Unit 4: Tests Evaluating Renal Functions

What are the 3 main physiologic functions of the kidneys?

• Glomerular filtration

• Tubular reabsorption

• Tubular secretion

What does glomerular filtration do?

Filters substances from the blood into the kidney tubules.

What does tubular reabsorption do?

Moves needed water and solutes from the filtrate back into the blood.

What does tubular secretion do?

Moves substances from the blood into the kidney tubules.

What part of the kidney is mainly affected in glomerular disorders?

The glomeruli.

What part of the kidney is mainly affected in tubular disorders?

The tubules.

What are common symptoms of glomerular disorders?

• Proteinuria

• Hematuria

• Edema

What are common symptoms of tubular disorders?

• Electrolyte imbalance

• Polyuria.

What are common causes of glomerular disorders?

• Autoimmune

• Metabolic

• Infectious causes

What are common causes of tubular disorders?

• Genetic

• Toxic

• Ischemic causes.

Which type of disorder is more common: glomerular or tubular?

Glomerular disorders.

Which type of disorder has a high risk of progressing to CKD?

Glomerular disorders.

What does serum creatinine come from?

Muscle metabolism.

What does an elevated serum creatinine usually indicate?

Reduced kidney filtration efficiency.

When does serum creatinine usually become elevated?

After GFR has already fallen significantly, around 50%.

Why isn't serum creatinine a perfect measure of kidney function?

It is affected by muscle mass, age, sex, diet, and some medications.

What medications can affect serum creatinine?

Cimetidine and trimethoprim.

What is BUN?

Blood urea nitrogen.

Where is urea produced?

In the liver during protein metabolism.

What happens to BUN when GFR decreases?

BUN rises.

What can the BUN/creatinine ratio help determine?

Whether kidney dysfunction is prerenal, renal, or postrenal.

What factors can affect BUN besides kidney function?

• Protein intake

• Liver disease

• Dehydration

• Catabolic states

What is albuminuria?

Albumin in the urine.

What is proteinuria?

Excess total protein in the urine.

How is albuminuria commonly measured?

With a spot urine albumin-to-creatinine ratio (ACR).

Why are albuminuria and proteinuria important?

They are used with eGFR to help stage and predict CKD.

What can temporarily cause proteinuria?

Fever, exercise, or dehydration.

Why should abnormal proteinuria be confirmed with repeat testing?

Because proteinuria can be temporary.

What do renal clearance tests measure?

Glomerular filtration rate (GFR).

Why are renal clearance tests useful?

• Detect kidney damage early

• Quantify GFR.

What does renal clearance measure?

The volume of plasma completely cleared of a substance by the kidneys per unit of time.

What is the unit for renal clearance?

mL/min.

What is the basic renal clearance formula?

Cx = (Ux × V) / Px.

What does Cx mean in the clearance formula?

Clearance of substance x.

What does Ux mean?

Urine concentration of substance x.

What does V mean?

Urine flow rate in mL/min.

What does Px mean?

Plasma concentration of substance x.

What are the characteristics of an ideal GFR marker?

• Appears in plasma at a constant rate

• Freely filtered

• Not reabsorbed, secreted, synthesized, or metabolized by the nephron.

What substances can be used to evaluate renal function?

• Creatinine

• Inulin

• Urea

• Beta-2 microglobulin

• Cystatin C

• Radionucleotides

What is the gold standard for measuring GFR?

Inulin clearance.

Why is inulin an ideal GFR marker?

It is freely filtered and is not reabsorbed, secreted, synthesized, or metabolized by the nephron.

What does inulin clearance equal?

True GFR.

Why isn't inulin clearance commonly used clinically?

It requires a constant IV infusion and is technically difficult.

What is creatinine?

A substance produced naturally from muscle.

Why can creatinine clearance slightly overestimate GFR?

A small amount of creatinine is secreted by the tubules.

What is the clinical standard for measuring clearance?

Creatinine clearance.

How is creatinine clearance measured?

With a 24-hour urine collection and plasma creatinine.

Why is creatinine clearance more practical than inulin clearance?

