Principles of Serological Testing Study Guide

Serological Testing Fundamentals and Definitions

• Serology Definition: Serology is the study of the fluid components in the blood, specifically focusing on antibodies.
• Serum: Serum is the liquid portion of the blood that remains after the coagulation factors have been removed. It is the most frequently encountered specimen in immunologic testing.
• Importance of Knowledge: Professional proficiency in specimen collection, preparation of dilutions, and pipetting is essential to all serological testing in the clinical laboratory.
• Key Terms:
  • Blowout pipette: A pipette with a frosted band around the opening that requires the last drop to be forced out for accurate delivery.
  • Diagnostic sensitivity: The proportion of people with a specific disease who have a positive test result.
  • Diagnostic specificity: The proportion of people without a specific disease who have a negative test result.
  • Diluent: The medium (usually saline, water, or buffer) making up the rest of the solution during a dilution.
  • Graduated pipettes: Pipettes with markings for varying amounts of liquid.
  • Micropipette: A mechanical pipette used for measuring very small volumes in the microliter ($\mu L$) range.
  • Negative predictive value: The probability that a person with a negative screening test truly does not have the disease.
  • Positive predictive value: The probability that a person with a positive test truly has the disease.
  • Serial dilution: A system where the dilution factor remains exactly the same in each successive step.
  • Serological pipette: A graduated pipette where markings go all the way down to the tip.
  • Solute: The substance being diluted (e.g., patient serum or a reagent concentrate).
  • Titer: An indicator of the amount of antibody present, expressed as the reciprocal of the last dilution showing a visible reaction.
  • Volumetric pipettes: Calibrated to deliver exactly one specific volume of liquid.

Blood Specimen Preparation and Storage

• Collection Method: Blood is collected aseptically via venipuncture into a sterile tube lacking anticoagulant (e.g., Red-top, gold-top, or serum separator tube).
• Hemolysis: Great care must be taken to avoid hemolysis (rupture of red blood cells), as this may produce false-positive test results.
• Clotting Process: Specimen is allowed to clot at room temperature or at $4^{\circ}C$, depending on specific procedural protocols.
• Centrifugation and Separation: The sample is centrifuged, and the serum is promptly separated into a secondary tube without transferring cellular elements.
• Complement Inactivation: While fresh serum is usually recommended, some tests require the inactivation of complement proteins because they interfere with results.
  • Procedure: Heat serum to $56^{\circ}C$ for $30$ minutes.
• Storage Requirements:
  • If testing is immediate: Fresh serum is best.
  • Delayed testing ($< 72$ hours): Store between $2^{\circ}C$ and $8^{\circ}C$.
  • Long-term storage ($> 72$ hours): Freeze at $-20^{\circ}C$ or colder.

Instrumentation for Volume Measurement

• Pipette Calibration: Pipettes are categorized by their markings (Volumetric or Graduated) and their delivery method ("TD" or "TC").
• Volumetric Pipettes:
  • Marked to deliver (TD) only one specific volume.
  • Feature an oval bulb in the center and a tapered dispensing end.
  • Dispense liquid exactly, leaving a small drop behind (not to be forced out).
  • Used with a suctioning device (rubber bulb) and held vertically against the surface of a container during gravity-flow delivery.
• Graduated (Measuring) Pipettes:
  • Have marks along their length to measure varying amounts.
  • Serological Pipettes: Marks extend all the way to the tip.
  • Blowout Pipettes: Identified by a frosted band. Labeled "TC" (to contain). Requires the last drop to be forced out using a pipette bulb or automatic filler.
  • Technique: Sighted at eye level to the bottom of the meniscus. Marks may increase from top to bottom (e.g., measuring $3\,mL$ by filling a $10\,mL$ pipette to the $7\,mL$ mark and emptying it entirely).
• Micropipettes:
  • Mechanical devices used for patient specimens, controls, and calibrators.
  • Used for volumes in the microliter ($\mu L$) range.
  • Use a plunger and single-use disposable tips.
  • Can be fixed-volume or adjustable range.

Simple Dilution Calculations

• Definition: A dilution is a ratio/fraction of solute to the total volume.
• Formula:$\frac{\text{Volume of Solute}}{\text{Total Volume}} = \text{Dilution}$$\text{Total Volume} = \text{Volume of Solute} + \text{Volume of Diluent}$
• Example 1: Finding Solute and Diluent Volumes
  • Problem: Prepare $2\,mL$ of a $1:20$ dilution.
  • Equation: $\frac{1}{20} = \frac{x}{2}$
  • Solve: $20x = 2$ → $x = 0.1\,mL$ serum.
  • Diluent calculation: $2.0\,mL - 0.1\,mL = 1.9\,mL$ diluent.
• Example 2: Finding Diluent Volume with Fixed Solute
  • Problem: $1:5$ dilution using $0.1\,mL$ serum.
  • Equation: $\frac{1}{5} = \frac{0.1}{0.1 + x}$
  • Solve: $x + 0.1 = 0.5$ → $x = 0.4\,mL$ diluent.
• Example 3: Buffer Concentrations
  • Problem: Mix $1$ part buffer with $19$ parts water using $50\,mL$ buffer.
  • Dilution: $\frac{1}{1 + 19} = 1/20$
  • Equation: $\frac{1}{20} = \frac{50}{50 + x}$
  • Solve: $50 + x = 1,000$ → $x = 950\,mL$ water.
• Percentage Solutions:
  • $10\%$ solution = $1/10$ dilution.
  • Volume/Volume (V/V): Liquid solute in liquid diluent.
  • Weight/Volume (W/V): Solid solute dissolved in liquid diluent.
  • Example 4: To make $500\,mL$ of a $10\%$ acetic acid solution: $\frac{1}{10} = \frac{x}{500}$. $x = 50\,mL$ acid; $500 - 50 = 450\,mL$ water.

