SN5
SHORT NOTE
5M
★ High-yield
Enumerate the renal function tests and give their normal values.
Definition
Renal function tests = investigations used to assess glomerular filtration, tubular function and the overall excretory capacity of the kidney.
Tests of Glomerular Function
| Test | Normal | Comment |
|---|---|---|
| Blood urea | 20–40 mg/dL | Affected by diet, GI bleeding, dehydration — not specific |
| Serum creatinine | 0.6–1.2 mg/dL | Better than urea; rises only after 50% loss of GFR |
| Creatinine clearance | 120–130 mL/min | The practical clinical measure of GFR |
| Inulin clearance | 125 mL/min | Gold standard; research use |
| Blood urea nitrogen : creatinine | About 10:1 | > 20:1 suggests prerenal failure |
Tests of Tubular Function
- Urine specific gravity — normal range 1.003–1.030; a fixed value of 1.010 (isosthenuria) indicates severe tubular damage
- Urine osmolality — after overnight water deprivation it should exceed 800 mOsm/kg
- Water deprivation and concentration test
- Urine acidification test — ammonium chloride loading; urine pH should fall below 5.3
- PAH clearance — measures renal plasma flow and tubular secretory capacity
Urine Examination
| Finding | Significance |
|---|---|
| Proteinuria > 150 mg/day | Glomerular damage; > 3.5 g/day = nephrotic |
| RBC casts | Glomerulonephritis |
| WBC casts | Pyelonephritis |
| Granular / muddy brown casts | Acute tubular necrosis |
| Glycosuria with normal blood sugar | Renal glycosuria, Fanconi syndrome |
Differentiating Prerenal from Renal Failure
| Index | Prerenal | Intrinsic renal (ATN) |
|---|---|---|
| Urine osmolality | > 500 mOsm/kg | < 350 mOsm/kg |
| Urine Na+ | < 20 mEq/L | > 40 mEq/L |
| Fractional excretion of Na+ | < 1% | > 2% |
| BUN : creatinine | > 20:1 | About 10:1 |
| Response to fluids | Improves | No improvement |
Applied Aspects
- EGFR from serum creatinine, age, sex and race is now used to stage chronic kidney disease (stages 1–5; stage 5 is GFR < 15)
- Drug doses of renally excreted agents (aminoglycosides, digoxin, metformin) must be adjusted to GFR
- Imaging and biopsy complement biochemical tests when the cause is unclear
All questions in Renal System
- Q1Define glomerular filtration rate. Describe the filtration barrier, the forces governing filtration, and the factors affecting GFR. Add a note on autoregulation. ★
- Q2Describe tubular reabsorption in the different parts of the nephron. Explain glucose reabsorption with transport maximum and renal threshold. ★
- Q3Describe the counter-current mechanism of urine concentration. Explain the role of the loop of Henle, vasa recta and ADH. ★
- Q4Describe the role of the kidney in the regulation of acid–base balance. Explain bicarbonate reabsorption, titratable acid and the ammonia mechanism. ★
- Q5Describe the nerve supply of the urinary bladder and the mechanism of micturition. Add a note on the abnormalities of micturition. ★
- SN1Define renal clearance. Describe inulin and creatinine clearance and their significance. ★
- SN2Describe the juxtaglomerular apparatus and the renin–angiotensin system. ★
- SN3Write a short note on antidiuretic hormone. ★
- SN4Classify diuretics according to their site of action and describe their mechanisms. ★
- SN5Enumerate the renal function tests and give their normal values. ★
- SN6Describe the principle and types of dialysis. Give its indications.
- SN7Enumerate the non-excretory functions of the kidney. ★