ABPath Chemical Pathology common mistakes
ABPath Chemical Pathology · Understand the traps and how to avoid them.
Treating a serum sodium that is low by the reference interval as true hyponatremia without first checking for hyperproteinemia or hyperlipidemia (pseudohyponatremia from indirect ISE methods), or ignoring a normal measured osmolality in a patient with low sodium.
Candidates reflexively map a low number to the named disorder and skip the confirmatory step of comparing measured vs calculated osmolality and reviewing the assay platform; indirect ion-selective electrode results are volume-displaced by excess protein or lipid.
What to do instead
For every dysnatremia, compute the osmolality gap (measured minus calculated) and ask whether the sample is lipemic or has very high protein; if measured osmolality is normal, call it pseudohyponatremia and identify the method (indirect vs direct ISE).
Reporting an anion gap or delta-delta without stating the reference interval and units used, or failing to correct the gap for hypoalbuminemia.
Different analyzers and textbooks use different normal ranges (e.g., 8-12 vs 3-11 mmol/L), and candidates memorize a single cutoff without appreciating that the gap is albumin-dependent and method-dependent.
What to do instead
Always report the gap with the lab's own reference interval, and apply the albumin correction (add ~2.5 mmol/L to the gap for every 1 g/dL albumin below 4.0) before interpreting a mixed acid-base picture.
Diagnosing a coagulation abnormality from a single prolonged aPTT or PT without a mixing study, or interpreting a mixing study without allowing for the time- and temperature-dependence of factor VIII and lupus anticoagulant effects.
Candidates treat prolongation as evidence of deficiency and forget that inhibitors and factor consumption produce the same screening result; immediate vs incubated mixing study interpretation is a frequent blind spot.
What to do instead
State the algorithm explicitly: prolonged aPTT/PT leads to a 1:1 mixing study, correction at 0 and 2 hours indicates deficiency, failure to correct indicates an inhibitor, and correction only at 0 hours suggests a time-dependent inhibitor such as a lupus anticoagulant.
Interpreting a tumor marker or hormone result without regard to assay standardization, biological variability, or the patient's clinical context (e.g., calling a mildly elevated PSA or CA-125 diagnostic).
Candidates conflate analytic measurement with diagnostic certainty and overlook that immunoassays are not harmonized across platforms and that markers are used for monitoring, not screening.
What to do instead
For each marker, state the intended use (screening, monitoring, prognosis), require serial measurements on the same platform, and describe confirmatory or imaging steps before assigning a diagnosis.
Applying adult reference intervals to neonates, pregnant patients, or the elderly, or misinterpreting newborn screening results without accounting for age-specific cutoffs and collection timing.
Reference intervals shift dramatically with age and pregnancy (e.g., alkaline phosphatase, TSH, creatinine, hemoglobin), and candidates default to the adult range they memorized.
What to do instead
Before interpreting any result, identify the patient's age and pregnancy status and select the age- and trimester-specific interval; for newborn screening, state the collection timing (e.g., 24-72 hours) and the follow-up protocol for an abnormal result.
Confusing analytic with total error when evaluating quality control, or accepting a QC rule based on a single control level without assessing the probability of error detection and false rejection.
Candidates memorize Westgard rule names but not the operational logic of selecting rules from the sigma metric, and they conflate imprecision (CV) with bias.
What to do instead
For each QC scenario, compute or state the sigma metric, select multirule strategies accordingly (e.g., 1-3s/2-2s/R-4s/4-1s with appropriate N), and distinguish random from systematic error when interpreting a rejected run.
Miscalculating creatinine clearance or eGFR by using the wrong formula, wrong units, or wrong body-size correction, or failing to recognize when an equation is invalid (e.g., non-steady state, extremes of muscle mass, pregnancy).
Candidates memorize one equation and apply it universally, and unit conversions (mg/dL vs µmol/L, mL/min vs mL/min/1.73 m²) are a frequent source of arithmetic error under time pressure.
What to do instead
Write the formula and units before substituting numbers, state the assumptions (steady state, stable creatinine), and flag when the equation is unreliable; for timed collections, verify completeness with a creatinine excretion check.
Interpreting therapeutic drug levels or toxicology screens without documenting the time of the last dose, the sampling time relative to dosing, or the specimen type and its limitations (e.g., urine immunoassay screen vs confirmatory GC-MS).
Candidates treat a concentration as an absolute value and forget that TDM requires trough sampling at steady state and that immunoassays have cross-reactivity and cutoffs.
What to do instead
For any drug level, record dose time and sampling time, confirm steady state and trough timing, and for toxicology screens distinguish screening (immunoassay, qualitative) from confirmation (chromatography-mass spectrometry) before drawing clinical conclusions.
Failing to recognize preanalytical causes of an abnormal result, such as hemolysis, prolonged tourniquet time, IV fluid contamination, or improper collection tube, before attributing the value to disease.
Candidates jump to the analytic or pathologic explanation and skip the specimen-quality review that would explain a spurious potassium, glucose, or calcium result.
What to do instead
Systematically check for hemolysis index, collection site relative to an IV line, tourniquet time, tube type and order of draw, and turnaround delay; if a preanalytical cause is plausible, state it and recommend a repeat specimen.
Answering a consultation question with a single number or diagnosis instead of a structured interpretation that includes the reference interval, the clinical context, the differential, and a recommended next step.
Candidates treat the exam as a recall test and omit the interpretive reasoning and follow-up that define laboratory consultation, which is what the patient-care decision-making domain rewards.
What to do instead
Use a fixed template for every consult item: state the result with units and reference interval, give the most likely explanation and key differential, identify confounding or preanalytical factors, and recommend a specific confirmatory test or clinical action.