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Medical Laboratory Scientist practice questions

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Question 1Blood Banking

A unit of packed red blood cells is issued for a patient with a historical anti-K. Which crossmatch result would be considered compatible for transfusion of this unit?

  1. AHG crossmatch shows 1+ agglutination that disappears after adding anti-IgG
  2. AHG crossmatch shows no agglutination, and the donor unit is K-positive
  3. Immediate spin crossmatch is negative, and the AHG phase is not performed
  4. AHG crossmatch shows no agglutination, and the donor unit is K-negative Correct answer.

Explanation

A clinically significant antibody such as anti-K requires the donor unit to lack the corresponding antigen and the AHG crossmatch to be negative. Serologic crossmatch alone cannot override a known antibody, so antigen-negative units must be selected.

Question 2Blood Banking

Which ABO type is considered the universal donor for packed red blood cell transfusion?

  1. AB positive
  2. O negative Correct answer.
  3. A positive
  4. B negative

Explanation

Group O red cells lack A and B antigens, and D-negative units avoid sensitization of D-negative recipients. This makes O-negative red cells suitable when the recipient's type is unknown or when supply constraints require it.

Question 3Blood Banking

A patient's serum reacts 3+ with A1 cells and 3+ with B cells at immediate spin, and the patient's own cells type as A. What is the most likely explanation?

  1. A2 subgroup with anti-A1 Correct answer.
  2. ABO typing error caused by cold autoagglutinin
  3. Acquired B antigen
  4. Loss of ABO antigens due to leukemia

Explanation

An A2 individual can produce anti-A1, which reacts with A1 reagent cells but not with the patient's own A2 cells. The reverse grouping shows unexpected anti-A1 reactivity while the forward type remains A.

Question 4Blood Banking

Which test is required before transfusing platelets to prevent transfusion-associated graft-versus-host disease in an at-risk patient?

  1. Use of a bedside leukoreduction filter
  2. Leukoreduction by filtration
  3. Washing the platelet unit with saline
  4. Irradiation of the platelet unit Correct answer.

Explanation

Gamma irradiation inactivates donor T lymphocytes, preventing transfusion-associated graft-versus-host disease. Leukoreduction reduces but does not eliminate viable T cells, so it does not substitute for irradiation.

Question 5Blood Banking

A patient with a history of anaphylactic reaction to IgA-containing blood products requires transfusion. Which product is most appropriate?

  1. IgA-deficient plasma
  2. Leukoreduced red blood cells
  3. Platelets stored in plasma
  4. Washed red blood cells Correct answer.

Explanation

Washing removes residual plasma proteins, including IgA, from cellular components. For patients with severe IgA deficiency and anti-IgA, washed red cells reduce the risk of anaphylaxis.

Question 6Blood Banking

What is the primary purpose of performing a direct antiglobulin test (DAT) on a patient with suspected autoimmune hemolytic anemia?

  1. To confirm the ABO type of the patient
  2. To detect antibody or complement coating the patient's red cells Correct answer.
  3. To measure the titer of anti-A and anti-B
  4. To identify the specificity of antibody in the patient's serum

Explanation

The DAT detects in vivo sensitization of red cells with IgG or complement. It is used to evaluate immune-mediated hemolysis, not to identify serum antibody specificity or ABO type.

Question 7Blood Banking

A unit of red cells is issued, but the patient develops fever and hypotension during transfusion. Which action should be taken first?

  1. Administer acetaminophen and continue the transfusion
  2. Slow the transfusion and observe
  3. Return the unit to the blood bank after completion
  4. Stop the transfusion and maintain intravenous access Correct answer.

Explanation

Any suspected acute transfusion reaction requires immediate cessation of the transfusion while intravenous access is maintained. Stopping the unit limits the dose of incompatible blood and allows evaluation for hemolysis or sepsis.

Question 8Blood Banking

Which antibody is most commonly associated with delayed hemolytic transfusion reactions?

  1. anti-K
  2. anti-Jka Correct answer.
  3. Anti-I
  4. anti-Lea

Explanation

Anti-Jka is a common cause of delayed hemolytic transfusion reactions because the antibody may fall below detectable levels before re-exposure. An anamnestic response then produces rapid hemolysis of transfused antigen-positive cells.

