ABPath Chemical Pathology cheat sheet

CLIA categorizes testing as waived, moderate, or high complexity; each complexity tier carries distinct personnel, QC, and proficiency testing obligations.

A Certificate of Waiver permits only waived testing; the laboratory must follow manufacturer instructions and perform any required QC.

Proficiency testing (PT) is required for regulated analytes; successful performance is generally defined as at least 80% correct for each analyte challenge.

CLIA '88 defines the director qualifications separately for each complexity tier; high-complexity directors require an MD/DO with board certification or equivalent experience.

Test menus and reference intervals must be validated before reporting; method verification is required for FDA-cleared tests, and full validation for laboratory-developed tests.

Critical (panic) value policies require documented notification of the responsible caregiver, read-back of the value, and a time log.

Document retention: records of test results, QC, and PT must be retained for at least 2 years; some states require longer.

Corrective action must be documented for any QC failure, PT failure, or complaint, including root cause analysis and follow-up verification.

Fasting specimens: 8–12 hours is standard for glucose and lipids; water is permitted, but coffee and smoking are not.

Order of draw for venous collection: blood culture, citrate (coagulation), serum (SST), heparin, EDTA, fluoride/oxalate, then other additives.

Tourniquet application should not exceed 1 minute; prolonged stasis increases potassium, lactate, and protein concentrations.

Hemolysis falsely elevates potassium, LDH, AST, and phosphate, and can falsely lower glucose and interfere with bilirubin assays.

EDTA contamination from a lavender tube falsely elevates potassium and calcium (as EDTA chelates) and lowers magnesium and ALP.

Timed collections (e.g., 24-hour urine) require documented start/end times and proper preservative; incomplete collection invalidates results.

Blood culture collection: 2–3 sets from separate sites, 20–30 mL per set in adults; draw before antibiotics when feasible.

Therapeutic drug monitoring trough levels are drawn just before the next dose; peak levels are drawn at a specified post-dose interval.

SI base units: mole (amount), kilogram (mass), meter (length), second (time), kelvin (temperature), ampere (current), candela (luminous intensity).

Enzyme activity is reported in U/L (μmol substrate consumed per minute per liter); IU is an international unit standardized to a reference enzyme preparation.

Osmolality is expressed in mOsm/kg of water; osmolarity in mOsm/L of solution; they differ by the volume of dissolved solids.

The Beer-Lambert law: A = εbc, where A is absorbance, ε is molar absorptivity, b is path length, and c is concentration.

Reference intervals are typically defined as the central 95% of a healthy reference population (2.5th–97.5th percentile).

Conversion between conventional and SI units requires a factor derived from molecular weight; for glucose, mg/dL × 0.0555 = mmol/L.

The mole is defined as 6.02214076 × 10^23 elementary entities (Avogadro constant).

Temperature conversion: °C = (°F − 32) × 5/9; Kelvin = °C + 273.15.

Mean = Σx/n; standard deviation = √[Σ(x − x̄)²/(n − 1)] for a sample.

Coefficient of variation (CV) = (SD/mean) × 100%; used to express imprecision.

Sensitivity = TP/(TP + FN); specificity = TN/(TN + FP); positive predictive value = TP/(TP + FP).

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻); normal ~8–12 mEq/L (or 3–11 if potassium is excluded).

Corrected calcium (mg/dL) = measured Ca + 0.8 × (4.0 − albumin g/dL).

Creatinine clearance (Cockcroft-Gault) = [(140 − age) × weight kg] / (72 × SCr mg/dL), × 0.85 for females.

Osmolal gap = measured osmolality − calculated osmolality (2×Na + glucose/18 + BUN/2.8); gap >10 suggests unmeasured osmoles.

Friedewald LDL = total cholesterol − HDL − (triglycerides/5), valid only when triglycerides < 400 mg/dL.

Levey-Jennings charts plot QC values over time with mean ± 1, 2, and 3 SD limits; trends and shifts are visually identified.

Westgard rules: 1-2s warning; 1-3s, 2-2s, R-4s, 4-1s, and 10x are rejection rules (the 1-2s rule is a warning, not a rejection).

A shift is a sudden change in mean; a trend is a gradual directional change; both suggest systematic error.

Random error increases imprecision (CV); systematic error shifts the mean (bias).

Six Sigma metrics combine bias, CV, and allowable total error (TEa) to grade process performance.

QC materials should span clinically relevant decision points (e.g., low, normal, high) and be analyzed at least once per shift.

Delta checks compare a result to the patient's prior value; large changes trigger verification before reporting.

External quality assessment (EQA)/PT compares performance to peer laboratories and is required for accreditation.

Spectrophotometry: absorbance is proportional to concentration per Beer-Lambert law; linearity is limited by stray light and detector saturation.

Atomic absorption spectroscopy quantifies trace metals (e.g., lead, zinc) by measuring absorption of ground-state atoms.

Ion-selective electrodes (ISEs) measure activity of ions; direct ISEs (undiluted) and indirect ISEs (diluted) differ in pseudohyperkalemia and hyperproteinemia.

