Analytical errors for Troponin -false positive fliers, and urine Albumin- false negatives due to antigen excess (prozone), have been discussed here recently.
Many of the methods in use will give these errors, infrequently, unless we have a strategy to detect or prevent them.
Troponin:
As Graham Jones points out, several studies have shown "random" poor reproducibility of Troponin results.
We presented a poster at the Australasian Association of Clinical Biochemists in 2009, comparing flier rates for TnI on Beckman Access and Abbott Architect. Beckman's rate was higher.
For 10 years, our procedure is, for the first positive Troponin of an admission, re-centrifuge @ 10,000 rpm for ten minutes, and re-assay in duplicate; report the mean of the duplicates.
We are using a conservative cutoff of 0.08 ug/L for the Abbott Architect TnI (our Emergency Department and cardilogists are thinking about the implications of lowering that to the recommended 0.03 ug/L).
In the past year, 0.7% of our Troponin results changed from positive (>0.08) to negative (<= 0.08) on repeat.
0.2% started at >0.1 ug/L and became reproducibly <= 0.08 on repeat.
Some examples:
0.102> 0.005,0.018 0.239> 0.030,0.047 0.574> 0.045,0.040,0.043 0.471> 0.016,0.002
At the borderline, some of the reclassifications are "statistical", due to analytical imprecision, but there is clearly a "clean up" effect for about 0.5 % of our specimens. This amounts to about 3% of initially positive Troponins here.
We use only BD PST II heparin gel vacuum tubes for Troponin. We have seen these clot after centrifugation. We have observed new ED staff take blood with a syringe, later transferring it to Vacutainers. Eventually they learn.
I think delayed anticoagulation is likely to be the major cause of our fliers. If so, this will vary with the collectors, and maybe the make of heparin tube.
There will be instrument differences. Non-microparticle methods may be less susceptible to fibrin entanglement preventing efficient washing. The Brisbane group reported a flier rate of 0.3 % for the Beckman Access, using stored serum; we found a similar incidence with distilled water as a sample.
The Abbott Architect had problems with fliers attributed to static electricity, and recently upgraded the i2000 photodetector electronics to minimise this.
Urine Albumin:
The necessity for dipstick testing of urine Albumin specimens for very high Albumin was queried.
Roche, and some other automated methods include an antigen addition step. This is a reliable prozone check, and is needed because the Roche prozone limit is only 2500 mg/L. Most methods do not claim a prozone limit greater than 10,000 mg/L, and this may vary with antibody lot.
If there is a prozone check, it needs to be tested with serum undiluted, and diluted to just above the prozone claim.
Simple end-point assays cannot check for prozone. If these are used, an extra test for high albumin needs to be one.
We run an endpoint assay on the Vitros Fusion, using Roche reagents, and run Vitros urine Protein on every sample. Initial Protein/Albumin ratios of >6 cannot be released, and are flagged to dilute the urine Albumin.
In our population, we get 5% of specimens > 2500 mg/L, and 0.3% >10,000 mg/L. Our record urine Albumin is 42,000 mg/L, Protein 90 g/L . The patient had newly diagnosed end-stage renal failure. The initial Albumin result was <3 mg/L.
Robert McFarlane
Network Supervising Scientist, Chemical Pathology, Royal Darwin Hospital
Northern Territory, Australia
08 89228079 0458 110196
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