Question 9Blood Banking

A patient's plasma reacts with all panel cells at the AHG phase, and the autocontrol is also positive. What is the most appropriate next step?

  1. Report the antibody as an alloantibody of undetermined specificity
  2. Select units that are crossmatch compatible by immediate spin only
  3. Perform an eluate and consider an autoantibody workup Correct answer.
  4. Transfuse O-negative units without further testing

Explanation

Panreactivity with a positive autocontrol suggests an autoantibody or other non-specific reactivity rather than a defined alloantibody. An eluate and additional workup help distinguish autoantibody from alloantibody and guide unit selection.

Question 10Blood Banking

What is the acceptable storage temperature for whole blood and red blood cell units?

  1. 1 to 6 degrees Celsius Correct answer.
  2. 37 degrees Celsius
  3. 20 to 24 degrees Celsius
  4. -18 degrees Celsius or colder

Explanation

Red cells and whole blood are stored at 1 to 6 degrees Celsius to slow metabolism and reduce bacterial growth. This range preserves red cell viability during the licensed storage period.

Question 11Blood Banking

A group O patient's serum shows weak reactivity with A1 cells and no reactivity with B cells. What is the most likely cause?

  1. Weak D phenotype Correct answer.
  2. Anti-A1 in a group O patient
  3. Acquired B antigen
  4. A2 subgroup

Explanation

A weak D phenotype affects Rh typing, not ABO reverse grouping. Weak or missing anti-A in a group O patient is more consistent with an immune deficiency or age-related change, so weak D is not the cause of the ABO finding.

Question 12Blood Banking

Which component is indicated for a patient with severe thrombocytopenia and active bleeding when platelets are unavailable?

  1. Packed red blood cells
  2. Cryoprecipitate Correct answer.
  3. Granulocyte concentrate
  4. Fresh frozen plasma

Explanation

Cryoprecipitate contains fibrinogen and factor VIII and is sometimes used to support hemostasis when platelets are unavailable. It is not a platelet substitute but provides clotting factors relevant to bleeding.

Question 13Blood Banking

What is the primary purpose of performing a minor crossmatch in some transfusion services?

  1. To confirm the ABO type of the recipient
  2. To detect antibodies in the donor unit that could react with recipient red cells Correct answer.
  3. To detect antibodies in the recipient serum against donor red cells
  4. To measure the hematocrit of the donor unit

Explanation

The minor crossmatch tests donor plasma against recipient red cells to detect donor antibodies that could cause hemolysis. It is not routinely required for red cell transfusion but may be used in specific situations.

Question 14Blood Banking

A patient with sickle cell disease requires chronic transfusion therapy. Which antigen matching strategy is recommended to reduce alloimmunization?

  1. Extended matching for common Rh and Kell antigens Correct answer.
  2. ABO matching only
  3. Crossmatch by immediate spin only
  4. ABO and D matching only

Explanation

Patients with sickle cell disease have a high risk of alloimmunization, so extended matching for Rh and Kell antigens is recommended. This reduces formation of antibodies against common antigens.

Question 15Blood Banking

Which of the following is a characteristic of the Rh blood group system?

  1. Antibodies are typically IgG and require exposure for formation Correct answer.
  2. Antibodies are naturally occurring IgM
  3. Antigens are present on the red cell membrane
  4. Antigens are soluble in plasma

Explanation

Rh antibodies are generally IgG and are formed after exposure to foreign Rh antigens through transfusion or pregnancy. They do not occur naturally and are not typically IgM.

Question 16Blood Banking

A unit of platelets is stored at 20 to 24 degrees Celsius with continuous agitation. What is the main reason for agitation?

  1. To activate platelets before transfusion
  2. To increase the oxygen content of the bag
  3. To prevent bacterial growth
  4. To maintain platelet viability and prevent clumping Correct answer.

Explanation

Continuous agitation maintains platelet viability by promoting gas exchange and preventing clumping. Platelets are stored at room temperature, so agitation is essential to preserve function.

Question 17Blood Banking

A patient's serum contains anti-D. Which red cell unit should be selected?

  1. D-positive unit
  2. D-negative unit only if the crossmatch is positive
  3. D-negative unit Correct answer.
  4. D-positive unit after washing

Explanation

Anti-D is a clinically significant antibody, so D-negative units must be selected. Transfusing D-positive cells would risk a hemolytic transfusion reaction.

