The blood samples (3-4 ml in purple top tubes) are counted using scintillation well-detector system. Since we need to determine the activity in these samples, it is necessary to make up a calibration standard, which can be counted at the same time. This involves the addition of a carefully assayed quantity of I-131 (approximately 3.7 MBq - 7.4 MBq {100-200 uCi}) to a total volume of 1000 ml. Such a small concentration is necessary in order to avoid saturating the detector. An alternative that we have implemented in the dosimetry program here at the Washington Hospital Center uses a Ba-133 rod source instead that has been cross-calibrated against the I-131 standard. With its relatively long half-life (10.3 years) and similar gamma emissions, Ba-133 is a suitable replacement for the prepared I-131 standard. At the conclusion of the data acquisition, two 1-ml aliquots of whole blood, and the "standard" are counted. Using this information, it is possible to calculate the % of administered dosage/liter of whole blood at each of the timed samples. A zero time point is calculated by dividing the total dosage by the patient's total blood volume. However, a patient-specific blood volume is not determined, but is assumed to equal 20% of the body weight.