Compound and Serial Dilutions

• Compound Dilution Formula: Use $C_1V_1 = C_2V_2$ where $C$ is concentration and $V$ is volume.
  • Example: Preparing $2.0\,mL$ of $1:20$ from a $1:5$ stock.
  • $(1/5)(V_1) = (1/20)(2.0)$
  • $V_1 = 0.5\,mL$ of the $1:5$ dilution. Diluent needed = $2.0 - 0.5 = 1.5\,mL$.
• Sequential Dilutions: Used for large ratios (e.g., $1:500$) to maintain accuracy and reduce waste.
  • Method: Multiply sequential dilutions ($1/5 \times 1/10 \times 1/10 = 1/500$).
• Serial Dilutions and Titers:
  • A constant dilution factor is used across a row of tubes.
  • Each successive tube is $1/n$ the concentration of the previous one (e.g., two-fold system = each is $1/2$ of the prior tube).
  • Agglutination: Antibodies combined with particulate antigens (e.g., Red blood cells) to form visible aggregates.
  • Titer Calculation: The reciprocal of the last dilution showing a visible reaction.

Comprehensive Serial Dilution Calculation Process

• Scenario: $10$ tubes; tube 1 has $0.8\,mL$ saline + $0.2\,mL$ serum; tubes 2-10 have $0.4\,mL$ saline. Transfer $0.4\,mL$ serially. Add $0.1\,mL$ antigen to all.
• Step 1: Dilution of Tube 1
  • $\text{Dilution} = \frac{0.2}{0.2 + 0.8} = 1/5$
• Step 2: Determine Dilution Fold ($df$)
  • $\frac{1}{df} = \frac{\text{Volume Transferred}}{\text{Volume Transferred} + \text{Volume Diluent}}$
  • $\frac{1}{df} = \frac{0.4}{0.4 + 0.4} = 1/2$
• Step 3: Calculate Dilution of Tube $n$ (Pre-antigen)
  • $\text{Dilution} = \text{Tube 1 Dilution} \times [1/df]^{(n-1)}$
  • For tube 4: $(1/5) \times [1/2]^{(4-1)} = (1/5) \times (1/8) = 1/40$
• Step 4: Incorporating Reagent Addition (Final Titer)
  • $C_1V_1 = C_2V_2$
  • $(1/40)(0.4\,mL) = C_2(0.5\,mL)$
  • $C_2 = (1/40) \times (0.4/0.5) = 1/50$
  • Final Titer: $50$

Diagnostic Test Parameters

• Diagnostic Sensitivity: The ability to identify individuals with a disease.
  • $\text{Sensitivity (\%)} = \left[ \frac{\text{True Positives}}{\text{True Positives} + \text{False Negatives}} \right] \times 100$
  • Note: analytic sensitivity refers to the lower limit of detection.
• Diagnostic Specificity: The ability to correctly identify individuals where the disease is absent.
  • $\text{Specificity (\%)} = \left[ \frac{\text{True Negatives}}{\text{True Negatives} + \text{False Positives}} \right] \times 100$
  • Note: analytic specificity refers to cross-reactivity detection.
• Predictive Values: Consider disease prevalence in the population.
  • Positive Predictive Value (PPV): $\left[ \frac{\text{True Positives}}{\text{True Positives} + \text{False Positives}} \right] \times 100$
  • Negative Predictive Value (NPV): $\left[ \frac{\text{True Negatives}}{\text{True Negatives} + \text{False Negatives}} \right] \times 100$
• Example Case Calculation:
  • Population: $200$ patients.
  • Diseased: $160$ Pos (TP), $1$ Neg (FN).
  • Non-diseased: $14$ Pos (FP), $25$ Neg (TN).
  • Sensitivity: $[160 / (160 + 1)] \times 100 = 99.4\%$
  • Specificity: $[25 / (25 + 14)] \times 100 = 64.1\%$
  • PPV: $[160 / (160 + 14)] \times 100 = 91.95\%$
  • NPV: $[25 / (25 + 1)] \times 100 = 96.2\%$

Antigen-Antibody Equivalence

• Lattice Structure: Large immune complexes form only when the relative proportions of antigen and antibody are similar.
• Prozone/Postzone Issues: If antigen or antibody molecules are in excess, molecules remain unbound and fail to create a visible reaction.
• Clinical Application: Serum containing too much antibody must be diluted to reach the endpoint (equilibrium) where visible reactions occur.

Case Study Analysis

• The Scenario: A supervisor uses a serological pipette to measure $0.1\,mL$ of serum by draining from the $0.9\,mL$ mark to the tip but leaves a drop behind. She adds $1.9\,mL$ of diluent.
• Mistake A (Pipetting): If the serological pipette is labeled "TC" (to contain), the last drop must be blown out. By leaving it behind, the volume of solute is less than the intended $0.1\,mL$.
• Mistake B (Calculation): The supervisor aimed for a $1:40$ dilution. Calculation for intended mix: $\text{Solute} / \text{Total} = 0.1 / (0.1 + 1.9) = 0.1 / 2.0 = 1/20$. The mix she prepared was a $1:20$, not a $1:40$.
• Correct Procedure for 1:40: To prepare a $1:40$ dilution using $0.1\,mL$ serum: $\frac{1}{40} = \frac{0.1}{x}$. Total volume $x = 4.0\,mL$. Diluent needed = $4.0 - 0.1 = 3.9\,mL$.