Indwelling catheters such as central venous lines provide

ready access to the patient circulation, eliminating multiple phlebotomies, and are especially useful in critical

care and surgical situations. Arterial catheters most often are placed in the radial artery. Indwelling catheters

are surgically inserted in the cephalic vein or internal jugular, subclavian, or femoral veins and positioned. They

are especially useful in selected patients for drawing venous blood, administering drugs or blood products, and

providing total parenteral nutrition. Continuous, real-time intra-arterial monitoring of blood gases and acid-base

status using fiberoptic channels containing fluorescent and absorbent chemical analyses has been used

successfully.

Placement of indwelling catheters, although not a routine laboratory function, is

of primary concern to the laboratorian blood specimens drawn from catheters may be contaminated with whatever was

administered or infused via the catheter. The solution, usually heparin used to maintain patency of the vein must

also be cleared. Sufficient blood minimum of 2 to 3 mL must be withdrawn to clear the line so laboratory data are

reliable. To obtain a blood specimen from the indwelling catheter, first draw 6 mL of intravenous fluid from the

line and discard. In a separate syringe, withdraw the amount of blood required for requested laboratory procedures.

Follow strict aseptic technique to avoid site or catheter contamination or both. Coagulation measurements

prothrombin time, activated partial thromboplastin time, and thrombin time are extremely sensitive to heparin

interference, so that even larger volumes of presample blood must be withdrawn before the laboratory results are

acceptable. The appropriate volume to be discarded should be established by each laboratory when performing

prothrombin times PT and activated partial thromboplastin times APTT, a minimum of 5.3 mL discard volume should be

used. The laboratory is sometimes asked to perform blood culture studies, using medical microscopes, on blood drawn

from indwelling catheters. This procedure is not recommended since, when viewed under a medical microscope, the

organisms that grow on the walls of the catheter can contaminate the blood specimen.

SPECIMEN

INTERFERENCES

The collection of specimens depends on proper identification of the patient, the appropriate

collecting method to procure the specimen, and the correct collection tubes. Timed collections must be verified to

ensure accuracy in generating laboratory data that will be used in the diagnosis or management of a patient. The

site of collection is generally not critical except for glucose tolerance test, in which it has been reported that

capillary glucose is 10 to 30 percent higher than venous blood glucose. Additionally, blood specimens collected

from an extremity with any type of catheter delivering parenteral solutions can generate artefactual results. If

this type of collection cannot be avoided, the venipuncture must be performed distal to the intravenous needle

site, with the tourniquet between the two. Blood-drawing equipment must be void of any residual detergents,

plasticizers, or other material that may interfere with laboratory determinations. Examples include specimens for

lead analysis, which must be collected in acid-washed, lead-free containers, and contamination from tissue

thromboplastin that may interfere with specific coagulation assays if a double-syringe technique first 5 mL of

blood is discarded is not used.

Lysis of red blood cells during the collection process or after phlebotomy,

before analysis is performed using medical microscopes, can contaminate the serum or plasma and alter results in

vitro hemolysis. Overzealous mixing of blood in collection tubes, residual alcohol left when cleansing skin,

prolonged exposure of tubes to heat or extreme cold freezing, and inadequate removal of red cells during

centrifugation may cause hemolysis. Even small amounts of lysed erythrocytes may have a significant impact on blood

plasma and serum analyte concentrations. In vitro hemolysis results may show increased levels of serum acid

phosphatase, zinc, magnesium, albumin, potassium, bilirubin spectrophometrically determined, and CK. Thrombolysis

can result in elevated serum potassium, magnesium, acid phosphatase, and aldolase. Granulocytosis releases

muramidase lysozyme, phosphohexose isomerase, arginase, glucose-6-phosphatase G6PD, and glutamate dehydrogenase.

Hemolysis may alter spectrophometric readings, such as those used in coagulation studies or hemoglobin

evaluations.