Blood Bank Principles Review

Gel Testing and Column Agglutination Technology

• Overview and Application:

  • Gel testing (column agglutination) is utilized for ABO blood typing, Rh phenotyping, antibody screening, and Direct Antiglobulin Testing (DAT).

  • Gel cards contain dedicated microtubes/wells pre-filled with specific reagents or gel matrices:

    • ABO Cards: Contain anti-A (faint blue color) and anti-B (faint yellow color) reagents within the gel microtubes.

    • IgG Cards: Used for antibody screening and DAT.

• Microtube Structure and Process:

  • Each gel card typically contains $6$ microtubes/wells.

  • Each tube consists of an upper mixing cup and a lower column containing a gel matrix.

  • Red blood cells and plasma/antisera are pipetted into the upper reaction chamber (mixing cup).

  • The card is incubated (if required) and centrifuged in a specialized gel card centrifuge.

  • During centrifugation, red blood cells are forced down through the gel matrix.

• Grading Reaction Strengths ($0$ to $4+$ and Mixed Field):

  • Grade $0$ (Negative): Unagglutinated red blood cells pass entirely through the gel matrix during centrifugation, forming a tight, solid cell pellet/button at the very bottom of the microtube.

  • Grade $1+$: Most red blood cells pass near the bottom, but a small portion remains dispersed slightly above the bottom pellet.

  • Grade $2+$: Agglutinated red blood cells are interspersed throughout the entire length of the gel matrix column.

  • Grade $3+$: Agglutinated red blood cells are distributed heavily throughout the upper half of the gel matrix column.

  • Grade $4+$: Red blood cell agglutinates remain trapped in a distinct, solid band at the top surface of the gel matrix, with no cell penetration into the column.

  • Mixed Field ($\text{MF}$)**: Characterized by a distinct dual-population reaction consisting of a line of agglutinated red cells trapped at the top of the gel matrix along with a solid pellet of unagglutinated cells at the bottom of the microtube. Mixed field reactions are substantially easier to detect in gel cards than in traditional tube agglutination.

• Advantages of Gel Testing:

  • Standardized testing procedures and endpoint stability.

  • Clear, well-defined, and consistent agglutination reactions.

  • Long-term reaction stability: Completed gel card reactions remain stable without changing for long periods (cards can be retained for $2$ to $4$ years as stable records or visual controls).

Microplate Agglutination Testing

• System Configuration:

  • Microplates contain $96$ small, flat-bottomed or U-bottomed wells.

  • Historically utilized in high-throughput donor testing environments (e.g., Blood Services donor testing).

• Procedure:

  • Red blood cells and specific antiserum are added directly to individual microplate wells.

  • The microplate is centrifuged in a specialized carrier centrifuge that spins the plates perfectly flat.

  • Following centrifugation, the plate is tilted at a slight angle to allow unagglutinated cells to flow.

• Interpretation and Reporting:

  • Positive Reaction: Agglutinated red blood cells remain as a concentrated, solid button in the center of the well.

  • Negative Reaction: Unagglutinated red blood cells stream downward from the center button toward the sidewall of the tilted well.

  • Reporting Protocol: Microplate results are reported strictly as binary positive ($+\text{ve}$) or negative ($-\text{ve}$) symbols without numerical grading ($1+$ through $4+$).

  • Quality Assurance Protocol: A double-read workflow is enforced. Reader $1$ performs the initial visual assessment ($+\text{ve}$ / $-\text{ve}$), and Reader $2$ independently reads the microplate to verify that Reader $1$ recorded every well correctly.

Hemolysis and Visual Grading

• Clinical Significance:

  • Visual hemolysis (lysis of red blood cells in test supernatants) is an indicator of an antigen-antibody reaction.

  • It indicates that an antibody activated complement to completion, producing the membrane attack complex and destroying red cells.

  • Hemolysis must never be overlooked, as missing it masks a clinically significant immune reaction.

• Visual Grading of Hemolysis:

  • Negative / Normal: Clear, transparent, golden-yellow supernatant.

  • Trace Hemolysis: Faint reddish tinge in the supernatant (often visually borderline negative/trace depending on the evaluator).

  • Moderate Hemolysis: Distinct pink-to-light-red translucent supernatant.

