John Hay
John Hay Biomedical
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Genetics of Disease (Medical Interventions)
Unit 1: How to Fight InfectionMI 1.1Bio-Molecular Technology (BMT)

Reading a serial-dilution / ELISA standard curve

Use a standard curve to turn an absorbance reading into a concentration.

Builds on (2 levels back)inferred · med confidence
  • Reading an x-y graph: A standard curve is an x-y plot; students must read a value off axes first.
  • Serial dilution (proportional reasoning): Standards come from serial dilutions; the concentration axis depends on understanding dilution.

Prerequisites are inferred: pending teacher review.

Re-learn the skill with worked practice and clear examples.

Build a standard curve from known concentrations, then read an unknown sample's concentration from its absorbance.

Step 1: Know the words
ELISA is a common lab test that makes a sample change color depending on how much of the target (like an antibody) is present. A machine reads that color as absorbance: how much light the sample soaks up. More target → more color → higher absorbance.
Step 2: Plot the knowns
Run standards of known concentration through the ELISA, plot absorbance (y) vs. concentration (x), and draw the best-fit line.
Step 3: Read the unknown
Take the unknown's absorbance on the y-axis, trace to the line, and drop to the x-axis for its concentration.
Step 4: Sanity check
If the unknown's absorbance is above the top standard, the reading is off the curve: dilute and re-run.
Practice

An unknown sample reads an absorbance of 0.6. Using the standard curve shown, the closest concentration is about:

Approved
Standard curve: absorbance (y, 0 to 1.0) vs concentration (x, 0 to 40). A straight best-fit line rises from the origin; a dashed guide at absorbance 0.6 meets the line, then drops to the concentration axis for the student to read.
  1. A.About 6 units/mL
  2. B.About 24 units/mL
  3. C.About 40 units/mL
  4. D.It cannot be read from this graph
Show the worked solution ▾

Answer: B. About 24 units/mL

  1. Step 1: Start at 0.6 on the y-axis: Find absorbance 0.6 and trace the dashed line across to the best-fit line.
  2. Step 2: Drop to the x-axis: From the curve, drop straight down; it lands near 24 units/mL.

Why it's right: Tracing absorbance 0.6 to the line and down to the x-axis gives roughly 24 units/mL.

Why the others miss:
  • A: 6 is far too low for an absorbance of 0.6 on this line.
  • C: 40 is the top of the axis, well above this reading.
  • D: It can be read: that is the purpose of a standard curve.

Aligned to BMT: standard curve reading · reading level ~grade 9

Where you'd see this
  • Quantifying antibody or antigen levels in a diagnostic ELISA from the plate-reader absorbance.
Careers that use this
Immunologist: Quantifies antibody titers from ELISA standard curves when testing vaccines.
Video library
Prerequisite: reading an x-y graph
Find x and y values from a graph
Joel Speranza Math
Prerequisite: serial dilution
What Are Serial Dilutions
FuseSchool
Remediation: ELISA & the standard curve
ELISA Assay Explained with Simple Models
StarFish Medical
Extension: off-range readings & dilution
High Dose Hook Effect
Bio-Resource
Guided notes

Fill these in as you work through the lesson.

Big idea: A standard curve turns a measured absorbance into a concentration: read up from the value, across to the line, then down to the axis.
Key terms: write the meaning
  • Standard curve (graph built from known concentrations):  
  • Absorbance (how much light the sample soaks up):  
  • Serial dilution (step-down in concentration):  
  • Linear range (the reliable part of the curve):  
The rule

To find an unknown's concentration: start at its   on the y-axis, trace across to the  , then drop down to the   axis.

Check yourself
  1. Why do you plot the known standards first, before reading an unknown? 
  2. An unknown reads an absorbance of 0.6 on the curve shown: about what concentration is that? 
  3. If an unknown reads higher than the top standard, what do you do before reporting a number? 
Work one example

An unknown's absorbance is above the highest standard. Describe the dilution fix and how you scale the result back to the true concentration.