Biochemical Assessment of Kidney Function and Damage Flashcards
What is the function of a nephron?
- NEPHRON = basic functional unit responsible for filtration of plasma and production of urine.
- There are around 1 000 000 nephrons per kidney
What are major functions of the kidney?
- Nitrogen Excretion
- Fluid (blood volume) Regulation
- Electrolyte regulation
- Acid-Base regulation
- Calcium Metabolism
How does the kidney carry Red Blood Cell Production?
- Erythropoetin is secreted mainly by the peri-tubular cells of the renal cortex. It is required to stimulate rbc production.
- Levels of EPO are increased when O2 delivery to the kidney is low.
- EPO stimulate bone marrow to make more rbc.
How is nitrogen excretion carried?
- Urea is major waste product. Filtered by the glomerulus.
- Some urea diffuses back into kidney from filtrate.
How does the kidney regulate fluid?
- Dehydration may be significant enough to reduce blood pressure – reduces GFR and fluid loss.
- RENIN secretion results in ANGIOTENSIN II production
- ALDOSTERONE increases Na+ reabsorption (water follows).
- ADH increases aquaporins in collecting duct
- BNP counteracts this – prevents fluid overload by reducing renin secretion and causing vasodilation to reduce blood pressure.
How does Electrolyte regulation take place?
- PTH regulates Ca2+ reabsorption and PO4- excretion
- Excretion of Mg2+ and other ions is largely passive – more being filtered = more being excreted
- In steady state, K+ is inversely related to Na+ (Aldosterone activity)
- Renal Failure – limited capacity to excrete potassium
How does ACID-BASE regulation take place?
- Metabolic process result in acid production (phosphoric, sulfuric and carbonic – CO2 is excreted via lungs).
- Carbonic Anhydrase is key to HCO3-
- Reabsorption in the PCT.
How is Calcium metabolism regulated?
- Cells in the kidney respond to PTH by increasing 1 α-hydroxylase activity
- Required to form 1, 25 (OH) Vit D (active form).
- This is required for normal bone metabolism and absorption of Ca2+ and PO4-
- PTH acts on the DCT to increase phosphate excretion and increase caclium reabsorption.
What is the effect of Renal disease on the glomerulus?
Reduced filtration
- Reduced urine volume
- Increased plasma creatinine /urea
- Hyperkalaemia
- Hyperphosphataemia
- Metabolic acidosis
Damage to glomerular membrane
- Proteinuria – large proteins
- Haematuria
What is the effect of renal disease on tubular?
Reduced reabsorption (general or specific inherited conditions).
- Polyuria, low urine osmolality
- Metabolic acidosis
- Proteinuria - small proteins (ß2 microglobulin, amino acids)
- Glycosuria
What is the effect of renal disease on hormonal disease?
- Anaemia
- Hypocalcaemia
What is the clinical presentation of Renal Disease?
Uraemic Syndrome
- Dermatological: Pruritus, Pigmentation, Slow wound healing
- Gastrointestinal: Vomiting, Nausea, Anorexia
- Genitourinary: Impotence, Polyuria/Nocturia
- Neurological: Lethargy, Headache, Peripheral neuropathy, Muscle weakness
- Immunological: Increased susceptibility to infection
- Cardiovascular: Hypertension, Percarditis, CCF, Anaemia
- Skeletal: Renal osteodystrophy, Stunted growth
- Azotaemia – increased urea
What is Biochemical Assessment useful for?
- Detection and staging of renal disease
- Identification and management of biochemical complications.
- Prognosis
- Monitoring disease progression
What is the use of kidney test?
Tests for Glomerular function
- Ability to remove waste products
- Integrity of glomerular membrane and ability to prevent large particles entering the filtrate
Tests for Tubular function
- Ability to adjust Na+, K+, H+ ions, water composition of filtrate & reabsorb small proteins, amino acids & glucose
Tests for hormonal production
- Vitamin D, erythropoietin
Urinalysis
- Detect disease
- Proteinuria
What is Albuminuria and Microalbuminuria?
MICROALBUMINURIA: Predictor of CVD, increased risk of diabetic nephropathy
- 24 hour urine sample 30 – 300 mg/L albumin (not detectable using a dipstick, measured using turbidometric immuoassay)
ALBUMINURIA
- > 300 mg
What is Albumin:Creatinie Ratio?
Used to screen diabetic patients, can use random urine sample.
