A Pre-op Troponin That Will Not Dilute
A 54-year-old man is scheduled for elective inguinal hernia repair and is sent for routine pre-operative blood work at an outpatient draw site. He has no chest pain, no dyspnea, and no cardiac history. High-sensitivity troponin I on the sample is 85 ng/L; the male 99th-percentile upper reference limit for this assay is 20 ng/L. The specimen is repeated on the same analyzer and then on a second analyzer in the main laboratory: 88, 86, and 84 ng/L. The 12-lead ECG shows normal sinus rhythm without ischemic change, and a resting echocardiogram is normal. eGFR is 90 mL/min/1.73 m2 and rheumatoid factor is 15 IU/mL (reference below 20). The laboratory performs a dilution study in the manufacturer's zero calibrator: neat 86 ng/L, 1:2 61, 1:4 41, 1:8 32. An aliquot re-assayed after treatment with a heterophile-blocking reagent gives 12 ng/L. At 17:00 the surgeon's office calls asking whether surgery can proceed tomorrow morning.
1. Analyze the dilution study. What does the pattern tell you, and what does the blocking-reagent result add?
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Expected versus observed. If the neat value of 86 ng/L were genuine analyte, a dilution in zero calibrator should fall proportionally: 1:2 expected 43, 1:4 expected 21.5, 1:8 expected 10.8.
Observed recoveries:
- 1:2: 61 / 43 = 142%
- 1:4: 41 / 21.5 = 191%
- 1:8: 32 / 10.8 = 296%
Interpretation. The sample is grossly non-linear on dilution, and the measured value rises relative to expectation as the sample is diluted. That is the classic signature of an interferent whose effect is concentration-dependent: dilute the matrix and the interference fades faster than the analyte, so the diluted result is 'too high' relative to the neat. A genuine high troponin dilutes in parallel within about 10-20%. Non-parallelism alone does not prove interference - it can occur near the lower limit of the measuring range - but here the effect is large and moves in one direction across three dilutions.
What the blocking reagent adds. A heterophile-blocking tube dropped the result from 86 to 12 ng/L, which is below the 20 ng/L upper reference limit. That is direct evidence that a heterophile antibody bridging the capture and detection antibodies was generating the signal. Heterophile antibodies are common, are not the same as rheumatoid factor (which is normal here at 15 IU/mL), and can persist for years.
Conclusion: the true troponin is approximately 12 ng/L, and the 85-88 ng/L results are assay interference, not myocardial injury. The non-parallel dilution and the blocking-reagent result agree, which is much stronger than either alone.
2. What do you report, and how do you communicate it to the surgeon's office and to the patient's record?
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Do not release 86 ng/L as a final result, and do not silently cancel it. Both are unsafe. A number in the chart without context will be acted on; a missing result invites a repeat that generates the same problem.
Reporting plan:
- Report the initial results as they were measured, flagged as not interpretable for clinical decision-making, with a comment: 'Results of 85-88 ng/L are discordant with serial measurement, dilution linearity, and clinical findings; suspected heterophile antibody interference. Repeat with a heterophile-blocking tube gave 12 ng/L.'
- Report the blocking-tube value as the interpretable result, with the method noted so future comparisons are made against the right assay.
- Document the dilution study and the blocking step in the LIS comment or an attached worksheet, so the reasoning is auditable.
Communication: call the surgeon's office before 17:30, not email after close. Identify the patient, state plainly that the elevated troponin is almost certainly an assay artifact, and give the evidence in one sentence - it did not dilute linearly and it disappeared with a blocking reagent. State explicitly that the laboratory cannot rule out cardiac disease on its own and that the decision to proceed is clinical, but that no lab evidence of myocardial injury remains.
Close the loop: ask the office to document the conversation, and send a written addendum to the chart so the interpretation survives the phone call. Advise the patient's record carry an alert that heterophile interference has been demonstrated, so future troponins are ordered with the blocking step from the start. Offer to discuss with cardiology if the surgeon wants a second opinion.
3. Suppose the same 86 ng/L had arrived at 02:00 from the emergency department for a man with chest pain. How does your handling change?
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The technical work does not change; the urgency and the framing do.
What changes:
- Timing. There is no time for a leisurely send-out. Run the blocking tube in-house immediately, and in parallel run a second, differently formatted troponin assay if one is available. Two independent methods agreeing is faster and more convincing than one.
- Do not delay a critical result. Report the measured value now with an explicit caveat, and call the ED physician: 'The troponin is 86, but it is not diluting linearly and I suspect interference; I am running a blocking tube and a second method, results in about 45 minutes.' Withholding the number entirely while the workup runs risks the clinician treating a patient on incomplete information.
- Do not let the caveat become reassurance. In the ED, a rising pattern across serial draws carries real weight, and an interfering antibody can mask a genuine rise. Serial sampling plus the clinical picture (ECG, symptoms, echo) must drive the decision.
The pre-operative case tests the same skill in a low-stakes setting, and the ED case is where getting it wrong causes harm in either direction: a false-positive troponin can trigger an unnecessary angiogram, and a false reassurance can miss a real infarct.
Follow-up either way: send a note to the laboratory medical director recommending that the interference be recorded in the patient's chart, that future troponin orders use the blocking step, and, if the pattern recurs across patients, that the interference rate for this lot and platform be reviewed with the manufacturer.