  • Marked Hemolysis: Dark red transparent supernatant with complete or near-complete absence of intact red cell button.

• Patient and Unit Management:

  • Sensitization and cell lysis occurring during compatibility or screening tests indicate active complement activation.

  • Any donor red blood cell unit causing hemolysis during testing is unsafe and must be destroyed rather than transfused.

Solid Phase Red Cell Adherence (SPRCA)

• Mechanism and Setup:

  • SPRCA utilizes microplate wells pre-coated with target red blood cell antigens (e.g., screening cells bound to the well bottom).

  • Patient plasma or donor serum is added to the antigen-coated microplate wells.

  • The microplate is incubated to allow antibody attachment, then thoroughly washed to remove unbound proteins.

  • Anti-IgG-coated indicator red blood cells are added to detect bound antibody.

• Interpretation of Results:

  • Strong Positive Reaction: Target antibodies bind indicator cells across the entire bottom of the well, creating a smooth, uniform monolayer covering the whole surface.

  • Weak / Least Positive Reaction: Indicator cells cover the bottom surface but stretch outward around the edges beyond the strict center.

  • Negative Reaction: Absence of bound target antibody allows indicator cells to settle directly to the bottom center, forming a tight, solid red cell button.

Mechanisms of Hemolysis: Intravascular vs. Extravascular

• Intravascular Hemolysis:

  • Complement cascade is activated through to full completion (C1 through C9).

  • Assembly of the Membrane Attack Complex (MAC) creates transmembrane pores in circulating red blood cells.

  • Red cells lyse directly inside blood vessels, releasing free hemoglobin and cellular contents into systemic circulation.

• Extravascular Hemolysis:

  • Complement cascade activation stops at the $C3b$ cleavage step without progressing to MAC assembly.

  • Red blood cells sensitized with IgG and/or $C3b$ pass through the reticuloendothelial system (spleen and liver).

  • Tissue macrophages recognize $C3b$ and IgG Fc receptors, phagocytosing and removing the sensitized red cells from circulation.

• Disseminated Intravascular Coagulation (DIC):

  • A severe pathologic condition characterized by widespread microvascular thrombi throughout the circulatory system.

  • Causes vessel occlusion, tissue ischemia, organ infarction, and high patient mortality.

• Pathophysiology of Renal Failure in Hemolytic Crisis:

  • Severe intravascular hemolysis triggers hypertension and intense renal vascular constriction.

  • High concentrations of circulating free hemoglobin, red cell stroma, and microvascular clotting block renal tubules, leading to acute kidney failure.

Clinical Manifestations and Signs of Intravascular Hemolysis

• Sudden Hypotension: Rapid drop in blood pressure caused by systemic release of anaphylatoxins ($C3a$ and $C5a$) during complement activation.

• Hemoglobinuria: Free hemoglobin filtered by renal glomeruli spills into urine, turning it dark pink or red. Testing requires obtaining a voided urine specimen immediately following a suspected hemolytic transfusion reaction.

• Hemoglobinemia: Free hemoglobin in blood plasma shifts plasma color from clear golden yellow to pink or dark red.

• Depleted Serum Haptoglobin: Free hemoglobin binds haptoglobin. When massive hemolysis exhausts available haptoglobin, plasma haptoglobin levels fall to near zero.

• Failure of Hemoglobin/Hematocrit Rise: Post-transfusion hemoglobin and hematocrit ($\text{Hgb/Hct}$) values fail to increase, reflecting rapid destruction of transfused red cells upon entering recipient blood vessels.

• Anaphylactic Transfusion Reaction: Massive complement and cytokine release causes severe anaphylaxis, frequently leading to death depending on whether lysis is slight, moderate, or marked.

Immunohematology Genetics and Mendelian Principles

• Key Genetic Terminology:

  • Allele: Alternate forms of a gene located at a specific gene locus on a chromosome.

  • Homozygous: Inheritance of identical alleles at a given locus from both parents (e.g., $M+N-$ or $Jk(a+b-)$).

  • Heterozygous: Inheritance of two different alleles at a given locus (e.g., $M+N+$ or $Jk(a+b+)$).

  • Polymorphic: Existence of multiple allele options at a single locus within a population.

