Glycosuria occurs when Plasma glucose level exceeds

KGMU 2024

Applied Physiology

Easy

Appeared in: KGMU 2024

Explanation

The correct option is 160-180 mg/dL, which represents the typical renal threshold for glucose.
The kidneys filter glucose from the blood but can reabsorb it back into circulation up to a certain limit.
This limit, known as the renal threshold, is the maximum blood glucose concentration the renal tubules can handle before glucose starts spilling into the urine.
When plasma glucose levels rise above 160-180 mg/dL, the transport mechanisms in the kidney tubules become saturated.
Consequently, the excess glucose that cannot be reabsorbed is excreted, resulting in glycosuria (glucose in the urine).

Option A: The range of 120-140 mg/dL is generally below the normal renal threshold for glucose. At these levels, the kidneys are typically able to reabsorb all the filtered glucose, so it does not appear in the urine.
Option B: The range of 100-120 mg/dL is considered impaired fasting glucose but is well below the renal threshold. The kidneys can easily reabsorb all glucose at this concentration.
Option C: The range of 130-150 mg/dL is still below the typical renal threshold of 160-180 mg/dL. Glycosuria would not be expected at this plasma concentration in most individuals.

Visual 1: Diagram of a Nephron - Illustrating the glomerulus where blood is filtered and the proximal tubule where glucose is reabsorbed. An arrow can show glucose spilling into the collecting duct when the threshold is exceeded.
Visual 2: Graph - Plotting plasma glucose concentration against the rate of glucose excretion in urine. The graph would show zero excretion until the plasma level hits the renal threshold (around 180 mg/dL), after which the excretion rate rises linearly.

Clinical Relevance

Nursing practice connection: Knowing Renal threshold for glucose and the pathophysiology of glycosuria helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
Glycosuria is a key sign of hyperglycemia and is often one of the first indicators leading to a diagnosis of diabetes mellitus.
Nurses frequently use urine dipstick tests to screen for glucose in the urine, especially in patients at risk for diabetes or those with established disease to monitor glycemic control.
What if? - If a patient has glycosuria but their blood glucose is normal (e.g., 95 mg/dL), the nurse should suspect renal glycosuria. This is a different condition where the kidney's threshold is abnormally low, and it does not indicate diabetes. Further investigation into renal tubular function would be necessary.

How to Approach the Question

First, understand the question is asking for the physiological limit at which glucose appears in the urine.
Identify the key term 'Glycosuria', which means glucose in the urine.
Recall the concept of the 'renal threshold for glucose'—the maximum blood glucose level at which the kidneys can reabsorb all filtered glucose.
Remember that this threshold is approximately 180 mg/dL.
Evaluate the given options to find the range that best corresponds to this physiological threshold.
Select the option 160-180 mg/dL, as this is the range where the threshold is met and exceeded, leading to glycosuria.

Concept Tested & Keywords

Concept Tested: Renal threshold for glucose and the pathophysiology of glycosuria.
Stem keywords: Glycosuria, Plasma glucose level
Lead-in keywords: exceeds

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