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SOP

Cardiac Markers Testing (hs-Troponin, CK, CK-MB)

SOP for high-sensitivity troponin, total CK, and CK-MB in the evaluation of acute coronary syndrome and myocardial injury.

Last verified 1 month ago 3 min read Clinical Chemistry Cardiac #troponin #hs-troponin #ck #ck-mb #myocardial infarction
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Document Control
Version
1.0
Effective
Aug 30, 2026
Reviewed
Aug 30, 2026
Department
Clinical Chemistry
Principle

Cardiac troponin I is a regulatory protein released on myocardial injury; high-sensitivity assays measure it near the 99th percentile with a coefficient of variation of about 10% or less, enabling early detection of small infarcts and dynamic delta changes. CK is a nonspecific muscle enzyme; CK-MB is enriched in myocardium and, combined with total CK, helps separate myocardial from skeletal muscle injury. Serial sampling captures the rise/fall pattern required to diagnose MI.

Standard Operating Procedure: Cardiac Markers Testing (hs-Troponin, CK, CK-MB)

1. Purpose and Scope

To measure high-sensitivity cardiac troponin I (hs-cTnI), total creatine kinase (CK), and CK-MB for the evaluation of acute coronary syndrome (ACS), myocardial infarction (MI), and myocardial or skeletal muscle injury.

2. Specimen Requirements

  • Type: Serum (plain tube or SST).
  • Volume: Minimum 1.0 mL per test.
  • Timing: Serial sampling is required for MI diagnosis. Common protocols draw at presentation and at 1-3 hours (hs-cTnI) using a rise and/or fall pattern.
  • Hemolysis: Reject markedly hemolyzed samples; hemolysis can interfere and CK-MB is unstable with cell contact.
  • Stability: Separate serum within 1 hour. Troponin is stable for days refrigerated; CK and CK-MB degrade substantially after 4 hours at room temperature if left on cells.

3. Safety and Precautions

  • Standard PPE (gloves, lab coat, eye protection). All blood samples are potentially infectious.
  • Dispose of sharps and biohazard waste per laboratory policy.
  • Do not pipette by mouth. Use mechanical pipetting devices.

4. Equipment and Reagents

  • Automated immunoassay analyzer for hs-cTnI (chemiluminescence or similar) and CK-MB mass assay.
  • Automated chemistry analyzer for total CK (enzymatic, coupled to hexokinase or creatine kinase N-acetyl-L-cysteine activated method, 340 nm).
  • Two levels of quality control material for each assay, including levels near the hs-cTnI 99th percentile.

5. Step-by-Step Procedure

  1. Centrifuge the specimen at 3000-3500 rpm for 10 minutes within 1 hour of collection.
  2. Load serum onto the immunoassay and chemistry analyzers per instrument instructions.
  3. Run two levels of control (including a low/normal level near the hs-cTnI decision point) before patient samples.
  4. Measure hs-cTnI by immunoassay and total CK enzymatically; CK-MB by mass immunoassay.
  5. Verify calibration and linearity; repeat results around the 99th percentile per lab reflex rules.
  6. For elevated CK without myocardial suspicion, offer optional CK-MB relative index (CK-MB mass/CK total x 100) to phenotype the muscle source.
  7. Release serial results with delta (change) flags integrated into the report.

6. Quality Control

  • Run two levels of QC for each assay daily, with each calibration, and with each new reagent lot.
  • QC around the hs-cTnI 99th percentile is mandatory; a low-control failure in this range invalidates the run.
  • Enforce Levey-Jennings rules; participate in an EQA program for all three analytes.

7. Decision limits

The interpretive thresholds below are clinical decision limits; the canonical adult intervals are displayed in the Reference Ranges panel.

  • hs-Troponin I: the 99th percentile upper reference limit is assay dependent; many modern assays report sex-specific 99th percentiles (approximately 34 ng/L for males and 16 ng/L for females on a common contemporary assay, but verify your own instrument). The laboratory report applies the canonical single adult upper limit of 0.03 ng/mL and flags values above it.
  • Acute MI (Fourth Universal Definition): detection of a rise AND/OR fall of cardiac troponin with at least one value above the 99th percentile and at least one supporting feature of myocardial ischemia (symptoms, ECG changes, imaging, coronary thrombus).
  • CK-MB relative index: CK-MB mass / total CK x 100 below 3-4% favours a skeletal muscle source; higher relative indices with elevated total CK suggest myocardial origin.
  • Non-ischemic elevations: troponin rises in sepsis, renal failure, pulmonary embolism, tachycardia, myocarditis, and cardio-toxic drugs; interpret with the clinical picture, never alone.

Reference Ranges

Browse all ranges

55–170 U/L

30–135 U/L

5 ng/mL

0.03 ng/mL

Frequently Asked Questions

A single troponin value cannot distinguish acute injury from chronic elevation. Diagnosis of MI requires a rise and/or fall with at least one value above the 99th percentile, which only serial draws (often at 0-1-3 hours) can demonstrate.
High-sensitivity troponin assays report lower upper reference limits for women than men (approximately 16 vs 34 ng/L for a common hs-cTnI assay). Using a single sex-combined cutoff would under-detect injury in women.
No. Troponin elevation indicates myocardial injury, which may be ischemic or non-ischemic (sepsis, renal failure, pulmonary embolism, myocarditis). MI requires injury plus evidence of ischemia and a dynamic pattern.