Technique and radiation

No specific patient preparation is required, but ensure that the patient is well hydrated and empties the bladder prior to injection. For GFR studies:

• Doses used are 10 MBq for 99mTc DTPA and 3 MBq for 51Cr EDTA

• A pre-tracer injection venous blood sample is taken for background activity

• The tracer is injected and the exact time noted

• A heparinized blood sample is taken at 90, 150, 210, 270 minutes following injection and corrected (minus background activity) plasma tracer concentration can be plotted against time. Studies have shown that a minimum of four blood samples are required for accurate results

• Plotting the log of tracer concentration will result in a linear curve. Extrapolation of this line back to time zero will indicate the effective volume of distribution. The GFR is then calculated as the product of the distribution volume and the slope of the linear log curve, using the formula

where X is the slope and V is injected tracer dose/distribution dose (see fig 4.3).

The Gates technique for GFR estimation, though not in common use, involves analysis of tracer activity in the kidneys between the 2- and 3-minute intervals following tracer injection. While the obvious advantage of this technique is the speed of the test and the absence of blood tests, its accuracy has been doubted. This technique has therefore fallen out of favor and most centers use a serial venous sampling method.

J Time after injection

In|ecte0 dose

FIGURE 4.3. Logarithmic plot of plasma decay curve in single injection clearance techniques (with permission from Comprehensive Urology, Weiss, Mosby, 2001, 137)

J Time after injection

In|ecte0 dose

FIGURE 4.3. Logarithmic plot of plasma decay curve in single injection clearance techniques (with permission from Comprehensive Urology, Weiss, Mosby, 2001, 137)

An alternative to blood sampling is using three urine samples over 3 hours to measure urinary tracer concentration, but difficulties and inaccuracies in specimen collection make this method unattractive.

A typical dose of 10 MBq for 99mTc DTPA and 3 MBq for 51Cr for EDTA for GFR studies corresponds to reasonably small effective radiation doses of 0.1 mSv and 0.007 mSv, respectively. It is therefore feasible to perform serial studies safely if clinically indicated.

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