CLT8 cheat sheet

Order of draw (CLSI): blood culture, sodium citrate (light blue), serum tube with or without clot activator (red/gold), heparin (green), EDTA (lavender), fluoride/oxalate (gray).

Light blue citrate tube must be filled to the marked volume: a short draw raises the citrate-to-blood ratio and falsely prolongs PT and aPTT.

EDTA tubes: invert 8-10 times; citrate tubes 3-4 times. Never shake - hemolysis falsely elevates potassium, LDH, and AST.

Serum separator tubes must clot 30 minutes before centrifugation; centrifuging before clot formation causes fibrin strands and instrument sampling errors.

Tourniquet time should not exceed 1 minute; prolonged stasis raises potassium, protein, and cell counts and causes hemoconcentration.

Fasting glucose specimen in a gray (fluoride/oxalate) tube inhibits glycolysis; without fluoride, glucose falls roughly 5-7% per hour at room temperature.

Blood cultures: collect before antimicrobials, 20-30 mL per adult set, inoculate aerobic bottle first; do not refrigerate.

24-hour urine requires the container to be refrigerated or chemically preserved, and the first morning void is discarded before starting collection.

Levey-Jennings chart plots control values against mean ± 1, 2, and 3 SD; Westgard rules flag random or systematic error.

1-3s rule: one control exceeds ±3 SD - reject (random error). 2-2s: two consecutive controls exceed the same ±2 SD - reject (systematic error).

R4s rule: difference between two controls within a run exceeds 4 SD - reject. 4-1s: four consecutive values exceed the same ±1 SD - reject.

10x rule: ten consecutive control values fall on one side of the mean - reject; indicates systematic bias.

Coefficient of variation CV = (SD / mean) × 100; lower CV means better precision.

Sigma metric = (TEa − |bias|) / CV; ≥6 is world class, <3 requires an alternative QC strategy.

Delta check compares a current result with the patient's prior value; a delta flag triggers specimen re-verification before reporting.

Proficiency testing specimens must be handled like patient samples; intentional result-sharing or peer comparison violates CLIA regulations.

Sodium is the major extracellular cation; hyponatremia with low osmolality suggests true hypotonicity, whereas normal osmolality suggests pseudohyponatremia (hyperlipidemia, hyperproteinemia).

Anion gap = Na − (Cl + HCO3), reference roughly 8-12 mmol/L; elevated gap indicates unmeasured anions (ketones, lactate, uremia, toxins).

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

Friedewald LDL = TC − HDL − (TG/5), valid only when triglycerides are below 400 mg/dL.

Cardiac troponin I and T are the preferred biomarkers for myocardial injury; CK-MB is less specific and rises after skeletal muscle injury.

HbA1c reflects mean glucose over roughly 120 days (erythrocyte lifespan); hemoglobinopathies and recent transfusion alter the result.

AST/ALT ratio above 2 suggests alcoholic liver disease; ALT predominates in viral hepatitis.

Creatinine is filtered and minimally secreted; cystatin C is less muscle-mass dependent and detects early GFR decline.

Reference ranges: hemoglobin 13.5-17.5 g/dL (men), 12.0-15.5 g/dL (women); MCV 80-100 fL; platelets 150-450 × 10^9/L.

MCV below 80 fL: iron deficiency, thalassemia, anemia of chronic disease, sideroblastic anemia. MCV above 100 fL: B12/folate deficiency, alcohol, hypothyroidism, reticulocytosis.

Elevated RDW with low MCV favors iron deficiency; normal RDW with low MCV favors thalassemia trait.

Reticulocyte count corrects for anemia: corrected retic % = retic % × (patient Hct / normal Hct); RPI divides by maturation factor.

PT measures extrinsic and common pathway (factors VII, X, V, II, fibrinogen); aPTT measures intrinsic and common (VIII, IX, XI, XII, X, V, II).

Heparin therapy prolongs aPTT; warfarin prolongs PT/INR. Mixing study corrects a prolonged aPTT in factor deficiency but not in a lupus anticoagulant.

Platelet count is falsely low in clumping (EDTA-dependent pseudothrombocytopenia); repeat in a citrate tube to confirm.

DIC shows prolonged PT and aPTT, low fibrinogen, elevated D-dimer, and schistocytes on smear.

Gram-positive cocci in clusters: Staphylococcus. Gram-positive cocci in chains: Streptococcus. Gram-negative diplococci: Neisseria.

Catalase test separates staphylococci (positive) from streptococci (negative); coagulase separates S. aureus (positive) from coagulase-negative staph.

