Assessment of renal function Jack Shepard Jayne Windebank

Functions of the kidney (6 marks) 1. Fluid and volume regulation 2. Electrolyte balance 3. Acid base regulation 4. Removal of toxins 5. Ca/phosphate homeostasis 6. RBC maturation via EPO

What tests/investigations can indicate kidney function? (4 marks) 1. Urine tests 2. Blood tests 3. Imaging (ultrasound common and routine) 4. Biopsy

Why assess kidney function? (3 marks) 1. To identify renal impairment 2. To monitor disease progress 3. To assess baseline measurements prior to starting treatment with certain drugs

Urine – What tests could you do? What are you looking for in each?  Dipstick  Blood  Glucose – Diabetes mellitus  Protein – proteinuria often indicates a failure of the glomerulus  Leucocytes – infection of urinary tract or kidney  Nitrites - infection of urinary tract or kidney  Microscopy  Cells – RBCs, WBCs  Crystals – potential kidney stones forming  Biochemistry  Essentially a posh dipstick  Fluid intake/Urine output  Does output roughly equal input?

Blood tests – What would you test and what are you looking for?  FBC – anaemia  Urea + electrolytes  Glucose - diabetes  eGFR or GFR – renal failure  Renal Screen - measurement of a number of indicators of systemic/autoimmune conditions that usually affect the glomerulus e.g. SLE

U&Es & eGFR

Urea & Electrolytes  U&E is often used as a screening test for patients who are generally unwell to detect abnormalities of blood biochemistry  Some key problems that can be identified:  Chronic kidney failure – increased creatinine and urea (creatinine increases more than urea) – creatinine used to estimate eGFR as it is a more accurate marker  Dehydration – increased creatinine and urea (urea increases more than creatinine)  Hyperkalaemia – increased potassium  potential cardiac arrest (medical emergency)  Hyponatremia - decreased sodium  confusion/coma  Metabolic acidosis – decreased bicarbonate  increased resp rate (tachypnea)

Estimated Glomerular Filtration Rate  eGFR is measured using serum creatinine levels.  What is the MDRD equation for? (2 marks)  A modification of eGFR that takes into account serum Cr, age, and adjusts if female and/or if African American.  At what eGFR would you consider renal transplant? (1 mark)  <15ml/min/1.73m²

Chronic kidney disease – Management basics StageGFRManagement I90+Observation and control of blood pressure. II60-89 Observation, control of blood pressure and cardiovascular risk factors. IIIa45-59 IIIb30-44 IV15-29Planning for end stage renal failure V<15Dialysis or kidney transplant

Imaging and biopsy  Size of kidneys  Position of kidneys (are they present?)  Condition of parenchyma  Is part of the urinary tract obstructed?  Renal vasculature  Presence/absence of  cysts  tumours

Remember… These investigations are just a guide. Always use in conjunction with a patients clinical presentation.

Some Bonus Questions!

Sara’s blood results show a large increase in urea, whilst creatinine is only slightly increased. What is the most likely cause? (1 mark)  Dehydration Microbiological analysis of the urine shows crystal formation. What is the patient at increased risk of developing? (1 mark)  Renal calculi (kidney stones) Why does urea increase more than creatinine in dehydration/hypovolaemia? (2 marks)  There is decreased renal perfusion and so GFR decreases, therefore both will go up, however some of the urea that is filtered gets reabsorbed, while creatinine is not. Therefore, serum urea increases more than the creatinine.

What is the best overall index of kidney function in health and disease? (1 mark)  Glomerular filtration rate (GFR) What are the units of measurement of eGFR and what is the key biochemical marker used in the equation? (2 marks)  ml/min/1.73m2  Creatinine

Martin, a 22 year old male comes into your clinic with polyuria and weight loss. What simple test would you do and what would you expect to be raised? (2 marks)  Urine dipstick – Glucose would be raised Pedro is suffering from chronic kidney failure and is complaining of feeling increasingly tired. On examination you notice pallor in his conjunctiva. Explain how his kidney failure is related to this finding (2 marks)  Reduced production of erythropoietin by the kidneys  normochromic, normocytic anaemia

What 3 things make creatinine an imperfect measure of GFR (3 marks) (2) creatinine production is not constant among individuals and over time (e.g. higher in people with larger muscle mass) Name a molecule that is freely filtered and not absorbed/secreted? (1 mark)  Inulin (3) measurement of creatinine is not accurate and reproducible across clinical laboratories (1) creatinine clearance approximates GFR: is freely filtered and not reabsorbed but a small amount is secreted

If you are bored this weekend… 

Thank you Any Questions?