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Fingerstick Specimens Can be EquivalentFingerstick Specimens Can be Equivalent
to Venous for Cholesterol Measurement
G. Russell Warnick, Pacific Biometrics, Inc, Seattle, WA
Questions of specimen equivalence with fingerstick blood collection have received more attention with the widespread use of compact analyzers in lipid screening. Compared to conventional venous collection, fingersticking requires less training and adverse consequences are less likely but paradoxically fingersticking is probably more sensitive to technique. At least this is a likely explanation for the contradictory results reported in numerous studies over the last twenty years. An oft-cited early study (Kupke et al, Clin Chim Acta 1979;34:426) suggesting cholesterol averaged lower by 9% in capillary fingerstick sera compared to venous sera has been the basis for the general perception that fingersticks are inherently lower. More recent studies cloud the issue. For example, another study (Miller, et al, Clin Chem 1990;36:963) reported higher cholesterol in capillary whole blood compared to venous, by 3% on the Reflotron and 6% on the VISION.A recent collaboration between our group and ChemTrak of Sunnyvale, CA observed no significant difference (less than 1 mg/dL) in cholesterol between fingerstick capillary and venous sera, either by an accurate enzymic method or by the non- instrumented AccuMeter device. Also the AccuMeter was in excellent agreement with the enzymic values on the specimens from both collection sites. Divergent results with other methods are probably due to technique differences in collecting the specimens and possibly to the interaction of specimen characteristics with a particular method. For example, a hemolyzed specimen, more likely in fingersticking especially with poor technique, may interfere causing bias with one method but not with another. Especially with the newer solid phase devices, whole blood, whether venous or capillary, usually gives different results compared to plasma or serum, which may confound studies comparing different types of specimen. Our study suggests that there is no inherent difference in cholesterol levels between capillary and venous specimens; with good fingerstick technique the specimens are equivalent. Prior to the study we consulted published recommendations for fingersticking and observed and interviewed experienced phlebotomists in order to compile detailed guidelines. Interestingly the NCCLS guidelines, H4-A and H14-A, although lengthy and detailed, provide few specifics about technique. General recommendations for blood collection - reviewed- in the NCEP Laboratory Guidelines-do apply i.e., seated 5 minutes prior, fasting preferred, etc. Some of the specific recommendations we consider important for good fingersticking follow:
- Have patient warm hands by rubbing or shaking. Massaging by the phlebotomist can help to relax the hand, straighten fingers and stimulate blood flow.
- Use the non-dominant hand. The ring finger is usually preferred because of less callus. The middle finger may be better on women or children with small hands.
- Cleanse end of finger with alcohol or antiseptic pad. Wipe away excess alcohol with a sterile gauze pad.
- Pinch end of finger from the side opposite the puncture site to distract patient and keep skin taut at the puncture site. Use a chisel type lances (important) and a spring loaded device with enough force to give a good puncture. (The disposable Becton Dickinson blue lances #366357 or the BMD Autoclix with Sherwood lances #8881-602422 are suitable.
The Autoclix platform as well as lances must be replaced for each patient to avoid disease transmission.) Holding the hand palm up, puncture on the upper side corner of the chosen finger up away from the nail bed. Orient the lances blade perpendicular to the finger. Hold the lancet tightly against the skin while activating and do not release the pressure during the puncture.
Remove lances and wipe away first drop of blood which can be contaminated with tissue fluid or contain alcohol from cleansing. With p alm down allow drops to form and touch into capillary or measurement device. Hold the capillary horizontal or tilted slightly upward to avoid air bubbles. Do not hold pipes directly against the site but rather allow droplets to form before touching the capillary to the droplet. When collection is complete, place a sterile pad over the puncture site and have the subject maintain pressure. When blood flow has stopped, a band-aid, preferably the spot type, can be placed over the site.If blood flow is slow, try lowering the hand. If necessary, express blood down from the hand toward the finger by progressively squeezing and releasing downward across the hand and finger in a "milking" motion. Avoid squeezing the puncture site directly which can cause dilution from tissue fluid.
Avoid leaving blood in a capillary tube for more than 2 or 3 minutes before analysis. Heparin can be washed to the top of the tube allowing clotting to occur at the fill end.With appropriate equipment and technique, capillary cholesterol should not, in our experience differ from the usual venous values. We welcome any additional tips on fingersticking technique.