Intraoperative Protocol Blood Sample Collection Time and Processing

In the operating room, a 14- or 16-gauge cannula is placed into an antecubital or other available peripheral vein. If not available, an arterial line can be used. This vascular access is maintained without heparin using a saline drip with extension tubing and a three-way stopcock at the head of the table. This allows blood sampling at the required intervals. Such access can also be used by the anesthesiologist as long as administered drugs are not collected with the blood samples used for PTH measurement. The blood volume required for the measurement depends on the surgeon's protocol and assay methodology. Usually, no more than 2 mL of whole blood is necessary, but it is wise to save a few milliliters of whole blood for control and future correlation if unexpected results are found postoperatively. For instance, if a patient has a sufficient PTH drop intraoperatively but has persistent hypercalcemia and elevated PTH (false positive), the saved intraoperative frozen plasma samples should be measured again and the intraoperative results reviewed for technical errors. Blood sample dilution, an incorrect standard assay curve, or high coefficient variation may lead to incorrect interpretation of hormone levels. In an attempt to avoid this problem, 4 to 5 mL of whole blood is drawn into a syringe after the intravenous tubing has been cleared of residual saline solution by a previous extraction with a separate syringe until undiluted blood is available. Once the syringe with whole blood is handed off the table, it is transferred into an ethylenediaminetetraacetic acid (EDTA) glass tube. The tube is inverted several times to ensure proper anticoagulation. Sampling can be done by anesthesia personnel at any time requested by the surgeon. The part of these blood samples that is not used for intraoperative measurement should be later separated by centrifugation and the plasma frozen for future control use.

Routinely, there are four samples drawn at specific intervals: before the skin incision (1); just before clamping the blood supply of a completely dissected suspicious parathyroid gland (2); and at the 5-minute (3) and 10-minute (4) intervals after the interruption of the blood supply and excision of this gland. The first sample is taken before the incision is made and serves as a baseline, or preincision, PTH level. A PTH level measured for diagnosis before the operation cannot be used as the preincision value or for hormone dynamic calculation. All samples must be measured with the same conditions and by the same assay standard curve at the time of the operation. The second sample, called pre-excision, is taken after complete dissection of the suspected tissue and just before clamping its blood supply. This sample is important since some patients have a substantial increase in the circulating hormone during manipulation, making this pre-excision sample necessary to meet the criterion of a 50% drop in 10 minutes, as shown in Figure 52-3. In some patients, the pre-excision level will be decreased if the blood supply to the hypersecreting gland is interrupted early during dissection. When this occurs, the higher preincision sample should be used to calculate the drop in PTH level at 10 minutes. If the pre-excision sample is taken too early (e.g., before or during gland manipulation), the peak of the hormone level might be missed, leading to a false delay in the hormone decay at the 10-minute interval. Such a false delay is probably due to an unmeasured, very high, pre-excision PTH level. This phenomenon has been observed by others as well.14 A 5-minute sample taken after gland resection is

Pre-incision Pre-excision 5 min 10 min Timed operative blood samples FIGURE 52-3. Representation of intraoperative hormone monitoring in a patient with a single gland involved. During resection of the parathyroid gland, the intact parathyroid hormone (PTH) level increased significantly, leading to a 76% drop in 10 minutes after gland resection and predicting operative success. This graph shows the importance of the pre-excision sample sample in some patients. If only the preincision sample had been measured, the criterion would not have been met in 10 minutes with a drop of only 40%. This false-negative result can and was prevented by a correct registration of the peak of the hormone caused by gland manipulation.

surgery, but absolute PTH values, especially at the low concentrations, are not always accurate. In our experience, the normal range can differ from the published directional insert, especially when different batches of antibodies are used. If the normal range is used as a part of the criterion, the laboratories should evaluate their own normal range for its population, which should be rechecked over a period of time and with each new batch of antibodies. A change of normal range does not affect intraoperative use as long as the standard assay curve is acceptable before the procedure and all samples are measured against the same curve.

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