Question 18Blood Banking

Which test is used to detect fetal-maternal hemorrhage and determine the dose of Rh immune globulin?

  1. Antibody titration
  2. Osmotic fragility test
  3. Kleihauer-Betke test Correct answer.
  4. Direct antiglobulin test

Explanation

The Kleihauer-Betke test detects fetal hemoglobin in maternal blood and estimates the volume of fetal-maternal hemorrhage. This estimate guides the dose of Rh immune globulin.

Question 19Blood Banking

A patient with a known anti-M requires transfusion. Which crossmatch approach is most appropriate?

  1. Immediate spin crossmatch only
  2. No crossmatch is needed because anti-M is not clinically significant
  3. Crossmatch with M-positive units if the antibody titer is low
  4. AHG crossmatch with M-negative units Correct answer.

Explanation

Anti-M can be clinically significant, so M-negative units and an AHG crossmatch are appropriate. Immediate-spin testing alone may miss IgG anti-M reactivity.

Question 20Clinical Chemistry

A serum sample shows a lipemic index above the assay's acceptable limit for a photometric assay. Which type of interference is most directly indicated?

  1. Light scattering from lipid particles that falsely elevates absorbance Correct answer.
  2. Competitive displacement of antibody from the solid phase
  3. Electrochemical interference at the ion-selective membrane
  4. Chemical interference from hemolysis releasing intracellular analytes

Explanation

Lipemia produces turbidity that scatters light in photometric measurements, raising apparent absorbance and biasing results. Hemolysis and icterus are separate index-based interferences with different mechanisms. Recognizing the specific mechanism guides whether to ultracentrifuge, dilute, or use an alternate method.

Question 21Clinical Chemistry

Which specimen handling practice best prevents in vitro hemolysis before serum chemistry analysis?

  1. Drawing through a small-gauge needle to slow the flow
  2. Using a 21-gauge needle and promptly separating serum after clotting Correct answer.
  3. Vigorously shaking the tube to speed clot formation
  4. Allowing the sample to clot on the clot activator for 2 hours before centrifugation

Explanation

Hemolysis is minimized by using an appropriately sized needle, avoiding excessive vacuum and turbulence, and separating serum promptly after complete clotting. Small-bore needles and vigorous shaking increase shear stress on red cells. Prolonged contact with the clot can also alter analyte concentrations.

Question 22Clinical Chemistry

In the Jaffe reaction for creatinine, which substance is the principal endogenous interferent that produces a positive bias?

  1. Glucose Correct answer.
  2. Urea
  3. Ascorbic acid
  4. Bilirubin

Explanation

The classic Jaffe method measures a chromogen formed when picrate reacts with creatinine, but glucose is a well-known non-creatinine chromogen that contributes a positive bias. Modern assays use kinetic reading or enzymatic methods to reduce this interference. Bilirubin and ascorbic acid tend to interfere in the negative direction.

Question 23Clinical Chemistry

An enzymatic glucose assay based on glucose oxidase and peroxidase is run on a severely icteric sample. What is the most likely effect of high bilirubin on the measured result?

  1. No effect because bilirubin is removed during deproteinization
  2. A falsely low result only if the sample is also lipemic
  3. Falsely high glucose due to increased chromogen absorbance
  4. Falsely low glucose due to bilirubin competing for peroxide Correct answer.

Explanation

In peroxidase-coupled Trinder-type reactions, bilirubin can compete for hydrogen peroxide and reduce chromogen formation, producing a negative bias. Interference depends on the specific chromogen and method. Laboratories should evaluate icteric samples against their assay's documented interference limits.

Question 24Clinical Chemistry

A patient's total calcium is 8.2 mg/dL and albumin is 2.0 g/dL. Using the conventional correction factor of 0.8 mg/dL per 1 g/dL albumin below 4.0, what is the albumin-corrected calcium?

  1. 8.6 mg/dL
  2. 9.8 mg/dL Correct answer.
  3. 10.6 mg/dL
  4. 7.4 mg/dL

Explanation

The correction adds 0.8 mg/dL for each 1 g/dL that albumin falls below 4.0. Here albumin is 2.0 g/dL, a deficit of 2.0 g/dL, so 2.0 × 0.8 = 1.6 mg/dL is added to 8.2 mg/dL, giving 9.8 mg/dL. Corrected values estimate the physiologically active fraction but are approximations.