- > 2.5 mg/mmol (male), > 3.5 mg/mmol (female)
- ACR 3 – 70 mg/mmol – confirm with early morning repeat
- ACR > 70 mg/mmol – no repeat required
MACR has high intraindividual variation (60 – 80 % CV)
What is Clinical Proteinuria?
Clinical Proteinuria:
- > 500 mg/24 hour
- +ve dipstick
- Increased ACR or PCR
What is overflow, tubular and Glomerular Proteinuria?
OVERFLOW (Mw ≤ Albumin)
- Abnormal small proteins over flow into urine (eg. multiple myeloma)
- Kappa/lambda light chains (Bence Jones Proteins)
TUBULAR
- Decreased reabsorption of filtered proteins
- N-Acetyl Glucosaminidase (NAG) – released from lysed tubular cells
GLOMERULAR (Mw ≥ Albumin)
- Increased glomerular permeability – larger proteins present
- IgG, Transferrin, retinol binding protein, α1-macroglobulin
How is proteinuria, severe proteinuria, heavy proteinuria and nephrotic syndrome catgorised?
What is clearance?
- Clearance (ability to remove waste products)
- The volume of plasma that is filtered by the kidneys and from which a substance is completely cleared per unit of time
What is the equation?
Clearance (mL.min<span>-1</span> )= Urine Marker concentration (µmol/L) x Urine Volume (mL) / Plasma Marker concentration (µmol/L) x time (min)
What makes an ideal marker of GFR?
Ideal Marker (clearance = GFR)
- Stable plasma concentration
- Freely filtered at the glomerulus
- Not secreted or reabsorbed by renal tubular cells
- Renal excretion ONLY (i.e. not metabolised by liver)
- Easy and cheap to measure
How are exogenous markers delivered?
- Bolus injection: disappearance from plasma
- IV infusion: clearance
What are some exogenous markers of GFR?
Inulin Clearance - gold standard.
- Difficult and expensive to measure
51Cr-EDTA - clinical standard
- Difficult and expensive to measure, not available for clinical use in the UK beyond March 2019
- IOHEXOL – non-ionic contrast media containing iodine.
99mTc-DTPA (diethylenetriaminepentacetic acid)
- Difficult and expensive to measure
- Allows imaging of kidney
P-aminohippuric acid
- Determines renal plasma flow
- Completely cleared in single passage through kidney
What are some endogenous markers of GFR?
- Urea
- Creatinine
- Cystatin C
What are characterisitics of Urea?
- End product of nitrogenous compound metabolism (esp amino acids)
- Freely filtered at glomerulus
- Quick, cheap & convenient
- NOT steady plasma levels: effected by diet and protein catabolism e.g. raised in GI bleed
- Low in liver disease
- Some passive reabsorption in renal tubules
- Under estimation of GFR
What are characterisitics of Creatinine?
- From breakdown of skeletal muscle cells.
- Normally filtered and excreted in the urine.
- Small amount secreted by PCT – creatinine slightly overestimates GFR
- Quick, cheap & convenient
- Dependent on body mass /ethnicity/diet
- Assay Interferences with Jaffe method (ketones, bilirubin & glucose). Can use Jaffe kinetic method to reduce interference.
- Enzymatic method (creatinase and creatininase coupled method)
N.B. GFR falls to <50ml/min before creatinine rises
What are characteristics of Cystatin C?
- Protein produced stadily by all nucleated cells
- Protease inhibitor: Lysosomal proteinases and extracellular cysteine proteinases
- Small MW 13kDa that is freely filtered by glomerulues
- Serum levels independant of age, muscle mass, weight, height and sex
- Expensive assay
How is Cystatin C affected by thyroid disease?
- Hypothyroid - Lower serum levels overestimates GFR
- Hyperthyroid - Higher serum levels underestimates GFR
How do you collect 24hr urine?
Day 1
- 8am empty bladder (discard output)
- Commence 24h urine collection
- All urine now passed until 8am next day must be collected into container.
Day 2
- 8am collect final urine output into container
Collect blood sample with 24 hr period
What is the equation of Creatinine Clearance?
Creatinine clearance (mL/min) = [Creat]u x V (mL/min) / [Creat]p
What is the eGFR (Estimated Glomerular Filtration rate)?
