Avoiding preanalytical errors in capillary blood gas testing

By Gitte Wennecke and Malene Dal Knudby, Radiometer Medical ApS.

Copyright 2009 Radiometer Medical ApS, Denmark.

Contents may be freely reproduced if the source is acknowledged.

Printed in Denmark by Radiometer Medical ApS, 2700 Brnshj, 2011.

ISBN978-87-91026-06-5

994-414. 201104E.

Data subject to change without notice.

Radiometer, the Radiometer logo, ABL, AQT, TCM, RADIANCE, PICO and

CLINITUBES are trademarks of Radiometer Medical ApS.

In vitro diagnostic medical device.

How to avoid preanalytical errors in blood gas testing

Up to 60 % of all errors in blood gas testing occur in the preanalytical phase [1]. Fortu-nately, many of these can be prevented.

This booklet offers you quick and straightfor-ward information about the most common errors in the preanalytical phase of capillary sampling and, most importantly, how you can prevent them.

The pocket-size format allows you to always have the booklet with you, making it a valu-able tool in your daily work.

For more information on how to avoid pre-analytical errors in blood gas testing, contact your local Radiometer representative.

Caution: In general results obtained from capillary samples, particularly pO2 values, should be interpreted with caution, as pO2 results might be biased due to the aerobic sampling technique. Alternatively, draw an arterial blood sample.

4Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

Patient ID

FIG. 1.

21955485156 Jens Jensen5548515

Jens Jensen

5Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

ConsiderationsIdentification of the patient must be done before collecting the specimen. Use of at least two patient identifiers is recommended [2].

To prevent a mix up of critical patient results, the samples must have a patient ID label attached before you leave the patient. The label can be attached as shown in Fig. 1.

The labels can contain information such as the patients name and date of birth or an accession number. This information is often also contained in a barcode, that ensures that data is entered correctly into the analyzer and that the results are delivered to the correct patient records after analysis.

Recommended procedure Useatleasttwopatientidentifierswhencollect-

ing capillary samples.

EnsurethatthecapillarytubehasanIDlabel attached when you leave the patient.

AlwaysenterpatientIDintotheanalyzer.

Barcodereadersareavailablebothforbedsideidentification and at the analyzer.

6Select the right size capillary tube

ConsiderationsChoosing the right size capillary tube in blood gas testing is important to ensure that the requested parameters are reported with the highest accuracy.

To ensure the highest accuracy, some Radiometer blood gas analyzers* have specific measuring modes prepared for different capillary tube sizes, unless a FLEXMODE is available**.

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

ctHb +

derivatives

pH pCO

2

pO2

cNa

+

cCrea

cK+

cCa

2+cC

l

cGlu

cLacctBil

125L

195L

95L

55L

70L

35L

35LFIG. 2.

Mixing wire

7Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

FLEXMODE has been developed for situations where the user does not know the size of the capillary tube or the exact blood volume.

If a smaller capillary tube than specified is used, the analyzer will abort the sample.

If a larger capillary tube than specified is used, it will render the internal analyzer corrections invalid and the results inaccurate.

Note: All volumes specified by Radiometer are assessed with a mixing wire present in the capillary tube, to ease the mixing of blood with heparin.

Recommended procedure Ensure that the chosen capillary tube size has a

volume that provides the requested parameters (see Fig. 2 and Capillary Tube Card (code no.: 994-415)).

Always match the capillary tube size with the specific measuring mode chosen on the analyzer, unless FLEXMODE** is available.

If varying volumes are available, use FLEXMODE**. FLEXMODE will report the param-eters possible with the specific volume of blood available in the capillary tube with the highest accuracy.

*ABL5/5XX, ABL6XX/ABL7XX/8XX

**ABL8XXFLEX

8Select the right type capillary tube

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

ConsiderationsOnly preheparinized capillary tubes are recommend-ed for blood gas sampling. However, capillary tubes with different heparin types and concentration are available.

Heparin in an appropriate concentration is vital to ensure that the capillary sample does not coagulate. However, using non-balanced heparin can lead to a bias in the electrolyte parameters, as heparin binds all positive ions in the blood, especially calcium ions.

Capillaryvolume Heparin Segment

35L 70 IU electrolyte balanced Adult

55L 70 IU electrolyte balanced Adult

70L 70 IU electrolyte balanced Adult

100L 70 IU electrolyte balanced Adult

125L 70 IU electrolyte balanced Adult

140L 80 IU Na-balanced* Adult

210L 70 IU electrolyte balanced Adult

55L 240 UI Na-balanced* NICU

85L 240 UI Na-balanced* NICU

*No electrolyte reporting

TABLE I.

9Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

To compensate for any bias in the electrolyte parameters, Radiometer offers capillary tubes with electrolyte-balanced heparin (see Tab. I).

