mg/min per 1.73 m^2 body surface area in a normal individual, but it can vary. In diabetes, the renal threshold for glucose can be higher due to various factors including increased glucose reabsorption capacity. However, once this threshold is exceeded, glucose appears in the urine.
## **Correct Answer:** . 1650 mg/min