Calculated result based on [Creatinine] in plasma/serum
- Creatinine-based eGFR
- Should use CKD-EPI equation
- When Cystatin C requested, should use CKD-EPI cystatin C equation
- Calculated eGFR is added to all U&E profiles
When is eGFR not validated?
NOT validated for use in
- Children
- Acute kidney injury
- Amputees
- Pregnancy
- Malnourished
- Odematous
- Muscle wasting
What are variables for eGFR?
- creatinine
- age
- sex
- ethnicity
What are features of MDRD equation?
- Study only included patients with known renal failure.
- Under-estimates GFR (>60 mL/min).
- The normal kidney patients have measured GFR that is slightly higher than the estimated GFR - MDRD equation underestimates GFR in persons with normal kidney function
What are features of CKD-EPI?
- Study used CKD and normal renal function patients
- More accurate than MDRD esp when GFR >60 mL/min
How is Tubular function assessed?
-Ability to excrete / retain water
- Determined by ability to concentrate / dilute urine
- Tests: Urine sodium, osmolality and volume/Serum sodium & osmolality/Fluid deprivation test
-Ability to maintain acid-base balance
- Usually see metabolic acidosis
- Renal tubular acidosis
- Urine / blood pH & Anion gap& Serum bicarbonate
-Ability to maintain electrolyte balance
- Serum sodium & potassium
- Random Urine sodium & potassium
- FRACTIONAL EXCRETION: Relative to creatinine, Excretion over same time and volume
-Ability to reabsorb small proteins, amino acids & glucose
- Tubular proteinuria – low molecular weight proteins (α1-microglobin, Retinol binding protein, RBP & β2-microglobin)
- Serum and urine aminoacids. (specific defects – aminoaciduria).
- Urine glucose – renal threshold = 11mmol/L
What is Fanconi syndrome and causes?
- Decreased reabsorption of solutes by PCT.
- Hypophosphataemia, Glycosuria, Hypouricaemia, aminoaciduria, low Mw proteinuria, Type 2 RTA
Caused by: cystinosis, Wilsons, Gentamicin, Cisplatin, heavy metal intoxication
What is the equation for Fractional Excretion?
FENa = 100 x(Urine [x] x Plasma [creat]) / (Plasma [x] x Urine [creat])
How are hormone production assessed?
-Ability to produce active Vitamin D (1,25 (OH) Vit D)
- PTH
- Serum adjusted calcium
-Ability to secrete erythropoetin
- EPO and FBC
What is Renal Osteodystrophy?
- Seen in dialysis patients/ CKD
- Hypocalcaemia and hyperphosphataemia
- Secondary hyperparathyroidism
What are some microscopy in urinalysis?
- Red cell
- Granular (cell debris)
- Hyaline (Tamm-Horsfall protein)
- White cell
What are types of Renal Stones?
Calcium oxalate ± phosphate (hypercalcaemia) - 67%
Triple phosphate/struvite (Mg, Ca, NH4) (UTI) - 12%
Calcium phosphate (alkaline urine) - 8%)
Uric acid (purine metabolism) - 8%
Cystine (cystinuria) - 1-2%
How are Renal Stones induced?
- Increased concentration of urinary constituents above natural solubility
- Lack of physiological inhibitors of stone growth (e.g mucopolysaccarides,citrate & pyrophosphate)
- Changes in urine pH (alkaline pH favours NH4 ion formation → precipitate)
- Seeding (i.e stone formed can be different to the nucleus that the stone started from & colonisation of bacteria can accelerate stone growth)
How is Urinalysis used for Renal Stones?
- Calcium
- Phosphate
- Urate
- Oxalate
- Cystine
- pH
- Sodium
- Magnesium
- Citrate
- Microbiology
- Stone Analysis
What are novel biomarkers of Renal Function?
- Neutrophil Gelatinase-Associated Lipocalin (NGAL) – 25 kDa epithelial protein, covalently bound to gelatinase from human neutrophils. Forms a complex with MMP-9
- Kidney Injury Molecule 1 (KIM-1) – type 1 transmembrane protein, with immunoglobulin and mucin domains. Expression is upregulated in PCT post injury.
- Liver Fatty Acid Binding Protein (L-FABP) – localises in PCT and is excreted in urine in response to oxidative stress
- Tissue Inhibitor Metalloproteinase 2 (TIMP2) and Insulin-Like Growth Factor Binding Protein 7 (IGFBP7) - Nephrocheck (Astute Medical [TIMP2] x [IGFBP7] = < 0.3