Fluorescence and chemiluminescence offer high sensitivity; used in immunoassays (e.g., TSH, troponin, tumor markers).

Chromatography separates analytes by differential partitioning between mobile and stationary phases; HPLC is used for HbA1c and drugs.

Mass spectrometry (LC-MS/MS) provides high specificity via mass-to-charge ratio; the reference method for many small molecules.

Centrifugation separates by density; relative centrifugal force (RCF) = 1.118 × 10⁻⁵ × r (cm) × rpm².

Nephelometry and turbidimetry quantify light scatter/absorbance by immune complexes; used for immunoglobulins and specific proteins.

Enzymatic assays: glucose oxidase, hexokinase, and glucose dehydrogenase methods differ in specificity and interference profiles.

Jaffe creatinine method is nonspecific; compensated Jaffe corrects for protein and ketone interference.

Immunoassay formats include competitive (small analytes) and sandwich (large analytes); hook effect occurs at very high antigen concentrations in sandwich assays.

Heterophile antibodies and rheumatoid factor can cause false-positive or false-negative immunoassay results.

Ion-exchange HPLC and capillary electrophoresis separate hemoglobin variants; immunoturbidimetry is used for HbA1c.

PCR amplifies DNA; real-time PCR quantifies target; sequencing identifies specific mutations.

Turbidimetric and chromogenic methods are common for coagulation (PT, aPTT); the INR standardizes PT results across thromboplastins.

Interference: hemolysis, icterus, and lipemia (HIL) are screened by automated indices; each analyte has a threshold above which results are unreliable.

Diabetes diagnosis: fasting glucose ≥126 mg/dL, 2-h OGTT ≥200 mg/dL, HbA1c ≥6.5%, or random glucose ≥200 mg/dL with symptoms.

HbA1c reflects average glucose over ~2–3 months; values are affected by hemoglobin variants, anemia, and renal disease.

Hypoglycemia in a fasting state with elevated insulin and C-peptide suggests insulinoma; low C-peptide suggests exogenous insulin.

Phenylketonuria: elevated phenylalanine with normal or low tyrosine; newborn screening is mandatory in most states.

Galactosemia: elevated galactose and galactose-1-phosphate; screen-positive newborns require urgent dietary restriction.

Maple syrup urine disease: elevated branched-chain amino acids (leucine, isoleucine, valine) and alloisoleucine.

Homocystinuria: elevated homocysteine and methionine; caused by cystathionine β-synthase deficiency.

Congenital hypothyroidism: elevated TSH and low T4 on newborn screen; early treatment prevents intellectual disability.

Normal arterial blood gas: pH 7.35–7.45, PaCO₂ 35–45 mmHg, HCO₃⁻ 22–26 mEq/L.

Respiratory acidosis: low pH with elevated PaCO₂; metabolic compensation raises HCO₃⁻ over days.

Metabolic acidosis: low pH with low HCO₃⁻; calculate anion gap to distinguish high-gap (MUDPILES) from normal-gap causes.

Winter's formula: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2; deviation indicates a mixed disorder.

Hyponatremia: assess serum osmolality and volume status; pseudohyponatremia occurs with hyperproteinemia or hyperlipidemia.

Hyperkalemia: rule out hemolysis and pseudohyperkalemia (thrombocytosis, leukocytosis); ECG changes include peaked T waves.

Hypokalemia: ECG shows U waves and flattened T waves; causes include GI loss and diuretics.

Hypercalcemia: PTH-dependent (primary hyperparathyroidism) vs PTH-independent (malignancy); check PTH and PTHrP.

Serum protein electrophoresis separates albumin, α1, α2, β, and γ fractions; a monoclonal spike in γ suggests a plasma cell dyscrasia.

Immunofixation identifies the heavy and light chain type of a monoclonal protein.

Free light chain ratio (κ/λ) is abnormal outside 0.26–1.65; used to detect light-chain disease.

Nephrotic syndrome: proteinuria >3.5 g/day, hypoalbuminemia, edema, hyperlipidemia.

Hemoglobin electrophoresis: HbA, HbA2, HbF, and variant hemoglobins; HbA2 >3.5% suggests β-thalassemia trait.

Sickle cell disease: HbS on electrophoresis; sickle cell trait is usually asymptomatic with ~40% HbS.

Methemoglobinemia: elevated MetHb; cyanosis unresponsive to oxygen; treat with methylene blue.

Carboxyhemoglobin: elevated COHb; suspect in smoke inhalation or fire victims; co-oximetry is required.

PT measures extrinsic and common pathway; aPTT measures intrinsic and common pathway.

INR = (PT patient / PT normal)^ISI; used to monitor warfarin therapy.

Heparin therapy is monitored by aPTT or anti-Xa activity; protamine sulfate reverses heparin.

Vitamin K deficiency prolongs PT before aPTT; factors II, VII, IX, and X are vitamin K dependent.

