Table 3.

Common methods for measuring renal blood flow and glomerular filtration rate in humans

Measure	Method	Advantages	Disadvantages
Renal blood flow	Para-aminohippurate clearance	Gold standard measure of renal plasma flow	Not a direct assessment of arterial blood flow, expensive, invasive, unable to measure dynamic changes, may require timed urine samples.
	Doppler ultrasound	Reliable, affordable, non-invasive, index of arterial blood flow, can measure dynamic changes	Can only measure blood velocity (not blood flow*) in renal, segmental, and/or interlobular arteries, has not been validated against other measures
	Magnetic resonance imaging	Direct assessment of renal blood flow and/or oxygenation, non-invasive	Not feasible in some experimental settings, expensive
Glomerular filtration rate	Inulin clearance**	Gold standard	Expensive, requires timed urine samples
	Creatinine clearance	Uses an endogenous substance, affordable	Requires timed urine samples, may be inaccurate during intense exercise and/or dehydration
	Estimated glomerular filtration rate	Simple, affordable, spot check of an endogenous substance without need to collect urine	May be inaccurate when glomerular filtration rate is rapidly changing

* Two studies have reported Doppler ultrasound derived measures of renal blood flow [171,172], but this has not been replicated by other laboratories nor has it been used during heat stress.

** Iohexol and mannitol clearance are also common and are based on the same principles as inulin clearance [189-192], but these have rarely been used during heat stress [264,290].