Measurement of kidney function Flashcards

Question

What is the equation for renal clearance?

Answer 1

* Measuring clearance means measurement of OVERALL nephron function i.e. all ~2 million nephrons in both kidneys * This gives the SUM of ALL transport processes occurring along nephrons… * …but no information about precise tubular sites or mechanisms of transport

Answer 2

This is the rate at which filtrate is produced in the kidneys = 125 mL/min (º 180 L/day)

Answer 3

GFR can be estimated by measurement of the clearance of CREATININE i.e. CREATININE CLEARANCE BUT creatinine is filtered AND secreted into tubule A more accurate estimation is provided by measurement of INULIN clearance – it is filtered but NOT secreted into tubule

Answer 4

•INULIN is a plant polysaccharide Mw 5200Da

Answer 5

•It is freely filtered - i.e. plasma and tubular concentration is same… …but it is NOT secreted and is NOT reabsorbed * Therefore rate of excretion in urine equals the rate of filtration by the kidneys * So Inulin clearance = GFR

Answer 6

•If a substance has clearance greater than inulin, then it must also be being secreted; …….less means that it must be being reabsorbed or not filtered freely at the glomerulus

Answer 7

•Most reliable method of measuring GFR, but not useful clinically WHY? * Inulin must be administered by IV to get relatively constant plasma or serum levels * Chemical analysis of inulin is technically demanding * Could use radiolabelled compounds instead, e.g. radioactive Vit B or EDTA * However, these may also bind to proteins and distort results

Answer 8

•Problems of IV infusion of GFR marker are avoided by using an endogenous substance with inulin-like properties – i.e. CREATININE

Answer 9

* Creatinine is filtered at the glomerulus… * …but some of it is also secreted into the proximal tubule * Therefore using the equation above would actually over-estimate GFR by about 20% in humans i.e. it would produce a GFR value of about 150 mL/min * However, the colorimetry methods used to measure creatinine (e.g. the Jaffe method) under-estimates creatinine concentrations by about 20 % * Luckily, these 2 errors cancel each other out, and calculated creatinine clearance ≈ inulin clearance (i.e. 125 mL/min)

Answer 10

* Cheap, easy, reliable, used clinically * Avoids IV infusion, just requires venous blood and urine samples * Creatinine usually produced by creatinine phosphate metabolism in muscle * Must remember to take into account if person has muscle disease/damage or has had large quantities of meat to eat * Usually measure over a 24 hr period to get reliable results and take samples before breakfast

Answer 11

Creatinine Clearance can also be adjusted to take account of body surface area

Answer 12

* PAH not normally present in blood * When given, almost all (~90%) cleared from kidney in one passage… …some is filtered in glomerulus and remainder secreted by proximal tubules ~10% by-passes tubule - travels from efferent arterioles into peritubular capillaries and then venous renal blood, and is not secreted •Uncorrected value for PAH clearance often used – known as “effective renal plasma flow” (ERPF)

Answer 13

•If a substance is completely cleared from the plasma, its clearance rate will be equal to the renal plasma flow (RPF)

Answer 14

·Indicators of renal function such as plasma creatinine or BUN increase only after there is significant loss of renal function: typically a 60% loss in renal function before plasma creatinine or BUN increases ·Urinary albumin/protein excretion can also be used as an indicator of chronic kidney disease · Currently a lot of research (and commercial) interest in identifying blood and urinary markers which increase in the early stages of renal failure and can be measured

Answer 15

·These are mostly proteins released into the plasma and/or urine: - Kidney injury molecule – 1 (KIM-1) (urine) - Interleukin (IL)-18 (urine) - Fatty-acid binding proteins (FABPs) (urine) - Neutrophil gelatinase-associated lipocalin (NGAL) (plasma & urine) - Cystatin C (plasma)