Oxidase-positive, non-fermenting gram-negative rods include Pseudomonas aeruginosa; oxidase-negative Enterobacterales ferment lactose on MacConkey.

Beta-hemolysis on blood agar is complete clearing; alpha is greenish partial; gamma is none.

MRSA detection uses cefoxitin disk or oxacillin MIC; ESBL producers are resistant to third-generation cephalosporins but inhibited by clavulanate.

Blood culture bottles are incubated at 35-37 °C and monitored for 5 days; most true positives grow within 24-48 hours.

Stool culture for Salmonella/Shigella uses selective media such as Hektoen or XLD; Campylobacter requires microaerophilic conditions at 42 °C.

Acid-fast staining (Ziehl-Neelsen) detects mycobacteria; auramine-rhodamine fluorescence is the higher-throughput screening stain.

Primary immune response: IgM appears first, followed by IgG; secondary response is faster, higher titer, and IgG-predominant.

ELISA formats: direct (antigen detection), indirect (antibody detection), sandwich (antigen capture), competitive (small molecules).

Western blot separates proteins by molecular weight and confirms reactivity; it is used as a supplemental test after screening.

False-positive syphilis screening can result from pregnancy, autoimmune disease, and other spirochete infections; treponemal and non-treponemal tests are paired.

Hepatitis B serology: HBsAg indicates infection, anti-HBs indicates immunity, anti-HBc IgM indicates recent infection, HBeAg indicates high infectivity.

Rheumatoid factor is an autoantibody against the Fc portion of IgG; anti-CCP is more specific for rheumatoid arthritis.

Complement C3 and C4 are consumed in active immune-complex disease; low levels support active SLE.

Flow cytometry identifies cell populations by surface markers (CD4 for helper T cells, CD8 for cytotoxic T cells, CD19 for B cells).

Urine specific gravity by reagent strip reflects ionic concentration and is inaccurate at high pH; refractometry measures refractive index and is less pH-dependent.

Dipstick blood detects peroxidase activity of heme; positive with intact or lysed RBCs and with myoglobin or hemoglobin.

Leukocyte esterase indicates granulocytes; nitrite positivity suggests nitrate-reducing gram-negative bacteria such as E. coli.

Microscopic RBC casts indicate glomerular bleeding; WBC casts indicate pyelonephritis or interstitial nephritis.

Hyaline casts are nonspecific; granular casts suggest tubular injury; broad waxy casts indicate chronic renal failure.

Transudate vs exudate by Light's criteria: pleural fluid protein/serum protein above 0.5 or LDH ratio above 0.6 indicates exudate.

CSF: bacterial meningitis shows elevated neutrophils, low glucose, and high protein; viral shows lymphocytes with normal glucose.

Urine pH below 5.5 with crystals can suggest uric acid stones; alkaline urine favors struvite (infection) stones.

Standard precautions apply to all blood and body fluids regardless of diagnosis; treat every specimen as potentially infectious.

Bloodborne pathogen exposure: wash the site with soap and water, report immediately, and initiate post-exposure prophylaxis evaluation within hours.

Tuberculosis transmission requires airborne precautions (N95 respirator, negative-pressure room); most routine lab work does not require this.

Chemical hazard communication requires Safety Data Sheets, labeling, and training under the Hazard Communication Standard.

Formaldehyde and xylene are carcinogens requiring fume hoods; spill kits and vapor monitoring are part of safe handling.

Sharps containers are filled to about three-quarters capacity and replaced; never recap needles by hand.

Centrifuges must be balanced and closed during operation; aerosol release from a broken tube requires waiting 30 minutes before opening.

Spill cleanup uses absorbent material and an appropriate disinfectant such as 10% bleach (0.5% sodium hypochlorite) for blood.

Beer-Lambert law: A = εbc; absorbance is proportional to concentration, path length, and molar absorptivity.

Spectrophotometry uses a light source, monochromator, cuvette, and detector; the linear range must be verified before reporting.

Ion-selective electrodes measure activity, not concentration; indirect ISE dilutes the sample and can be affected by hyperlipidemia.

Calibration establishes the relationship between signal and concentration; calibration verification confirms performance across the reportable range.

Linearity studies define the analytical measuring range; results outside it require dilution and reanalysis.

Reference interval verification requires at least 20 healthy subjects per CLSI; method comparison uses at least 40 patient samples across the range.

Random access analyzers process samples in any order; batch analyzers process groups, which affects turnaround time.

Autoverification rules must include delta checks, critical value flags, and instrument error codes before results are released.