Highly-heparinized* capillary tubes are available for samples with a higher risk of in-vitro clotting, e.g. fetal scalp samples or too slow filling of the capillary tube.

* 240 IU/mL Sodium Heparin

Recommended procedure Always use capillary tubes preheparinized with

electrolyte-balanced heparin when electrolyte parameters, such as cNa+, cK+, cCa2+, are to be reported.

Use highly-heparinized capillary tubes for samples with a higher risk of in-vitro clotting. Do not report electrolyte values.

Caution: Do not use non-balanced highly-heparin-ized capillary tubes for reporting electrolytes as they might be biased [3].

10

Arterialization of the puncture site

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

FIG. 3.

11

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

ConsiderationsArterialization of the puncture site is performed by warming the site prior to puncture. The procedure is carried out to increase the arterial blood flow to the site. However, it is debatable as to whether or not this procedure [4] has any effect on blood gas parameters in neonates and children.

Although studies show that prewarming might not be necessary when using a skin incision device, increasing capillary blood flow may be necessary to prevent the need for milking or massaging the punc-ture area and thereby avoiding the risk of hemolysis and/or contamination with tissue fluids.

Recommended procedure Use a warm, moist towel (see Fig. 3) or other

warming device at a temperature no higher than 42 C to cover the site for three to five minutes prior to the puncture [4].

Caution: In general, results obtained from capil-lary samples, particularly pO2 values, should be interpreted with caution, as pO2 results might be biased due to the aerobic sampling technique. Alternatively, draw an arterial blood sample.

12

Skin puncture techniquesNeonates

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

ConsiderationsThe recommended technique for skin puncturing in neonates and children up to 12 months is heel punc-turing. The following procedures should be followed strictly to avoid preanalytical errors or permanent damage to the puncture site.

Preanalytical errors to avoid are: Hemolysis and falsely increased cK+ caused by

squeezing or milking the puncture area during collection or by admixture of tissue fluid.

Dilution from tissue fluid, causing erroneous low pCO2.

Contamination with disinfection agent that may cause hemolysis.

Damage to the puncture site: A puncture that is too deep can cause bone damage. Caution should be taken, especially with very small premature infants where the bone may be less than 2 mm beneath the plantar heel-skin surface [4].

FIG. 4.

13

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

Recommended procedure Skin puncture device [4,5]:

Use a disposable capillary blood sampling device that offers a permanently retractable blade or needle feature that minimizes the possibility of injury and reuse.

Disinfection of puncture site [6]: Clean the puncture area with 70 % isopropanol and allow it to dry completely prior to collection.

Puncture site [4]: The medial or lateral aspect of the plantar surface

of the foot, see Fig. 4.

Puncture depth: Max. 2 mm. Use caution with premature infants.

Blood flow from puncture: Gently press the arterialized area without milking or massaging. Some institutions recommend the use of silicone cream or Vaseline to make the drops form more easily.

Local anesthetics: To relieve pain, drops of glucose solution can be given orally before the puncture. There is no documentary evidence to indicate that local anesthetics, such as EMLA, have any effect on the degree of pain experienced during skin puncture in the heel [7].

Note: Crying during collection of blood may be as-sociated with increased glucose and lactate concen-trations [8].

14

Skin puncture techniquesAdults and children older than three months

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

ConsiderationsThe recommended technique for skin puncturing in adults and children older than 3 months is finger or earlobe puncturing. The procedures described below should be followed very strictly to avoid preanalytical errors and permanent damage to the puncture site.

Preanalytical errors to avoid are: Hemolysis and falsely increased cK+ caused by

squeezing or milking the puncture area during collection or by admixture of tissue fluid.

Dilution from tissue fluid, causing erroneous low pCO2.

Contamination with disinfection agent that may cause hemolysis.

Damage to the puncture site: A puncture that is too deep can cause bone damage.

FIG. 5.

15

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

Recommended procedure Skin puncture device [4,5]:

Use a disposable capillary blood sampling device that offers a permanently retractable blade or needle feature that minimizes the possibility of injury and reuse.

Disinfectionofpuncturesite[6]: Clean the puncture area with 70 % isopropanol and allow it to dry completely prior to collection.

Puncture site: The puncture must be on the palmar surface of

the distal phalanx, see Fig. 5. The puncture should be performed across the

fingerprints, not parallel to them. The middle finger and ring finger are preferred.

Puncture depth: 2 4 mm

Blood flow from puncture: Gently press the arterialized area without milking or massaging. Some institutions recommend the use of silicone cream or vaseline to make the drops form more easily.

16

Fill the capillary tube

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

ConsiderationsBefore filling the capillary tube, it is important to remove the first drop of blood with a gauze pad or similar, as this is most likely to contain tissue fluid.

