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Published on: 6/27/2003
Last Visited: 7/9/2006
But Dr. Barth, a consultant in chemical pathology and metabolic medicine at Leeds General Infirmary in Great Britain, reminded an audience at the July 2003 meeting of the American Association for Clinical Chemistry that the price of interference can be steep.
"Seven or eight years ago, a company said they didn't have a problem with interference with their hCGs," he said in a presentation on erroneous results.
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Interference has been well recognized for many years with competitive immunoassays, Dr. Barth said."The way it was identified in the early days was by serial dilution."With the development of immunometric (noncompetitive) assays a decade ago, case reports of interference have appeared sporadically, he noted.
The difference between the immunometric assays, particularly the competitive, automated ones, and the noncompetitive assays is that the latter are not done at equilibrium.He pointed out that two-site assays are believed to be more prone to interference than single-site ones.
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"I'm not talking about samples elevated by twofold, but those that remain within the reference range," Dr. Barth noted.The question is whether a diagnosis will be made or not made on the basis of an abnormal result.
Among the causes of interference are venous stasis, the presence of hormones and drugs such as anticoagulants, and to an unknown extent, other preanalytical factors, he said.
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"On the basis of this grossly elevated estradiol, this lady had a hysterectomy and a bilateral oophorectomy, and after the operation her gonadotropins had elevated, and her estradiol remained elevated," Dr. Barth said.
"After the laboratory measured her estradiol by organic extraction, it realized that this lady had her complete pelvis removed on the basis of an assay interferent or an estradiol nonspecificity problem."
Under a national external quality assurance scheme, he recounted, a second trimester pregnancy sample was distributed around the British countryside.
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Digoxin is one of the most important drugs laboratories measure therapeutically, Dr. Barth continued, because the side effects of overtreatment are similar to the effects of undertreatment.
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Insulin antibodies were the culprit in a case of interference that came through Dr. Barth's laboratory recently.
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"She had the lowest plasma glucose I've ever seen in my life," Dr. Barth recalled.
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"Rheumatoid factor is something that we think of almost in the same breath as interference antibodies in immunoassay," Dr. Barth continued.
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Dr. Barth emphasized that interfering antibodies may be human anti-animal antibodies, but he said, "I don't think necessarily that anti-mouse or anti-animal antibodies are the key to interference."
He described a broader interfering phenomenon called the idiotype cascade-a series of checks and balances within the immune system to make sure nothing gets out of control.
"I think what's happening is the immune system has got a whole system of protections to prevent certain reactions, in the same way if you think of moving your arm up and down, you have a coordinated action by opposing muscles to provide a smooth movement.So the immune system has a series of checks and balances to maintain a smooth control of antibody action."
"My feeling is that it's problems with this whole system that interfere with our immunoassays rather than anti-mouse antibodies," Dr. Barth contended.
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"Most of you would send this patient into a coronary care unit, and that's exactly what happened to him," Dr. Barth reported.
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Dr. Barth cited a recent study that found just under half of patients, once they had antibodies, had them for a long time, while other studies have reported three weeks' or eight weeks' duration.
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Dr. Barth outlined another case that illustrated the difficulties of managing interference.
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The consequences can be serious, Dr. Barth added."There was a huge cost to the patient both in terms of the number of consultations and in the number of investigations she had."
So while identifying abnormal results is important, results are not always abnormal, he stressed.
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Dr. Barth and a colleague went through 5,000 patient samples at his institution and pulled out 59 as being abnormal on the basis of clinical details and previous results (if they were available).
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All of these factors underscore the need for caution in interpreting laboratory results, Dr. Barth warned.The case notes in his hospital have a huge "D" written on them if the patient is a diabetic, and signs are posted on the outside of the case notes if the person is allergic to drugs.
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For more on immunoassay interference, you can read the comments of Dr. Barth's co-presenter, Larry Kricka, PhD, in the October 2003 issue of CAPTODAY, page 76.
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