Norm of Inulin Clearance Test
|>70 years||Decreased clearance up to 45% of adult value|
|Male Female||124 ± 25.8 mL/min 119 ± 12.8 mL/min|
|<11 years||82–122 mL/minute|
|11–20 years||86–126 mL/minute|
Usage of Inulin Clearance Test
Assessment of glomerular filtration rate (GFR).
Decreased results occur when more than 50% of renal nephrons are damaged, thus indicating impaired glomerular filtration; acute tubular necrosis, atherosclerosis (renal artery), congestive heart failure, decreased renal blood flow, dehydration, glomerulonephritis, malignancy (bilateral renal), nephrosclerosis, obstruction (renal artery), polycystic kidney disease, pyelonephritis (advanced bilateral chronic), renal lesions (bilateral), renal failure, shock (cardiogenic, hypovolemic), thrombosis (renal vein), tuberculosis (renal), and ureteral obstruction (bilateral). Drugs include aminoglycosides, amphotericin B, penicillins, and phenacetin.
Description of Inulin Clearance Test
GFR is the standard measure of renal function and is critical for the management of renal diseases. The GFR in humans can be determined exactly by measurement of the clearance of an ideal filtration marker, such as inulin, which is a soluble dietary plant fiber that is freely filtered by the kidney and does not undergo metabolism, tubular secretion, or absorption. This method, however, is not suitable for routine clinical practice. The classic method of measuring inulin clearance in humans includes constant intravenous infusion of the compound and timed collections of urine. To avoid the need for timed urine collections, several alternative procedures have been devised. All these methods use only determination of inulin in plasma or serum. From these, the total body inulin clearance is obtained by means of pharmacokinetic calculations. To measure total body clearance, usually called “plasma clearance,” inulin is given either as a constant intravenous infusion or as a bolus infusion. Both procedures overestimate GFR because of incomplete distribution of inulin during the study periods. The error may be minimized by means of model-independent pharmacokinetic calculations.
Professional Considerations of Inulin Clearance Test
Consent form NOT required.
- Determination of systemic inulin clearance by the standard technique of constant intravenous infusion has long been accepted as a reliable method for measuring GFR without urine collection.
- Obtain alcohol wipes, a tourniquet, 5 needles, 5 syringes, 5 green-topped tubes, 5 clean specimen containers with lids, and a 500-mL bag of D5W with Y tubing.
- Establish intravenous access, using 5% dextrose in water.
- Number each set of tubes and specimen containers 1, 2, 3, 4, and 5, and use them sequentially throughout the test.
- Obtain 25 mL of 10% inulin and 500 mL of 1.5% inulin along with an intravenous pump. Prepare the inulin by shaking the vial and heating it to promote crystal dissolution.
- Use this test cautiously in clients with cardiac disease because intravenous fluids given during the test may induce congestive heart failure.
- See Client and Family Teaching.
- Do NOT perform test or draw specimen during hemodialysis.
- Draw a 10-mL blood sample for inulin level, without hemolysis. Record the time of collection on each label.
- Infuse intravenously 25 mL of 10% inulin over 4 minutes.
- An hour later, begin an infusion of 1.5% inulin, 500 mL at 240 mL/hour.
- Draw a 10-mL blood sample for inulin level, without hemolysis, at 40, 60, 80, and 95 minutes after the initiation of the 1.5% inulin solution. Record the time of collection on each label.
- The test takes 2 hours.
- Write the time and amount of each dose of inulin on the laboratory requisition.
- Send each blood sample immediately to the laboratory.
- Resume previous diet.
Client and Family Teaching
- Fast from food for 4 hours before the test, and refrain from exercise the morning of the test.
- The test takes 2 hours to complete.
Factors That Affect Results
- Hemolysis of blood will falsely decrease results.
- Failure to infuse intravenous inulin at the prescribed rate will decrease results.
- The initial bolus of inulin should be administered within 1 hour of preparation.