R.NCS.T. Exam common mistakes
R.NCS.T. Exam · Understand the traps and how to avoid them.
Reporting a sensory nerve action potential amplitude as the peak-to-peak value when the laboratory protocol specifies baseline-to-negative-peak measurement.
Candidates memorize a single amplitude value without tracking which measurement convention the protocol uses, and the two conventions differ substantially for the same waveform.
What to do instead
For every sensory study, state the measurement convention aloud before reading the amplitude, and confirm it matches the laboratory's written protocol and normative table.
Calling a conduction block when the only abnormality is temporal dispersion from a proximal stimulation site.
Candidates apply the amplitude-drop criterion without accounting for the expected widening and amplitude loss that occurs with dispersion across a long segment, especially in acquired demyelinating disease.
What to do instead
Compare duration as well as amplitude between proximal and distal responses; if duration increases substantially, label the finding as dispersion rather than block, and apply the protocol's block criteria explicitly.
Using a fixed distance of 14 cm for the median motor study regardless of the patient's limb length or the protocol's landmark-based method.
Textbook distances are easy to remember and candidates default to them instead of measuring to the actual anatomical landmark on the patient.
What to do instead
Measure from the active electrode to the stimulation site with a tape measure for each study, and document the distance used so conduction velocity can be verified.
Misidentifying the ulnar nerve at the wrist and stimulating the median nerve instead.
The ulnar nerve lies medial to the ulnar artery and the landmarks are close together; candidates rely on surface appearance rather than palpating the artery and using bony landmarks.
What to do instead
Palpate the ulnar artery and place the stimulating cathode just medial to it, then confirm correct nerve activation by observing the expected intrinsic hand muscle twitch before recording.
Confusing the root value of the posterior tibial nerve with the common fibular nerve when answering plexus or root localization items.
Both arise from the sacral plexus and share overlapping root contributions, so candidates conflate their root values and the muscles they supply.
What to do instead
Build a table pairing each major lower-limb nerve with its root values, the muscles it innervates, and the sensory territory, and review it until the divisions are automatic.
Leaving the filter settings at the default broadband values for a sensory study, then reporting a low amplitude caused by the settings rather than the nerve.
Candidates set filters once and do not revisit them, not realizing that wide-open low-frequency filters admit noise and that inappropriate high-frequency cutoffs attenuate the small sensory potential.
What to do instead
Set low- and high-frequency filters according to the study type before recording, and note the settings on the report so amplitude values are interpretable.
Recording with a stimulator that is not delivering supramaximal stimulation because the current was increased only until a visible twitch appeared.
A visible muscle twitch occurs well below supramaximal levels, so candidates stop titrating too early and underestimate amplitude.
What to do instead
Increase stimulus intensity until the response amplitude stops increasing, then increase a further 10 to 20 percent to confirm supramaximal stimulation.
Failing to warm a cold limb before performing the study and then interpreting the slowed velocity as demyelination.
Candidates focus on electrode placement and forget that limb temperature directly affects conduction velocity and distal latency, so cold-induced slowing mimics pathology.
What to do instead
Measure and record limb temperature before the study, warm the limb to the laboratory's target range, and recheck temperature if values fall outside it.
Continuing a study on a patient with an implanted cardiac pacemaker or defibrillator without confirming the device's safety status or the laboratory's policy.
Candidates treat electrical stimulation as universally safe and do not screen for implanted devices during intake.
What to do instead
Screen for implanted electronic devices during the history, follow the laboratory's written policy for stimulation near such devices, and consult the device manufacturer or cardiology when the policy requires it.
Documenting only the final numeric values and omitting electrode placement, stimulation sites, distances, temperature, and filter settings from the report.
Candidates treat the report as a results sheet rather than a defensible record, so the study cannot be reproduced or audited.
What to do instead
Use a standardized report template that requires each technical parameter to be entered before the study is signed, and verify completeness against the template before finalizing.