Question 25Clinical Chemistry

In an arterial blood gas result, which parameter is measured directly by the blood gas analyzer rather than calculated?

  1. pCO2 Correct answer.
  2. Bicarbonate
  3. Anion gap
  4. Base excess

Explanation

Modern blood gas analyzers directly measure pH and pCO2 with electrodes, and often pO2 as well. Bicarbonate is typically calculated from the Henderson-Hasselbalch relationship, base excess is derived, and the anion gap is a calculated value from multiple analytes.

Question 26Clinical Chemistry

A blood gas sample is analyzed 45 minutes after collection without ice. Which change is expected in the measured values?

  1. pO2 increases and pCO2 decreases
  2. pCO2 decreases and bicarbonate increases
  3. pH increases and lactate decreases
  4. pH decreases and pCO2 increases Correct answer.

Explanation

Continued cellular metabolism in a delayed sample consumes oxygen and produces carbon dioxide and lactic acid. The accumulating CO2 raises pCO2 and the acid production lowers pH. Prompt analysis or ice immersion slows but does not entirely stop these changes.

Question 27Clinical Chemistry

Which lipoprotein fraction is measured when a laboratory reports LDL cholesterol using the Friedewald calculation?

  1. A value derived from apolipoprotein B concentration
  2. A directly measured LDL particle concentration
  3. A calculated value derived from total cholesterol, HDL, and triglycerides Correct answer.
  4. A value obtained by ultracentrifugation of the LDL fraction

Explanation

The Friedewald formula estimates LDL cholesterol as total cholesterol minus HDL cholesterol minus triglycerides divided by 5, when triglycerides are expressed in mg/dL. It is a calculation, not a direct measurement, and becomes unreliable at high triglyceride concentrations. Direct assays and apolipoprotein B measurement are alternatives.

Question 28Clinical Chemistry

A Friedewald-calculated LDL cholesterol is requested on a sample with triglycerides of 520 mg/dL. What is the most appropriate interpretation?

  1. The calculated LDL is reliable if the total cholesterol is above 200 mg/dL
  2. The calculated LDL is reliable because HDL was measured directly
  3. The calculated LDL is invalid at this triglyceride level and a direct method should be considered Correct answer.
  4. The calculated LDL may be falsely low because of triglyceride interference in the formula

Explanation

The Friedewald formula assumes a fixed triglyceride-to-VLDL cholesterol ratio, which fails at high triglycerides, so the calculated LDL is unreliable. Laboratories typically do not report Friedewald LDL above approximately 400 mg/dL triglycerides. A direct homogeneous assay or alternative approach is preferred.

Question 29Clinical Chemistry

Which enzyme is most specific for hepatocellular injury rather than skeletal muscle injury?

  1. Aspartate aminotransferase
  2. Alanine aminotransferase Correct answer.
  3. Lactate dehydrogenase
  4. Creatine kinase

Explanation

Alanine aminotransferase is concentrated in hepatocytes and is relatively more liver-specific than aspartate aminotransferase, which is also abundant in muscle and red cells. Creatine kinase is a muscle marker. Lactate dehydrogenase is widely distributed and lacks specificity.

Question 30Clinical Chemistry

A patient with suspected acute pancreatitis has a lipase activity twice the upper reference limit. How does lipase compare with amylase for diagnostic specificity?

  1. Both enzymes are equally specific and interchangeable
  2. Amylase is more specific because it is produced only by the pancreas
  3. Lipase is more specific for pancreatic disease and remains elevated longer Correct answer.
  4. Lipase is less specific because it also rises in renal failure only

Explanation

Lipase is regarded as more pancreas-specific than amylase, which also originates from salivary glands and other tissues. Lipase also tends to remain elevated longer after an episode, improving diagnostic sensitivity in later presentations. Neither enzyme magnitude reliably predicts severity.

Question 31Clinical Chemistry

A troponin result is reported as 0.04 ng/mL with a 99th percentile upper reference limit of 0.02 ng/mL and an assay 10% coefficient of variation at 0.06 ng/mL. How should this result be interpreted?

  1. It is a false positive because it is below the 10% CV concentration
  2. It is diagnostic of myocardial infarction without serial testing
  3. It is below the limit of detection and should be reported as zero
  4. It exceeds the 99th percentile but lies below the assay's functional limit of quantitation Correct answer.