Creatinine is produced naturally by the body and the test is easier to perform.

What is the creatinine clearance formula?

Ccr = (UCr × V) / (PCr × T).

What does UCr mean?

Urine creatinine concentration.

What does V mean in the creatinine clearance formula?

Total urine volume collected.

What does PCr mean?

Plasma or serum creatinine concentration.

What does T mean?

Total urine collection time in minutes.

How many minutes are in a 24-hour urine collection?

1,440 minutes.

What is the simplified creatinine clearance formula for a 24-hour collection?

Ccr = (UCr × V) / (PCr × 1440).

What is the normal creatinine clearance range in young adults?

About 90–130 mL/min/1.73 m².

What does BSA stand for?

Body surface area.

Why is BSA used with creatinine clearance?

To correct the result based on the patient's body size.

What information is needed to calculate BSA using the Mosteller formula?

Height in cm and weight in kg.

What is the Mosteller BSA formula?

BSA = √[(height × weight) / 3600].

What does eGFR stand for?

Estimated glomerular filtration rate.

What is eGFR used to assess?

Kidney function.

What is eGFR calculated mainly from?

• Serum creatinine

• Demographic information

What factors are included in eGFR equations?

Age, sex, and body size.

What are common eGFR equations?

MDRD and CKD-EPI.

What is the main clinical use of eGFR?

Staging and monitoring chronic kidney disease.

What is one limitation of eGFR?

It is less accurate in people with extreme body composition or unusual diets.

What is cystatin C?

A low-molecular-weight protein produced by all nucleated cells at a constant rate.

What is an advantage of cystatin C?

It is less affected by muscle mass, age, or diet.

When can cystatin C be especially useful?

In elderly, malnourished, pediatric, or very muscular patients.

What is CKD defined as according to these notes?

An eGFR below 60 or a UACR of 30 or greater for more than 3 months.

Is there one serum or plasma test that directly measures tubular function?

No

How is tubular function mainly assessed?

• Urine tests

• Clearance studies

What do tubular reabsorption tests evaluate?

How well the renal tubules reclaim water and solutes from the filtrate.

What are the two main tests for tubular reabsorption?

• Urine concentration testing

• Urine dilution testing.

What does the urine concentration test assess?

The kidney's ability to concentrate urine.

What is another name for the urine concentration test?

Water deprivation test.

What parts of the nephron are involved in urine concentration?

• The proximal tubule

• Loop of Henle

• Collecting duct

How long is water usually restricted during the water deprivation test?

About 8 hours.

What is measured during the water deprivation test?

Urine osmolality and specific gravity.

What is a normal urine osmolality after water deprivation?

Greater than 800 mOsm/kg.

What is a normal specific gravity after water deprivation?

Greater than 1.022.

What does failure to concentrate urine indicate?

• Possible chronic kidney disease

• Nephrogenic diabetes insipidus

• Sickle cell nephropathy.

What does USG stand for?

Urine specific gravity.

What does urine specific gravity measure?

The density of urine compared with water.

What does urine specific gravity reflect?

Urine concentration.

How can specific gravity be measured?

With a refractometer or dipstick.

Which is more accurate for measuring specific gravity: refractometer or dipstick?

Refractometer.

What is the normal adult range for urine specific gravity?

1.003–1.030.

What does Uosm stand for?

Urine osmolality.

What does urine osmolality measure?

The total solute concentration per kilogram of water in urine.

What instrument measures urine osmolality?

An osmometer.

What is the normal range for random urine osmolality?

300–900 mOsm/kg.

What does the urine dilution test assess?

The kidney's ability to dilute urine.

What is another name for the urine dilution test?

Water loading test.

What parts of the nephron are mainly assessed by the urine dilution test?

The distal tubule and collecting duct.

What does the patient do during a water loading test?

Drinks 20 mL/kg of water.

How often is urine collected during the water loading test?

Every 30–60 minutes.

What is a normal urine osmolality during the dilution test?

Less than 100 mOsm/kg.

What is a normal specific gravity during the dilution test?

≤ 1.003.