MLS cheat sheet

ABO compatibility: group O red cells may be transfused to any ABO recipient; group AB plasma may be given to any recipient because it lacks anti-A and anti-B.

A group B recipient can receive group B or group O red cells; a group A recipient can receive group A or group O red cells.

A 300 µg dose of Rh immune globulin neutralizes approximately 15 mL of fetal red cells (about 30 mL whole blood); Kleihauer-Betke or flow cytometry quantifies larger fetomaternal hemorrhage.

The indirect antiglobulin test detects unexpected serum antibodies; a positive antibody screen requires panel identification and antigen-negative crossmatched units.

Ficin or papain treatment enhances Rh, Kidd, Lewis, and P1 antibodies but denatures Duffy and MNS antigens.

Storage limits: red cells 1-6°C for up to 42 days in additive solution; platelets 20-24°C with agitation for 5 days; fresh frozen plasma at or below -18°C for 12 months.

Acute hemolytic reactions are most often caused by clerical error and ABO incompatibility; febrile nonhemolytic reactions are the most common type overall.

The direct antiglobulin test is positive when red cells are coated in vivo with IgG or complement, as in autoimmune hemolysis, drug-induced hemolysis, and hemolytic disease of the newborn.

Anion gap = Na - (Cl + HCO3); reference 8-12 mEq/L; an elevated gap with metabolic acidosis suggests ketoacidosis, lactic acidosis, renal failure, or toxin ingestion.

Calculated osmolality = 2Na + glucose/18 + BUN/2.8; an osmolal gap above 10 suggests methanol, ethylene glycol, or isopropanol.

Corrected calcium = measured calcium + 0.8 x (4.0 - albumin in g/dL); roughly half of serum calcium is bound to albumin.

Friedewald equation: LDL = total cholesterol - HDL - triglycerides/5; the calculation is invalid when triglycerides exceed 400 mg/dL.

Cockcroft-Gault creatinine clearance = [(140 - age) x weight in kg] / (72 x serum creatinine); multiply the result by 0.85 for females.

Hemolysis falsely raises serum potassium, LDH, AST, and phosphate; separate serum promptly when these analytes are critical.

HbA1c reflects mean glycemia over roughly 120 days; estimated average glucose = 28.7 x A1c - 46.7.

In hyperglycemia, measured sodium falls about 1.6 mEq/L for every 100 mg/dL glucose above 100 due to water shift; TSH is the most sensitive first-line thyroid test.

MCV = (Hct / RBC) x 10 fL; MCH = (Hgb / RBC) x 10 pg; MCHC = (Hgb / Hct) x 100 g/L.

Microcytic anemia (MCV below 80 fL): iron deficiency, thalassemia, anemia of chronic disease, sideroblastic anemia; macrocytic (MCV above 100 fL): B12 or folate deficiency, reticulocytosis, liver disease, alcohol.

Corrected reticulocyte count = reticulocyte percent x (patient Hct / 45); a value below 2% indicates inadequate marrow response.

PT and INR assess the extrinsic pathway (factor VII) and monitor warfarin; aPTT assesses the intrinsic pathway and monitors unfractionated heparin.

A mixing study that corrects the prolonged clotting time indicates a factor deficiency; failure to correct indicates an inhibitor such as a lupus anticoagulant or a specific factor antibody.

DIC pattern: thrombocytopenia, prolonged PT and aPTT, low fibrinogen, elevated D-dimer and fibrin degradation products, and schistocytes on the smear.

Iron deficiency: low ferritin, high TIBC, low transferrin saturation; anemia of chronic disease: low TIBC, normal or high ferritin, low saturation.

Reference platelet count 150-400 x 10^9/L; the ESR is nonspecific and rises with anemia, pregnancy, and inflammation.

Gram stain sequence: crystal violet, iodine, alcohol decolorizer, safranin; gram-positive cells retain crystal violet and appear purple, gram-negative cells appear pink.

Staphylococcus aureus is coagulase and catalase positive; streptococci are catalase negative and arranged in chains or pairs.

Streptococcus pneumoniae is optochin sensitive and bile soluble; group A streptococci are bacitracin sensitive; group B streptococci are bacitracin resistant and hippurate positive.

Oxidase-positive organisms include Pseudomonas aeruginosa and Neisseria species; Enterobacterales are oxidase negative and ferment glucose.

MacConkey agar inhibits gram-positive organisms; lactose fermenters such as E. coli form pink colonies, while nonfermenters remain colorless.

Adult blood cultures: collect 2-3 sets totaling 20-30 mL before antibiotics; most clinically significant isolates grow within 5 days.