Explanation

High-sensitivity troponin interpretation requires comparison with the 99th percentile upper reference limit and awareness of the functional limit of quantitation, the lowest concentration measurable with a 10% CV. A value above the 99th percentile but below that limit indicates myocardial injury but has greater analytical uncertainty. Serial testing and clinical context are required for infarction diagnosis.

Question 32Clinical Chemistry

Which condition most commonly causes a falsely elevated serum potassium result in a non-hemolyzed specimen?

  1. Prolonged tourniquet application with fist clenching Correct answer.
  2. Collection in a tube containing EDTA
  3. Immediate centrifugation after clotting
  4. Storage of serum at 4 degrees Celsius

Explanation

Prolonged tourniquet use and fist clenching raise local potassium release from muscle, producing pseudohyperkalemia. EDTA contamination is another cause but typically also lowers calcium and raises EDTA-related changes. Recognizing preanalytical causes prevents inappropriate treatment.

Question 33Clinical Chemistry

An ion-selective electrode for sodium is calibrated with aqueous standards but patient samples are measured in a protein-rich matrix. Which phenomenon can cause a systematic bias?

  1. The liquid junction potential of the internal reference
  2. The Donnan equilibrium effect on red cells
  3. The Nernstian slope exceeding 59 mV per decade
  4. The protein error from the sample matrix Correct answer.

Explanation

Matrix differences between aqueous calibrators and protein-containing samples can produce a bias known as the protein error in some direct ISE systems. Modern direct ISEs are designed to minimize this, but the principle explains why matrix-matched comparison is important. The Nernstian slope and junction potential are separate analytical considerations.

Question 34Clinical Chemistry

Which formula correctly expresses the anion gap using routinely measured electrolytes?

  1. Na+ − (K+ + Cl−)
  2. (Na+ + K+) − (Cl− + HCO3−)
  3. Na+ − (Cl− + HCO3−) Correct answer.
  4. (Na+ + Cl−) − (K+ + HCO3−)

Explanation

The anion gap is conventionally calculated as sodium minus the sum of chloride and bicarbonate, representing unmeasured anions. Some laboratories include potassium, but the standard formula omits it. An elevated gap suggests accumulation of unmeasured anions such as lactate or ketones.

Question 35Clinical Chemistry

A patient with diabetic ketoacidosis has a pH of 7.20, pCO2 of 25 mmHg, and bicarbonate of 10 mmol/L. Which acid-base disturbance is present?

  1. Mixed metabolic acidosis and metabolic alkalosis
  2. Primary respiratory acidosis with metabolic alkalosis
  3. Primary respiratory alkalosis with metabolic compensation
  4. Primary metabolic acidosis with respiratory compensation Correct answer.

Explanation

The low pH and low bicarbonate indicate a primary metabolic acidosis, and the low pCO2 reflects hyperventilation as respiratory compensation. Expected compensation can be checked with Winter's formula. A mixed disorder would show compensation outside the predicted range.

Question 36Clinical Chemistry

Which preanalytical factor most strongly affects measured plasma ammonia concentration?

  1. Exposure of the sample to light
  2. Use of a lithium heparin tube
  3. Delay in separation of plasma from red cells Correct answer.
  4. Collection after an overnight fast

Explanation

Ammonia concentrations rise rapidly in whole blood because red cells and platelets release ammonia during storage. Plasma should be separated promptly and the sample kept cold or analyzed quickly. Light exposure and fasting status are not the principal concerns for ammonia.

Question 37Clinical Chemistry

In a two-point calibration of a spectrophotometric assay, what is the primary purpose of the blank measurement?

  1. To establish the slope of the calibration curve
  2. To verify the wavelength accuracy of the monochromator
  3. To determine the assay's limit of detection
  4. To set the absorbance contributed by reagents and cuvette to zero Correct answer.

Explanation

A reagent or sample blank corrects for absorbance from reagents, cuvette, and matrix so that the measured signal reflects the analyte. The calibrator, not the blank, establishes the slope. Limit of detection and wavelength accuracy require separate experiments.

Question 38Clinical Chemistry

A serum sample has a total protein of 9.8 g/dL, and the laboratory suspects a paraprotein. On serum protein electrophoresis at pH 8.6, in which region would a monoclonal IgG paraprotein most typically produce a discrete, narrow band?

