CLS ASCP + CALIFORNIA LAW EXAMINATION
200 PRACTICE QUESTIONS WITH ANSWERS AND
RATIONALES
2026–2027 TESTING CYCLE
PART 1: ASCP MLS (ASCP) EXAMINATION (Questions 1–150)
SECTION 1: BLOOD BANKING (IMMUNOHEMATOLOGY) (Questions 1–25)
Question 1: A patient is typed as Group A, Rh(D)-negative. Which antibody is present in
this patient's serum?
Answer: Anti-B.
Rationale: A Group A individual has A antigens on their red blood cells and naturally
occurring anti-B antibodies in their serum. The patient does not have anti-A, as they have A
antigens. The Rh(D)-negative status does not affect the ABO antibodies present. Anti-B is an
IgM antibody that can cause hemolytic transfusion reactions if the patient receives group B or
AB red cells.
Question 2: A patient's blood type is AB positive. Which of the following blood types can
this patient receive?
,Answer: All blood types (A, B, AB, O, Rh positive or negative).
Rationale: AB positive individuals have no antibodies against A or B antigens and no
antibodies against the Rh(D) antigen. They are the universal recipients. Type O negative is the
universal donor. While AB positive patients can receive any blood type, they should ideally
receive AB positive blood to conserve O negative units for emergencies.
Question 3: The gold standard for crossmatching blood before transfusion is:
Answer: The antiglobulin (Coombs) crossmatch.
Rationale: The antiglobulin crossmatch (also called the indirect Coombs crossmatch) detects
clinically significant unexpected antibodies in the patient's serum that could react with donor
red blood cells. It is the most sensitive crossmatch method and is required for patients with a
history of antibodies or transfusion reactions.
Question 4: A patient has a positive direct antiglobulin test (DAT). This indicates:
Answer: Antibodies are bound to the patient's red blood cells in vivo.
Rationale: A positive DAT indicates that antibodies and/or complement are bound to the
patient's red blood cells in the body. This can occur in autoimmune hemolytic anemia,
hemolytic transfusion reactions, or hemolytic disease of the newborn. The indirect
antiglobulin test (IAT) detects antibodies in the patient's serum, not on the cells.
,Question 5: The most common cause of hemolytic disease of the newborn (HDN) is:
Answer: Rh(D) incompatibility.
Rationale: Rh(D) incompatibility occurs when an Rh-negative mother carries an Rh-positive
fetus. Maternal anti-D antibodies can cross the placenta and cause hemolysis in the fetus.
ABO incompatibility can also cause HDN but is usually less severe. Kell and other antibodies
can also cause HDN but are less common.
Question 6: A patient has a positive antibody screen. The next step is:
Answer: Antibody identification using a panel of reagent red cells.
Rationale: A positive antibody screen indicates the presence of unexpected antibodies. The
next step is to identify the specific antibody(ies) using a panel of reagent red cells with known
antigen profiles. Once identified, compatible blood (negative for the corresponding antigen)
must be selected for crossmatching.
Question 7: Which of the following is a characteristic of the Rh blood group system?
Answer: Rh antigens are proteins; antibodies are typically IgG and can cause HDN.
Rationale: Rh antigens are integral membrane proteins (not carbohydrates like ABO
antigens). Rh antibodies are typically IgG and can cross the placenta, causing hemolytic
, disease of the newborn. Rh antibodies are clinically significant and can cause hemolytic
transfusion reactions.
Question 8: The reagent used for Rh(D) typing is:
Answer: Anti-D serum (monoclonal or polyclonal).
Rationale: Anti-D serum is used to detect the presence of the D antigen on red blood cells.
Monoclonal anti-D reagents are commonly used. A positive reaction indicates the patient is
Rh(D)-positive; a negative reaction indicates Rh(D)-negative. Weak D testing may be required
for donors or in some patient populations.
Question 9: A patient is group O, Rh(D)-negative. Which blood type is the patient's
"compatible" red blood cell type?
Answer: O negative.
Rationale: O negative individuals have anti-A, anti-B, and anti-D (if previously sensitized)
antibodies. They can only receive O negative red blood cells. Transfusion of any other blood
type would cause a hemolytic transfusion reaction. Type O negative blood is the universal
donor and is reserved for emergencies.