  • Antithetical Antigens: Antigens encoded by allelic genes that act as logical pairs/opposites:

    • $C$ and $c$

    • $E$ and $e$

    • $\text{Fy}^a$ and $\text{Fy}^b$

    • $M$ and $N$

    • $S$ and $s$

    • $Jk^a$ and $Jk^b$

  • Antigens Lacking Antithetical Partners: $P1$ has no antithetical partner. $D$ has no antithetical partner (there is no "little d" antigen; "d" indicates the absence of $D$).

• Modes of Inheritance:

  • Codominance: Most blood group antigens show codominant expression, where both inherited alleles are expressed equally on red blood cells.

  • Recessive vs. Dominant: Group $O$ is a recessive allele (expressed only in the homozygous $OO$ state). Group $A$ and Group $B$ alleles are dominant over $O$.

• Mendelian Laws:

  • Law of Independent Segregation: Each parent passes one allele per locus to offspring.

  • Law of Independent Assortment: Genes on different chromosomes are distributed into gametes independently of one another.

Antigen Dosage Effect

• Definition: Dosage occurs when an antibody reacts more strongly with red blood cells expressing a double dose of an antigen (homozygous inheritance) than with cells expressing a single dose (heterozygous inheritance).

• Comparison Example:

  • Cell A phenotype: $M+N-$ (Homozygous for $M$; two copies of $M$ allele).

  • Cell B phenotype: $M+N+$ (Heterozygous; one $M$ allele and one $N$ allele).

  • Anti-M antiserum produces a significantly stronger agglutination reaction with Cell A ($M+N-$) than with Cell B ($M+N+$).

• Blood Group Systems Demonstrating Dosage:

  • Duffy System: $\text{Fy}^a$ and $\text{Fy}^b$

  • Rh System: $C$, $c$, $E$, $e$ (Note: $D$ does NOT display dosage because it lacks an antithetical allele).

  • MNS System: $M$, $N$, $S$, $s$

  • Kidd System: $Jk^a$ and $Jk^b$

Genetic Linkage, Haplotypes, and Cis/Trans Effects

• Gene Linkage and Haplotypes:

  • Genes located close together on the same chromosome are linked and inherited together as a unit called a haplotype (e.g., MNS system genes, Rh genes).

  • Most Common Rh-Positive Haplotype: $DCe$ ($R^1$).

• Cis vs. Trans Position Effects:

  • Cis Position: Alleles inherited on the same chromosome.

  • Trans Position: Alleles inherited on opposite homologous chromosomes.

  • Trans Suppressive Effect: When the $C$ gene is inherited in trans to the $D$ gene, steric interactions weaken $D$ antigen expression on red cells. The test yields reduced agglutination strength ($2+$ or $3+$) compared to normal strong ($4+$) $D$ expressions.

Silent Genes, Null Phenotypes, and Suppressor Genes

• Amorphs (Silent Genes):

  • Genes that produce no detectable antigen expression (e.g., the $O$ gene, or silence at Rh loci).

• Rh Null Phenotype:

  • Complete absence of all Rh antigens ($D$, $C$, $c$, $E$, $e$).

  • Transfusion Requirement: Rh null patients produce antibodies against Rh antigens if exposed. They must receive exclusively Rh null red blood cell units, requiring specialized regional blood supplier management.

• Suppressor/Regulator Genes:

  • Genes operating at a distinct locus that inhibit the expression of blood group antigens at another locus:

    • $In(Jk)$ suppressor gene yields a $Jk(a-b-)$ null phenotype.

    • $In(Lu)$ suppressor gene yields a $Lu(a-b-)$ null phenotype.

Parentage and Relationship Testing (Direct and Indirect Exclusion)

• Obligatory Gene: The gene that must have been contributed by the true biological father to the offspring.

• Direct Exclusion:

  • Occurs when an offspring possesses a genetic marker/antigen that is completely absent in both the mother and the putative father, or when a child lacks an antigen that a homozygous parent must pass on.

  • Example: Mother phenotype is Group $O$ ($OO$); Putative Father phenotype is Group $B$ ($BB$ or $BO$); Offspring phenotype is Group $A$ ($AO$). The offspring's obligatory $A$ allele could not come from either parent, directly excluding the putative father.