  1. Prealbumin region
  2. Albumin region
  3. Gamma region Correct answer.
  4. Alpha-1 region

Explanation

At pH 8.6 serum proteins are negatively charged and migrate toward the anode, with albumin moving fastest and immunoglobulins slowest. Most monoclonal immunoglobulins, including IgG, are cathodal and appear as a restricted, dense band in the gamma region, which is the classic M-spike pattern. A narrow band elsewhere would be unusual and should prompt consideration of other protein abnormalities.

Question 39Coagulation

Which coagulation test is most appropriate for monitoring a patient receiving continuous unfractionated heparin therapy?

  1. Prothrombin time (PT)
  2. Activated partial thromboplastin time (aPTT) Correct answer.
  3. Thrombin time (TT)
  4. Fibrinogen assay

Explanation

Unfractionated heparin potentiates antithrombin and prolongs the intrinsic and common pathways, which the aPTT measures, so aPTT is the standard monitoring test. PT reflects the extrinsic pathway and is insensitive to therapeutic heparin. TT is affected by heparin but is not used for dose titration because it is too sensitive and nonspecific.

Question 40Coagulation

A citrated plasma sample for routine coagulation testing is received in the laboratory 5 hours after collection and has been stored at room temperature. What is the most appropriate action?

  1. Recalibrate the analyzer and proceed
  2. Reject the sample and request a new collection Correct answer.
  3. Centrifuge twice and freeze the plasma
  4. Analyze immediately and note the delay

Explanation

Uncentrifuged citrated whole blood for coagulation testing is stable for only about 4 hours at room temperature, so a 5-hour-old sample may give falsely prolonged or variable results. The standard practice is to reject the specimen and request a fresh collection. Analyzing it with a comment reports a potentially invalid result.

Question 41Coagulation

Which plasma protein is the precursor of fibrin and the substrate measured in a Clauss fibrinogen assay?

  1. Plasminogen
  2. Factor XIII
  3. Fibrinogen Correct answer.
  4. Prothrombin

Explanation

Fibrinogen (factor I) is cleaved by thrombin to form fibrin monomers that polymerize into the clot. The Clauss method measures clottable fibrinogen by adding excess thrombin and timing clot formation. Prothrombin is a zymogen of thrombin, factor XIII cross-links fibrin, and plasminogen is the precursor of the fibrinolytic enzyme.

Question 42Coagulation

In the coagulation cascade, which factor is the only one that circulates as an active serine protease rather than as a zymogen?

  1. Factor VIII
  2. Factor X
  3. Factor VII Correct answer.
  4. Factor IX

Explanation

Factor VII circulates in plasma as an active enzyme (about 1% of it in the active form), which allows rapid initiation of the extrinsic pathway upon binding tissue factor. Factors VIII, IX, and X circulate as inactive zymogens requiring proteolytic activation. This property makes factor VII the trigger of coagulation in vivo.

Question 43Coagulation

Which finding characterizes the laboratory profile of disseminated intravascular coagulation (DIC)?

  1. Shortened PT and aPTT with elevated fibrinogen and normal D-dimer
  2. Prolonged PT with normal aPTT and isolated factor VIII deficiency
  3. Prolonged PT and aPTT with decreased fibrinogen and elevated D-dimer Correct answer.
  4. Normal PT and aPTT with isolated thrombocytopenia and normal D-dimer

Explanation

DIC consumes clotting factors and fibrinogen while generating cross-linked fibrin that is degraded, producing elevated D-dimer. The result is prolonged PT and aPTT, low fibrinogen, thrombocytopenia, and raised D-dimer. This consumptive profile distinguishes DIC from isolated factor deficiencies or simple thrombocytopenia.

Question 44Coagulation

A patient on warfarin has an INR of 5.8 and no active bleeding. Which interpretation of the INR is correct?

  1. It is a ratio of thrombin time to a normal plasma and monitors fibrinogen
  2. It reflects the extrinsic pathway and is used to monitor vitamin K antagonist therapy Correct answer.
  3. It is a platelet function measure used to monitor antiplatelet drugs
  4. It reflects the intrinsic pathway and is used to monitor heparin therapy

Explanation

The INR is derived from the PT, which measures the extrinsic and common pathways, and is standardized against a reference thromboplastin to monitor vitamin K antagonists. Warfarin reduces functional factors II, VII, IX, and X, and factor VII has the shortest half-life, so the PT/INR rises first. It does not monitor heparin or platelet function.

