- Last Update On : 2013-01-13
Many inpatients have intravenous (IV) catheters. While IV lines provide a means of direct vascular access for infusing fluids, collection of specimens through these lines can result in contamination of the specimen with the contents of the line.
Whenever possible, specimens should be collected from the arm opposite the line to avoid contamination. Specimens should not be collected distal to a catheter because fluids tend to pool in the periphery of the limb. Collection of samples proximal to a catheter will be diluted by the infusion fluid.
When vascular access is limited, a specimen may need to be collected from a line. This decision should only be made after weighing the risk of specimen contamination versus the risk of phlebotomy from another site. Before drawing a specimen from a line, the infusion fluid should be completely stopped for several minutes and an amount of blood equal to three or more times the deadspace of the catheter should be discarded. Ideally, a specimen should never be drawn from a line that is being used to infuse the same analyte that the laboratory will be measuring because even three volumes of discarded blood may not be an adequate amount to ensure complete flushing of the line. A perfect example is the collection of a specimen for plasma glucose measurement from a line being used to infuse D5W. Although 5% dextrose does not sound like a lot, it denotes a glucose concentration of 5000 mg/dL. Contamination of the blood specimen with just one part in twenty of this highly concentrated solution can falsely elevate the glucose concentration by as much as 100 mg/dL. Additionally, electrolytes measured on this contaminated specimen will be falsely decreased due to dilution.
The laboratory may not catch every IV contaminated specimen. Below are some parameters that may be helpful in determining if a blood sample has been contaminated with fluid from an IV catheter:
- Na <130 mEq/L and Cl <100 mEq/L and K >5.5 mEq/L
- Glucose >800 mg/dL and creatinine <0.6 mg/dL
- Na >180 mEq/L and K <2.5 mEq/L
(Hernandez, James, Mayo Medical Laboratories, Clinical Laboratory News, April 2011)
These findings may be particularly helpful in interpreting a sudden shift in laboratory results that cannot be readily explained by a change in clinical condition. If a CBC specimen was submitted at the same time as a chemistry panel, its results should also be reviewed to determine if the hemoglobin or hematocrit is consistent with previous. An unexplained decrease may be due to IV contamination. Another parameter worth examining is MCV which should not fluctuate more than 1-2 fL within an individual. A sudden shift of 4-5 fL, in the absence of a recent transfusion, is another reliable indicator of IV fluid contamination.
Drug levels drawn from IV lines should also be interpreted cautiously. Some drugs, such as cyclosporine, tacrolimus and vancomycin, are very hydrophobic and adhere to tubing. Falsely elevated drug levels can be seen even after discarding 10 or more volumes of blood. Physicians need to carefully review unexpected drug levels collected from patients with IVs.