• Indirect Exclusion:

  • Occurs when an offspring fails to inherit an antigen for which the putative father is homozygous, or offspring is homozygous for an antigen the father lacks.

  • Example: Mother phenotype is $\text{Fy}(a-b+)$; Putative Father phenotype is $\text{Fy}(a+b-)$; Offspring phenotype is $\text{Fy}(a-b+)$.

  • Genotypic Analysis: Mother is $\text{Fy}^b \text{Fy}^b$; Offspring is $\text{Fy}^b \text{Fy}^b$; Putative father is $\text{Fy}^a \text{Fy}^a$. The putative father must pass $\text{Fy}^a$ to all offspring. Since the child lacks $\text{Fy}^a$, the putative father is indirectly excluded.

Antigen Frequencies and Donor Screening Calculations

• Population Antigen Frequencies (Caucasian Population):

  • $D$ positive: $851\%$ ($15\%$ negative)

  • $C$ positive: $70\%$ ($30\%$ negative)

  • $c$ positive: $80\%$ ($20\%$ negative)

  • $E$ positive: $30\%$ ($70\%$ negative)

  • $e$ positive: $\approx 98\%$ ($2\%$ negative)

  • $K$ (Kell) positive: $9\%$ ($91\%$ negative)

  • $Jk^a$ positive: $77\%$ ($23\%$ negative)

• Calculation Procedure for Antigen-Negative Units:

  1. Determine the positive frequency of each target antigen.

  2. Calculate the negative frequency for each target antigen:        $\text{Negative Frequency} = 1.00 - \text{Positive Frequency}$

  3. Multiply the negative decimal frequencies together to determine the combined probability of finding a compatible unit:        $\text{Combined Frequency} = P(\text{Ag}_1^-) \times P(\text{Ag}_2^-) \times \dots \times P(\text{Ag}_n^-)$

  4. Convert combined decimal to a percentage.

  5. Calculate donor units required ($x$) for $N$ compatible units:        $\frac{\text{Combined Frequency}}{100} = \frac{N}{x} \implies x = \frac{N \times 100}{\text{Combined Frequency}}$

• Worked Example 1:

  • Scenario: Find $2$ units negative for $E$ ($30\%$ positive), $c$ ($80\%$ positive), and $\text{Fy}^a$ ($65\%$ positive).

  • Negative Frequencies:

    • $E^- = 100\% - 30\% = 70\% = 0.70$

    • $c^- = 100\% - 80\% = 20\% = 0.20$

    • $\text{Fy}^{a-} = 100\% - 65\% = 35\% = 0.35$

  • Combined Frequency:         $0.70 \times 0.20 \times 0.35 = 0.049 = 4.9\% \approx 5\%$

  • Units to Screen:         $\frac{5}{100} = \frac{2}{x} \implies 5x = 200 \implies x = 40\text{ donor units}$

• Worked Example 2:

  • Scenario: Find $4$ units negative for $K$ ($9\%$ positive), $c$ ($80\%$ positive), and $Jk^a$ ($77\%$ positive).

  • Negative Frequencies:

    • $K^- = 100\% - 9\% = 91\% = 0.91$

    • $c^- = 100\% - 80\% = 20\% = 0.20$

    • $Jk^{a-} = 100\% - 77\% = 23\% = 0.23$

  • Combined Frequency:         $0.91 \times 0.20 \times 0.23 = 0.04186 = 4.186\% \approx 4\%$

  • Units to Screen:         $\frac{4}{100} = \frac{4}{x} \implies 4x = 400 \implies x = 100\text{ donor units}$

Control Selection for Quality Control Testing

• Positive Control Selection Principles:

  • Positive control red blood cells must display heterozygous antigen expression (single dose) for the target antigen.

  • Heterozygous cells provide the weakest true positive reaction, ensuring the antisera reagent can reliably detect low-density antigens or low-titer antibodies.

  • Example: When running positive controls for anti-E reagent, select an $E+e+$ cell (heterozygous), rather than an $E+e-$ cell (homozygous).

• Negative Control Selection Principles:

  • Negative control red cells must completely lack the target antigen (antigen negative).

  • Expression of non-target partner antigens is irrelevant as long as the cell is negative for the antigen under evaluation.

  • Example: When running negative controls for anti-E reagent, select an $E-e+$ cell.