DIC: prolonged PT and aPTT, low fibrinogen, elevated D-dimer, and thrombocytopenia.

von Willebrand disease: prolonged aPTT (if vWF low), low vWF antigen and activity, and normal or low factor VIII.

Hemophilia A: prolonged aPTT with normal PT; factor VIII deficiency; hemophilia B is factor IX deficiency.

Platelet function disorders: normal platelet count with prolonged bleeding time or abnormal PFA-100; von Willebrand disease is the most common inherited bleeding disorder.

Acute kidney injury: rise in creatinine ≥0.3 mg/dL within 48 h or ≥1.5× baseline within 7 days.

eGFR (CKD-EPI) estimates kidney function; values <60 mL/min/1.73 m² for ≥3 months indicate CKD.

Liver injury patterns: hepatocellular (AST/ALT predominant) vs cholestatic (ALP/GGT predominant).

Cardiac troponin (hs-cTn) is the preferred biomarker for myocardial infarction; serial measurements show rise and/or fall.

BNP and NT-proBNP are elevated in heart failure; obesity and renal disease affect interpretation.

Reproduction: hCG rises in early pregnancy; β-hCG doubling time ~48 h in viable early pregnancy.

Pregnancy screening: first-trimester combined test (PAPP-A, free β-hCG, nuchal translucency) and second-trimester quad screen (AFP, hCG, uE3, inhibin A).

Newborn screening: heel-stick blood spots collected 24–72 h after birth; tests for PKU, congenital hypothyroidism, CAH, galactosemia, and hemoglobinopathies.

POC glucose meters are waived tests; accuracy is affected by hematocrit, altitude, and interfering substances.

POC INR devices are used for warfarin monitoring; results should be confirmed by laboratory INR when clinically discordant.

POC blood gas and electrolyte analyzers use ISEs; calibration and QC must follow manufacturer and regulatory requirements.

POC urine pregnancy tests detect β-hCG in urine; sensitivity is lower than serum quantitative hCG.

POC cardiac troponin assays have higher detection limits than laboratory hs-cTn; negative results do not exclude MI.

POC testing requires documented operator training, competency assessment, and QC records per CLIA.

POC infectious disease tests (e.g., rapid HIV, strep) require confirmatory testing for positive screening results per guidelines.

Connectivity and result transmission to the LIS must be validated to prevent transcription errors.

IgM indicates recent or acute infection; IgG indicates past infection or immunity; paired sera show a fourfold rise.

HIV screening uses a fourth-generation antigen/antibody assay; reactive results require supplemental differentiation (e.g., HIV-1/2 antibody differentiation or NAT).

Hepatitis B serology: HBsAg indicates infection; anti-HBs indicates immunity; anti-HBc IgM indicates acute infection.

Hepatitis C: anti-HCV screening; HCV RNA confirms active infection.

Syphilis: nontrepidemal tests (RPR, VDRL) for screening and monitoring; treponemal tests (FTA-ABS, TP-PA) for confirmation.

Immunodeficiency: low immunoglobulin levels, abnormal lymphocyte subsets, and functional assays (e.g., neutrophil oxidative burst).

Allergy testing: specific IgE (ImmunoCAP) or skin testing; total IgE is nonspecific.

Autoimmune disease: ANA screening with reflex to specific antibodies (anti-dsDNA, anti-Smith) for connective tissue disease.

Therapeutic drug monitoring is indicated for drugs with narrow therapeutic index (e.g., digoxin, lithium, vancomycin, phenytoin, tacrolimus).

Trough levels are drawn before the next dose; peak levels at a specified interval after dosing.

Ethanol: legal limit for driving is 0.08 g/dL (80 mg/dL) in most US states; measured by enzymatic or GC methods.

Acetaminophen overdose: Rumack-Matthew nomogram guides N-acetylcysteine therapy based on time since ingestion and level.

Salicylate toxicity: mixed respiratory alkalosis and metabolic acidosis; check serum level and anion gap.

Drugs of abuse screening immunoassays are presumptive; confirmatory testing uses GC-MS or LC-MS/MS.

False-positive immunoassay screens occur with structurally related compounds (e.g., amphetamine with pseudoephedrine).

Lithium: narrow therapeutic range 0.6–1.2 mEq/L; toxicity causes tremor, ataxia, and renal effects.

Reference intervals should be age- and sex-specific where appropriate; pediatric and geriatric populations require separate intervals.

Critical value thresholds are established by the laboratory in consultation with clinical staff and reviewed periodically.

Delta checks flag results that differ significantly from prior values; verify before reporting.

Test utilization: choose tests with high pre-test probability and interpret in clinical context; avoid redundant testing.

Consultation on unexpected results should consider preanalytical, analytical, and biological causes before clinical interpretation.

Biological variation affects serial results; reference change value (RCV) accounts for analytical and biological variation.

Turnaround time (TAT) is monitored from collection to result reporting; outliers require investigation.

Patient safety: results requiring urgent action (e.g., critical potassium, glucose) must be communicated and documented.