A mixing wire is placed in the capillary tube prior to collection (except for fetal scalp sampling). If done afterwards, it will supersede blood from the capillary tube. Remember that to obtain the specified volume of blood, a mixing wire should be present in the capillary tube.

During collection, the capillary tube should be held at an angle that allows the capillary forces to fill the capillary tube. However, if the blood does not flow freely from the puncture site, it might be necessary to tip the fill-end of the capillary tube downwards until the next drop forms, so that no air is allowed into the capillary tube.

FIG. 6.

17

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

After filling, seal the capillary tube with the supplied caps. Using cap is vital when mixing the sample to avoid contact with the blood during transportation, etc.

Recommended procedure After the puncture, remove the first drop of blood

with a gauze pad or similar.

Place a mixing wire in the capillary tube and loosely attach a cap at one end.

Allow the capillary forces to fill the capillary tube by holding the tube at a downward angle, while blood flows freely from the puncture site (see Fig. 6).

If blood does not flow freely, consider repunctur-ing, but note that:

The blood flow is enhanced by holding the puncture site downwards and gently applying intermittent pressure to the surrounding tissue (or proximal to the puncture site when blood is obtained from a finger).

A scooping motion to collect blood, milking or massaging the puncture area must be avoided, as these procedures might result in hemolysis or tissue-fluid contamination of the specimen.

Seal the capillary tube with the second cap.

18

Mix the sample

ConsiderationsThe preheparinized capillary tubes should always be mixed immediately after they have been filled and sealed. This will allow the heparin to dissolve and mix with the blood to avoid the formation of clots.

Mixing should always be done carefully due to the risk of hemolysis especially in fragile samples, e.g. neonatal samples. This can result in falsely increased cK+ and decreased cNa+ results [9,10]. Three mixing procedures are described below. A procedure for samples collected from adults, a procedure for samples fragile to hemolysis, e.g. neo-natal samples [9,10], and a procedure for handling capillary tubes for FLEXMODE analysis that are not completely filled.

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

FIG. 7. FIG. 8.

19

Recommended procedureMixing of adult samples: Hold the sealed capillary tube between two fin-

gers.

Gently move a magnet back and forth along the length of the tube 10 times each way. Allow the mixing wire to move all the way from end to end with every single stroke with the magnet as shown in Fig. 7.

Mixing of neonatal samples or samples fragile to hemolysis: Hold the sealed capillary tube between two fin-

gers.

Invert the capillary tube slowly 20 times. Allow the mixing wire to move all the way from end to end with every single inversion as shown in Fig. 8.

Mixing of capillary tubes not completely filled: Hold the sealed capillary tube between two fin-

gers.

Gently move a magnet back and forth along the filled part of the tube 10 times each way. Do not move the mixing wire outside the filled part of the capillary tube, as this will create air bubbles in the sample and affect the pO2.

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

Caution: In general, results obtained from capil-lary samples, particularly pO2 values, should be interpreted with caution.

20

Transport & Storage

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

ConsiderationsStorage time and temperature of capillary tubes de-pend on which type of capillary tube is being used. However, the general recommendation is to analyze the capillary tube immediately.

Plastic capillary tubes: If it is not possible to analyze the sample immediately, analyze the sample within 10 minutes. Keep the sample at room temperature.

Glass capillary tubes: If it is not possible to analyze the sample immediately, analyze the sample within 10 minutes when stored at room temperature. If storage is unavoidable, store the sample horizontally at 0 4 C (32 39 F) for a maximum of 30 minutes.

Prolonged storage affects multiple parameters. Espe-cially cGlu, cLac, pO2 can be significantly biased.

Do not store the capillary tube directly on ice, as this may lead to hemolysis.

Jens Jensen

Jens Jensen

FIG. 9.

21

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

Recommended procedure Analyze the capillary sample immediately after

collection and mixing.

If storage is unavoidable, see the above recom-mendations for plastic and glass capillary tubes [11].

Caution: In general results obtained from capillary samples, particularly pO2 values, should be inter-preted with caution.

22

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

Analysis

ConsiderationsBefore analyzing the sample, ensure that it has been mixed. Follow the mixing procedure described on page 19.

Small volumes of blood in capillary tubes with a relatively large surface settle very fast. Consequently mixing again after storage is very important in order to avoid biases in hemoglobin/hematocrit measure-ments.

Mixing should always be done carefully due to the risk of hemolysis especially in fragile samples, e.g. neonatal samples. This can result in falsely increased cK+ and decreased cNa+ results [9,10].

If clots are suspected, use a clot catcher in order to prevent blockage of the measuring chamber.

If Vaseline or similar is used at the puncture area, introduce the capillary sample into the analyzer from the opposite end (see Fig. 10).