Bacterial meningitis CSF: neutrophils predominate, glucose is below 40% of serum, and protein is elevated; viral meningitis shows lymphocytes with normal glucose.

MRSA is confirmed by cefoxitin disk, oxacillin MIC, or detection of mecA; ESBL producers resist most penicillins and cephalosporins but remain susceptible to carbapenems.

Primary immune response: IgM appears first after a lag of days; the secondary response is faster, larger, and IgG predominant with higher affinity.

The classical complement pathway is activated by IgG or IgM immune complexes and proceeds C1, C4, C2, C3; the alternative pathway is antibody independent.

Screening assays are optimized for sensitivity; confirmatory assays such as Western blot or nucleic acid tests are optimized for specificity.

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

HIV fourth-generation screens detect p24 antigen plus antibody; reactive results are confirmed with an HIV-1/HIV-2 antibody differentiation immunoassay.

Syphilis: non-treponemal RPR and VDRL are used for screening and monitoring activity; treponemal tests such as FTA-ABS and TP-PA confirm the result.

Anti-dsDNA and anti-Sm are specific for systemic lupus erythematosus; rheumatoid factor and anti-CCP support a diagnosis of rheumatoid arthritis.

PPV = TP/(TP+FP) and NPV = TN/(TN+FN); PPV rises as disease prevalence increases while sensitivity and specificity remain constant.

Urine specific gravity 1.005-1.030 and pH 4.5-8.0; values outside these ranges suggest concentrating defects, diet, or infection.

Dipstick blood detects peroxidase activity of hemoglobin and myoglobin; ascorbic acid can cause false-negative results and delayed reading can cause false-positive blood.

Nitrite positivity suggests nitrate-reducing gram-negative bacteriuria; leukocyte esterase detects pyuria; both negative make urinary tract infection unlikely but do not exclude it.

Red cell casts indicate glomerulonephritis; white cell casts indicate pyelonephritis or interstitial nephritis; broad waxy casts suggest chronic renal failure.

Struvite (triple phosphate) crystals form in alkaline urine with urease-producing organisms such as Proteus; cystine crystals are hexagonal and suggest cystinuria.

Normal CSF: glucose about 60% of serum, protein 15-45 mg/dL, white cells 0-5/microliter, predominantly lymphocytes.

Light's criteria classify pleural fluid as an exudate if the protein ratio exceeds 0.5, the LDH ratio exceeds 0.6, or fluid LDH exceeds two-thirds the upper normal serum LDH.

Gout: needle-shaped monosodium urate crystals with negative birefringence; pseudogout: rhomboid calcium pyrophosphate crystals with positive birefringence.

PCR cycle: denaturation at 94-95°C, primer annealing at 50-65°C, extension at 72°C using thermostable Taq polymerase.

In real-time PCR the cycle threshold is inversely proportional to the starting target quantity; a lower Ct value means more target present.

Sanger sequencing uses dideoxynucleotide chain termination with capillary electrophoresis to read the sequence and detect point mutations or short insertions and deletions.

FISH uses fluorescent probes to detect gene rearrangements, gene amplification, and aneuploidy in interphase cells.

JAK2 V617F supports polycythemia vera and other myeloproliferative neoplasms; BCR-ABL1 confirms chronic myeloid leukemia; F5 R506Q is Factor V Leiden.

NAAT such as PCR detects Chlamydia trachomatis and Neisseria gonorrhoeae in urine or swabs with greater sensitivity than culture.

A no-template control that amplifies indicates contamination and invalidates the run; extraction controls and positive controls monitor efficiency.

In agarose gel electrophoresis, negatively charged DNA migrates toward the anode, and smaller fragments travel farther than larger ones.

Westgard rules: 1-2s warns, 1-3s rejects, 2-2s rejects, R-4s rejects when two controls differ by 4 SD, and 4-1s or 10x reject systematic drift.

CV = (standard deviation / mean) x 100; a lower CV indicates better precision for that analyte.

Sensitivity = TP/(TP+FN); specificity = TN/(TN+FP); PPV = TP/(TP+FP); NPV = TN/(TN+FN).

CLIA test categories are waived, moderate complexity, and high complexity; the laboratory must hold a CLIA certificate matching the tests it performs.

Proficiency testing is required for regulated analytes; unacceptable results require investigation and documented corrective action.

Critical values must be reported promptly to the responsible caregiver and documented with read-back, time, and the name of the person notified.

Specimens are rejected when mislabeled, unlabeled, collected in the wrong tube, grossly hemolyzed, or clotted when anticoagulated blood is required.

The OSHA bloodborne pathogen standard requires engineering controls, hepatitis B vaccination, and personal protective equipment; standard precautions apply to all specimens.