Question 45Coagulation

A mixing study is performed on a patient with a prolonged aPTT. The aPTT corrects into the reference range immediately after mixing with normal plasma and remains corrected after 2 hours of incubation at 37°C. What is the most likely cause?

  1. A specific factor VIII inhibitor
  2. Heparin contamination of the sample
  3. A lupus anticoagulant
  4. A factor VIII deficiency Correct answer.

Explanation

Immediate and sustained correction with normal plasma indicates a factor deficiency, because normal plasma supplies the missing factor. A lupus anticoagulant or a specific inhibitor typically fails to correct or shows time-dependent correction on incubation. Heparin contamination would not be corrected by simple mixing in this pattern and usually prolongs the thrombin time as well.

Question 46Coagulation

Which laboratory finding best distinguishes von Willebrand disease from mild hemophilia A?

  1. Prolonged bleeding time or PFA closure time with reduced von Willebrand factor activity Correct answer.
  2. Prolonged thrombin time with normal fibrinogen
  3. Selectively prolonged PT with normal aPTT
  4. Reduced factor IX activity with normal factor VIII activity

Explanation

von Willebrand disease impairs platelet adhesion, so platelet-function tests such as the bleeding time or PFA closure time are prolonged, and von Willebrand factor antigen and activity are reduced. Hemophilia A shows reduced factor VIII activity with normal vWF and normal platelet function. Both may prolong the aPTT, so vWF testing is the discriminator.

Question 47Coagulation

A patient with severe liver disease has a prolonged PT and aPTT. Which additional finding is most consistent with impaired hepatic synthesis of coagulation factors?

  1. Elevated factor VIII activity with low fibrinogen
  2. Isolated prolonged thrombin time with normal factor levels
  3. Decreased factor VII activity with normal platelet count Correct answer.
  4. Normal factor VII activity with elevated D-dimer

Explanation

Factor VII has the shortest half-life among the vitamin K–dependent factors, so its activity falls early when hepatic synthesis is impaired, prolonging the PT first. Fibrinogen and other factors decline with progressive disease. Elevated factor VIII is more typical of an acute-phase response and does not indicate impaired synthesis.

Question 48Coagulation

Which preanalytical error most commonly causes a falsely prolonged aPTT due to an underfilled citrate tube?

  1. Reduced citrate concentration causing premature clotting
  2. Hemolysis releasing platelet factor 4
  3. Excess citrate diluting the plasma and lowering calcium availability Correct answer.
  4. Excess thrombin generation from contact activation

Explanation

Citrate tubes must be filled to the marked volume to maintain the correct blood-to-anticoagulant ratio. Underfilling leaves excess citrate relative to plasma, which binds more calcium and prolongs clotting times such as the aPTT and PT. Overfilling or short draws are therefore rejected according to laboratory criteria.

Question 49Coagulation

Which test is most specific for detecting the presence of a lupus anticoagulant?

  1. Thrombin time with a reptilase correction
  2. Fibrinogen assay by the Clauss method
  3. Prothrombin time with a 1:1 mixing study
  4. Dilute Russell viper venom time with a phospholipid neutralization step Correct answer.

Explanation

The dilute Russell viper venom time (dRVVT) directly activates factor X and is sensitive to phospholipid-dependent inhibitors. Confirmation with a phospholipid neutralization step demonstrates that the prolongation is due to an antiphospholipid antibody. The PT, thrombin time, and fibrinogen assay are not designed to detect or confirm lupus anticoagulants.

Question 50Coagulation

A patient with a known factor VIII inhibitor has a prolonged aPTT that does not correct on mixing. Which additional test best quantifies the strength of the inhibitor?

  1. Euglobulin lysis time
  2. Bethesda assay with serial dilutions of patient plasma Correct answer.
  3. PT with a 1:1 mix
  4. Platelet aggregation with ristocetin

Explanation

The Bethesda assay incubates serial dilutions of patient plasma with normal plasma and measures residual factor VIII activity, with one Bethesda unit defined as the dilution that leaves 50% residual activity. This quantifies inhibitor titer and guides therapy. The other tests do not measure inhibitor strength.