GFR and Renal Clearance Flashcards
What is meant when it is said a test is specific, or accurate, or sensitive?
What are the ratios / fractions used to calculate the specificity, accuracy and sensitivity of a test?
Specific = when a disease is not present, is the test able to not detect it? Can be calculated using the ratio: [false negative] / [true negative + false positive]
Accurate = is the test correct? Can be calculated using the ratio: [true negative + true positive] / [true negative + true positive + false negative + false positive]
Sensitive = when disease is present, is the test able to detect it? Can be calculated using the ratio: [true positive] / [true positive + false negative]
How is it decided if a test is useful to screen or confirm, and why?
Useful to:
Screen = highly sensitive test with a low number of false negatives; do not want to miss if the patient needs that treatment
Confirm = highly specific test with a low number of false positives; do not want to subject the patient to unneccessary / ineffective treatment
What is the difference between a screening test and a confirming test?
Screening test = it can be useful to detect or rule out some diseases or infections, often followed by a confirmatory test (generally confirms the suspected diagnosis)
What are the false results for a highly sensitive test?
What are the false results for a highly specific test?
if highly sensitive = low false negatives but higher false positives
if highly specific = low false positives but higher false negatives
What is the difference between a Type I and Type II error?
Type I error = false positive; data collected leads to rejection of the null and acceptance the alternative hypothesis, when infact the null hypothesis is correct
Type II error = false negative; data collected leads to the rejection of the alternative and acceptance of the null hypothesis, when infact the alternative hypothesis is correct
What is the difference between precision and accuracy?
Precision = how close 2 values are to each other
Accuracy = how close the value is to the true value
What is GFR?
What is meant by the term ‘freely filtered’?
Glomerulus filtration rate (GFR) is the sum of all the filtration rates (volume of fluid filtered from the glomerulus to the bowman’s capsule) in all the functioning nephrons per unit time - measured in mL/min usually
Freely filtered = solute can be found in equal concentrations in the plasma and ultrafiltrate as the solute is neither reabsorbed nor secreted in the nephron
Why measure GFR?
Important to measure renal function, most renal diseases
What is the normal GFR value in mL/min per 1.73m2 for a young adult?
What is the normal range of GFR values in mL/min per 1.73m2 for young adults?
After the age of 40, GFR falls by how much mL/min per decade?
copy off
How can renal clearance rate be calculated? Assuming the substance is labelled ‘z’.
Measure the concentration of z in the arterial plasma
Find the amount (no. of moles) of z in the urine by measuring the concentration of z and multiplying it with the volume of urine
Renal clearance = amount of z in the urine / concentration of z in the arterial plasma
How can GFR be measured?
It is impossible to measure directly as nephrons are tiny and there are millions of htem in the kidney - instead GFR can be measured indirectly
It can be measured by using the renal clearance of a freely filtered substance, that is not reabsorbed or excreted along the nephron, so the amount filtered = amount excreted (i.e. the no. of moles of the substance excreted per unit time is the same as the no. of moles of the substance filtered per unit time)
What are the equations used to calculate GFR?
Assuming the substance that is freely filtered is given the name ‘Y’.
Calculate the GFR using the values below:
Why is inulin the ideal molecule to use to measure renal clearance and so GFR?
What are its drawbacks?
Freely filtered, neither reabsorbed nor secreted, non toxic, measurable int he urine and blood
A plant polysaccharide - therefore not naturally found in mammals so must be infused
If a substance is freely filtered, yet its clearance is less than that of inulin, what has occurred?
If a substance is freely filtered, yet its clearance is greater than that of inulin, what has occurred?
Reabsorption of the substance along the nephron
Secretion of the substance along the nephron
