Q & A

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cap today

May 2003

Richard A. Savage, MD

Q. The pediatricians in our facility want us to perform a sweat chloride test in our laboratory. Our test volume does not warrant including such tests in our menu. What is the best alternative we can offer our concerned pediatricians and obstetricians?

A. Your concern about adequate testing volume is valid. Not all laboratories should perform sweat chloride testing to confirm the diagnosis of cystic fibrosis. Laboratories need to perform a sufficient number of sweat tests on a frequent basis to maintain competency. Points to consider in meeting the needs of your local pediatricians and obstetricians include:

  1. How close is the nearest accredited CF care center? In many situations, patients can be referred to a nearby accredited CF care center. These centers are accredited by the Cystic Fibrosis Foundation as having met criteria for sweat testing volume, methodology, and quality assurance.
  2. Is a screening test appropriate? The CF Foundation has approved the use of sweat conductivity (Macroduct and Sweat-Chek, Wescor Inc., Logan, Utah) as a screening test for community hospitals. A patient with a sweat conductivity of 50 mmol/L or greater should be referred to an accredited CF care center for a quantitative sweat chloride test.1 When evaluating sweat conductivity results, it should be noted that values from sweat conductivity methods are approximately 15 mmol/L higher than sweat chloride concentrations.2 Laboratories performing screening tests need to provide the appropriate reference intervals and must clearly inform physicians on the report that the test is for screening, not diagnosis, and that the test measures conductivity and not chloride.3
  3. Is the laboratory providing in-house DNA testing or referral testing for CF carrier status? The American College of Medical Genetics, American College of Obstetricians and Gynecologists, and National Human Genome Research Institute recommend that Caucasian couples be offered preconception CF carrier screening using a pan-ethnic panel of the 25 most common mutations. They also recommend that non-Caucasian couples be informed of the availability of such screening.4

References

  1. CF Center Directors Update No. 1. Bethesda, Md.: Cystic Fibrosis Foundation; 1990.
  2. Hammond KB, Turcios NL, Gibson LE. Clinical evaluation of the Macroduct sweat collection system and conductivity analyzer in the diagnosis of cystic fibrosis. J Pediatr. 1994;124:255-260.
  3. LeGrys VA. Common errors in sweat testing reporting for cystic fibrosis. Lab Med. 2001;33:55-57.
  4. Grody WW, Cutting GR, Klinger KW, et al. Laboratory standards and guidelines for population-based cystic fibrosis carrier screening. Genet Med. 2001;3:149-154.

Vicky A. LeGrys, DrA
Professor, School of Medicine
Division of Clinical Laboratory Science
University of North Carolina at Chapel Hill
Consultant, CAP Chemistry Resource Committee

 Q. Before initiating thrombolytic therapy or heparin therapy, our protocols require preprocedure coagulation laboratory testing, which includes activated partial thromboplastin time and prothrombin time. However, therapy is initiated before our laboratory issues the results. How reasonable is this practice? If an abnormal result is obtained or if the specimen is unsatisfactory, it is no longer possible to recollect after starting therapy.

A. Hemostasis is a balance between bleeding and thrombosis. Since bleeding is a major side effect of thrombolytic therapy,1 the risk of bleeding should be carefully assessed before starting thrombolytic therapy.

When urokinase was used for thrombolytic therapy in one study, researchers found that 37 out of 82 patients had bleeding, with 22 patients experiencing severe bleeding episodes.2 One contraindication for thrombolytic therapy is known preexisting intracranial disease. A separate study found there was a 1.9 percent risk of bleeding among 312 patients who received thrombolysis for pulmonary embolism.3

Thrombolytic therapy should be approached with caution if a patient has coagulopathy. A patient’s fibrinogen level typically should be at least 100 mg/dL. If it is below this level, cryoprecipitate is transfused to increase the fibrinogen to more than 100 mg/dL. This is a reasonable approach since the fibrinogen level may decrease after thrombolytic therapy due to fibrinogenolysis and may cause bleeding. There are no guidelines or recommendations for acceptable PT or partial thromboplastin time for thrombolytic therapy. A prolonged PTT obviously does not mean the patient has a risk for bleeding when it is associated with lupus anticoagulant or factor XII deficiency. The presence of lupus anticoagulant is not uncommon for patients who need thrombolytic therapy for pulmonary embolism since lupus anticoagulant is one of the risk factors for thrombosis.

In conclusion, there are no standard recommendations regarding whether PT and PTT should be checked before thrombolytic therapy. Though definitive evidence is lacking regarding the utility of assessing coagulopathy beforehand, I would recommend that PT, PTT, fibrinogen, and complete blood count be evaluated before initiating thrombolytic therapy.

References

  1. Thabut G, Thabut D, Myers RP, et al. Thrombolytic therapy of pulmonary embolism: a meta-analysis. J Am Coll Cardiol. 2002;40:1660-1667.
  2. Goldhaber SZ. Thrombolysis in pulmonary embolism: a debatable indication. Thromb Haemost. 2001;86: 444-451.
  3. Kanter DS, Mikkola KM, Patel SR, et al. Thrombolytic therapy for pulmonary embolism. Frequency of intracranial hemorrhage and associated risk factors. Chest. 1997;111:1241-1245.

Jun Teruya, MD, DSc
Director of Blood Bank and Coagulation
Texas Children’s Hospital
Baylor College of Medicine, Houston

Q. I need information regarding CAP or CLIA requirements for precalibrated assays on automated instrumentation. The requirement for a calibrated assay is usually to recalibrate at least every six months. Are precalibrated assays from the manufacturer exempt from this requirement?

A. I assume by “precalibrated” that you mean an FDA-cleared measurement system that does not use calibration materials that simulate a patient specimen but instead has a process to identify the reagent lot for a disposable unit use test module, which has encoded the calibration settings. In this situation, the requirement is to follow the manufacturer’s instructions. The manufacturer usually provides some type of control material or process intended to verify the correct operation, including calibration, of the measurement system. Additional guidelines can be found in NCCLS document EP18-A, Quality Management for Unit-Use Testing; Approved Guideline (2002).

Greg Miller, PhD
Professor, Department of Pathology
Virginia Commonwealth University, Richmond
Consultant, CAP Chemistry Resource Committee