FIG. 10.

Clot catcher

Aspiration end

VaselineMixing wire

23

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

Recommended procedure Mix the sample according to the procedure de-

scribed on page 19.

Enter or scan patient ID.

Place the mixing wire in the end opposite to the aspiration (see Fig. 10)*.

Remove the cap from the end that will be used for aspiration and attach a clot catcher.

Loosen the second cap to allow air to pass when the sample is aspirated.

Place the capillary tube with the clot catcher in the inlet before pressing the aspiration button on the analyzer.

When the sample has been aspirated, remove the capillary tube and clot catcher and close the inlet.

* For analysis on ABL77/80 remove the mixing wire from the safeCLINITUBES

tube prior to analysis.

24

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

Fetal blood sampling

ConsiderationsFetal scalp sampling is a difficult procedure that requires experience and training. The result of the intervention is crucial in the indication of the need for a cesarian section.

The recommended procedure below describes best practice as defined by the Sandbjerg working group in Denmark [12] and described by Dr. J.S. Jrgensen, Odense University Hospital, Denmark [13].

Recommended procedure It is a prerequisite that the fetal membranes have

ruptured and the water has broken. The cervix must be at least 3 4 cm dilated.

FIG. 11.

25

Preparation

Sam

pling and handling

Transport and storage

Analysis

Fetal scalpsam

pling

An amnioscope is led transvaginally towards the babys scalp and the skin is illuminated. If blood, amniotic fluid or fetal fat are present it should be wiped away. The visible piece of the scalp is rubbed to create hyperemia and to make blood collect under the skin.

Smear the puncture site with vaseline or silicone to make the blood form drops.

Use of a scalpel is recommended, reaching a depth of 1.4 mm (max. allowable depth is 2 mm). The blade is oblique; you tip the knife slightly to increase the cut surface and to make the blood run fast enough. If the fetus moves, it might be necessary to make a new cut.

Remove the first drop of blood and collect the sample. The capillary tube is introduced attached to another rod. It is important to keep the tip of the tube inside the blood drop during the whole sampling procedure and hold the rod with the tip of the tube upwards.

Two capillary tubes are filled.

After sampling, insert a mixing wire in the capil-lary tube, seal it and mix the sample according to the procedure on page 19. (Mixing of neonatal samples fragile to hemolysis).

Follow the procedures described on page 20 23 for storage and analysis.

26

References:

1. Carraro P, Plebani M. Errors in Stat Laboratory: Types and Frequencies 10 years after. Clin Chem 2007; 53, 7: 1338-42.

2. Joint Commission on Accreditation of Healthcare Organizations. 2008 National Patient Safety Goals. Available at: http://www.jcipatientsafety.org.

3. Sampling. In: ABL800 FLEX Operators Manual. Ed. 200802I. Brnshj: Radiometer Medical ApS, 2008: Chapter 12, 7-8. Code no. 989-942.

4. Reiner CB, Meites S, Hayes JR. Optimal sites and depths for skin puncture of infants and children as assessed from anatomical measurements. Clin Chem 1990; 36, 3: 547-49.

5. Meites S, Hamlin CR, Hayes JR. A study of ex-perimental lancets for blood collection to avoid bone infection of infants. Clin Chem 1992; 38, 6: 908-10.

6. Ernst DJ, Ballance LO, Becan-McBride KE et al. Procedure and devices for the collection of diag-nostic capillary blood specimens. Fifth edition. CLSI (former NCCLS) publication H4-A5. Wayne, Pennsylvania: CLSI, 2004.

7. AstraZeneca. Quick Guide to EMLA anaesthetic times. http://www.anaesthesia-az.com/sites/156/imagebank/typeArticleparam509796/10180.pdf Assessed February 2008.

8. Young DS. Preanalytical issues in neonatology. Blood Gas News 2002; 11,1: 14-18.

9. Unpublished data from the Biochemistry depart-ment in Aalborg Hospital by Kristensen SR and Pedersen J.

27

10. Meites S. Skin-puncture and blood-collecting technique for infants: Update and problems. Clin Chem 1988; 34,9: 1890-94.

11. Skurup A. Storage recommendations for blood gas samples. Radiometer Publication bulletin no. 31 - 2006. Copenhagen: Radiometer Medical ApS, 2006. Code no. 918-686.

12. Brooks L, Hvidman L, Jrgensen JS et al. Bestem-melse af pH, SBE og laktat i ftale scalp-blod-prver under fdslen. Sandbjerg 2001. http://www.ringamt.dk/SU/Haandboeger/DokHaand-bogGynObs.nsf/Sandbjerg2001.pdf

13. Interview with Jrgensen JS. Get in control of fetal scalp blood sampling. 2001. www.bloodgas.org