Introduction to kidney tests - acid-base metabolism

Question 1

Significance of K and PO4 in kidney function tests

Answer 1

K and PO4 retained with decreased GFR

Question 2

Urea and GFR

Answer 2

• Urea is the end product of protein metabolism and is retained with decreased GFR

Question 3

Creatinine and GFR

Answer 3

• Creatinine is the end product of creatine breakdown and is retained with decreased GFR

Question 4

Kidney blood flow

Answer 4

• 1500mL/min

Question 5

GFR

Answer 5

• Most frequent test of kidney function

Based on serum creatinine:

• Starts to rise - 50% glomeruli lost
• Marker of progressive kidney damage
• Poor sensitivity - minor kidney damage

Question 6

Hierarchy of kidney function tests - inaccurate to accurate

Answer 6

• s-Urea
• 24h creatinine clearance
• s-Creatinine
• eGFR is best index of glomerular function, estimated from s-Creatinine(EPI, MDRD, cockcroft-gault)
• Direct GFR measurement

Question 7

What is creatinine dependent on

Answer 7

• Muscle mass

Question 8

Changes in creatinine levels with age

Answer 8

With age muscle mass decreases
- Elderly lower muscle mass than younger

With age, kidney function decreases
With age, glomeruli decrease
- Elderly less glomeruli than younger

Blacks from Africa, USA, West Indies have greater muscle mass - than other ethnic groups

Question 9

Formula for estimating GFR

Answer 9

• CKD-EPI (based on the largest population of any calculator, best for ‘normal sized’ people)
• Account for outliers in muscle mass
• Creatinine clearance = ( Urine Cr * urine volume)/ (plasma Cr* time)

Question 10

Creatinine origin

Answer 10

Muscle mass proportional to creatine proportional to creatinine

• Creatine + ATP - creatine-P + ADP
• There is a constant decay of creatine to creatinine
• CK - creatine kinase is the enzyme that carries out the above reaction
• Creatine-p - high energy phosphate

Question 11

Urea origin

Answer 11

• Protein proportional to amino acids proportional to urea
• Alanine –ALT–> urea + pyruvate

Aspartate –AST–> Urea + oxalo-acetate –> glucose

• Enhanced by stress: cortisol (glucocorticoid)

Question 12

Ureamia

Answer 12

• Condition having ‘urea in the blood’
• Denotes a very high serum urea that is the result of kidney failure
• Describes the pathological and symptomatic manifestations of severe loss of GFR

Question 13

AKI

Answer 13

• GFR compromised - low BP, shock

- Decreased blood flow to kidneys

Question 14

Which waste products accumulate in AKI

Answer 14

• Urea, creatinine, acid, PO4

Question 15

What should be considered in AKI

Answer 15

Review urgently
Consider drugs: ACEI/ARB, NSAID, diuretics (most important drugs of relevance)
- Also obstruction, hydration, infection

Question 16

Complications of AKI

Answer 16

• Volume overload, raised K+, H+, PO4
• Initial assessment - volume status(possible dehydration, CHF)
• s-K, HCO3, PO4, calcium, albumin
• s-uric acid, magnesium
• FBC

Question 17

ACEI pathophysiology in AKI

Answer 17

• Reduce systemic vascular resistance
• Renoprotective - diabetic and renal disease
• Kidney adverse effects (intrarenal efferent vasodilation, fall in filtration pressure and GFR)
• Fall in filtration pressure (antiproteinuric and long term renoprotection)
• GFR dependency on angiotensin II- mediated efferent vascular tone is lost

Question 18

Most common cause of lactic acidosis in hospitalised patients

Answer 18

• Sepsis
• Usual cause is tissue hypoperfusion
• Impaired tissue oxygenation, leading to increased anaerobic metabolism
• Hypovolemia
• Cardiac failure
• Sepsis
• Cardiopulmonary arrest

Question 19

Sepsis 6

Answer 19

1. Deliver O2 to target 94-98% or 88-92% in COPD
2. Take blood culture
3. Give IV (intravenous) antibiotics
4. Start IV resuscitation
5. Measure serum lactate
6. Monitor urine output

Question 20

Maintenance of acid base balance

Answer 20

• Lungs remove CO2 (fast)
• Kidneys reabsorb HCO3- and secrete H+
  H+ + HCO3- –> H2CO3 –> H2O + CO2
• Lungs - CO2 removal shifts equation to right with loss of H+ and HCO3=
• Kidneys - PCT - HCO3- reabsorption
• DCT - H+ secretion, urine acidified

Question 21

What happens in a patient with metabolic acidosis

Answer 21

• The fall in pCO2 will mitigate the fall in pH caused by reduced HCO3
• In all simple acid-base disorders, the primary abnormality will cause a compensatory response such that the HCO3 concentration and pCO2 will move in the same direction
• Both will decrease or both will increase
• These directional changes will act to return the pH toward normal
• The compensatory responses are mediated by alterations in pH within the respiratory center
• Respiratory compensation for metabolic acidosis generates a linear relationship between arterial pCO2 and bicarbonate

Question 22

Main cation in extracellular fluid

Answer 22

Sodium

Question 23

Main cation in intracellular fluid

Answer 23

Potassium

Question 24

How are electrolytes usually measured

Answer 24

• Usually measured with creatinine, urea

- Also chloride, bicarbonate required with acid/base disturbance to determine anion gap

Question 25

What is anion gap due to

Answer 25

• Albumin- phosphate- sulphate-

Question 26

Formula for anion gap

Answer 26

Measured cations - measured anions

Question 27

What is the increased AG in acidosis due to

Answer 27

• Ketone- , lactate- ,salicylate- ,ethylene, glycol-

Question 28

DKA - acid added

Answer 28

• Na+ + HCO3- exist in serum
• Addition of H+
• Combines with HCO3-
• Leaving Na+ salt of the acid
• Na+ ketone-

To maintain pH or H+

• H+ combines with HCO3- forming H2CO3
• CO2 blown off
• Resulting loss HCO3- and H+
• In serum ketone- exists instead of HCO3-