Question 10: The principle of the indirect antiglobulin test (IAT) is:
200 PRACTICE QUESTIONS WITH ANSWERS AND
RATIONALES
2026–2027 TESTING CYCLE
PART 1: ASCP MLS (ASCP) EXAMINATION (Questions 1–150)
SECTION 1: BLOOD BANKING (IMMUNOHEMATOLOGY) (Questions 1–25)
Question 1: A patient is typed as Group A, Rh(D)-negative. Which antibody is present in
this patient's serum?
Answer: Anti-B.
Rationale: A Group A individual has A antigens on their red blood cells and naturally
occurring anti-B antibodies in their serum. The patient does not have anti-A, as they have A
antigens. The Rh(D)-negative status does not affect the ABO antibodies present. Anti-B is an
IgM antibody that can cause hemolytic transfusion reactions if the patient receives group B or
AB red cells.
Question 2: A patient's blood type is AB positive. Which of the following blood types can
this patient receive?
,Answer: All blood types (A, B, AB, O, Rh positive or negative).
Rationale: AB positive individuals have no antibodies against A or B antigens and no
antibodies against the Rh(D) antigen. They are the universal recipients. Type O negative is the
universal donor. While AB positive patients can receive any blood type, they should ideally
receive AB positive blood to conserve O negative units for emergencies.
Question 3: The gold standard for crossmatching blood before transfusion is:
Answer: The antiglobulin (Coombs) crossmatch.
Rationale: The antiglobulin crossmatch (also called the indirect Coombs crossmatch) detects
clinically significant unexpected antibodies in the patient's serum that could react with donor
red blood cells. It is the most sensitive crossmatch method and is required for patients with a
history of antibodies or transfusion reactions.
Question 4: A patient has a positive direct antiglobulin test (DAT). This indicates:
Answer: Antibodies are bound to the patient's red blood cells in vivo.
Rationale: A positive DAT indicates that antibodies and/or complement are bound to the
patient's red blood cells in the body. This can occur in autoimmune hemolytic anemia,
hemolytic transfusion reactions, or hemolytic disease of the newborn. The indirect
antiglobulin test (IAT) detects antibodies in the patient's serum, not on the cells.
,Question 5: The most common cause of hemolytic disease of the newborn (HDN) is:
Answer: Rh(D) incompatibility.
Rationale: Rh(D) incompatibility occurs when an Rh-negative mother carries an Rh-positive
fetus. Maternal anti-D antibodies can cross the placenta and cause hemolysis in the fetus.
ABO incompatibility can also cause HDN but is usually less severe. Kell and other antibodies
can also cause HDN but are less common.
Question 6: A patient has a positive antibody screen. The next step is:
Answer: Antibody identification using a panel of reagent red cells.
Rationale: A positive antibody screen indicates the presence of unexpected antibodies. The
next step is to identify the specific antibody(ies) using a panel of reagent red cells with known
antigen profiles. Once identified, compatible blood (negative for the corresponding antigen)
must be selected for crossmatching.
Question 7: Which of the following is a characteristic of the Rh blood group system?
Answer: Rh antigens are proteins; antibodies are typically IgG and can cause HDN.
Rationale: Rh antigens are integral membrane proteins (not carbohydrates like ABO
antigens). Rh antibodies are typically IgG and can cross the placenta, causing hemolytic
, disease of the newborn. Rh antibodies are clinically significant and can cause hemolytic
transfusion reactions.
Question 8: The reagent used for Rh(D) typing is:
Answer: Anti-D serum (monoclonal or polyclonal).
Rationale: Anti-D serum is used to detect the presence of the D antigen on red blood cells.
Monoclonal anti-D reagents are commonly used. A positive reaction indicates the patient is
Rh(D)-positive; a negative reaction indicates Rh(D)-negative. Weak D testing may be required
for donors or in some patient populations.
Question 9: A patient is group O, Rh(D)-negative. Which blood type is the patient's
"compatible" red blood cell type?
Answer: O negative.
Rationale: O negative individuals have anti-A, anti-B, and anti-D (if previously sensitized)
antibodies. They can only receive O negative red blood cells. Transfusion of any other blood
type would cause a hemolytic transfusion reaction. Type O negative blood is the universal
donor and is reserved for emergencies.
Question 10: The principle of the indirect antiglobulin test (IAT) is:
