Chemistry QC, Interferences, Specimen Handling, and Result Correlation

QC is part of chemistry interpretation

The official MLT chemistry outline repeatedly pairs test procedures with specimen collection, processing, troubleshooting, and interfering substances. That means QC is not separate from chemistry. It is the gatekeeper for whether patient chemistry results are reportable.

Westgard-style rule map

The exam usually gives a short QC story and asks for the best action or rule. First decide if QC is acceptable. If not, do not release patient results from that run until the lab's corrective process is complete.

Interference patterns

Hemolysis adds intracellular contents and can falsely increase potassium, LD, AST, and phosphorus. It can also interfere spectrally or chemically depending on the assay. Icterus can interfere with photometric methods and is especially relevant when bilirubin is high. Lipemia creates turbidity and volume displacement issues, especially for photometric assays and some indirect ion-selective electrode results.

EDTA contamination has a recognizable pattern: potassium may be very high while calcium and magnesium are unexpectedly low. IV contamination can dilute analytes or add glucose, electrolytes, or medications from the line. Delayed separation lowers glucose and can alter potassium. Light exposure can affect bilirubin. Ammonia and lactate are time-sensitive and technique-sensitive.

Result correlation workflow

Use this reporting screen before releasing an unexpected chemistry result:

1. QC acceptable? Check current controls, calibration status, reagent lot, maintenance, and analyzer flags.
2. Specimen acceptable? Review hemolysis, icterus, lipemia, clotting, insufficient volume, wrong tube, delay, and collection site.
3. Delta plausible? Compare with prior values when available and allowed by policy.
4. Pattern coherent? Ask whether related analytes move together, such as high potassium with hemolysis or high ALP with high GGT.
5. Policy triggered? Follow critical-value, repeat, recollect, and documentation rules.

Lot changes and calibration

A new reagent lot should not be placed into routine patient reporting without verification. Lot-to-lot comparison, QC recovery, calibration checks, and acceptable bias limits protect against systematic shifts. When multiple methods or analyzers exist, method differences can also explain deltas, so the MLT should verify the source before assuming patient change.

Practical exam cue

If an option says to report an implausible critical value without checking specimen quality or QC, it is usually unsafe. If an option says to delete the result without documentation, it is also unsafe. The better MLT answer usually follows policy: hold or verify the result, repeat if appropriate, recollect if specimen integrity is compromised, document corrective action, and notify according to critical-result procedure.