qPCR vs Digital PCR (dPCR) — FRCPath Medical Microbiology Comparison

qPCR vs Digital PCR (dPCR)

Feature	qPCR (Real-Time PCR)	Digital PCR (dPCR)
Principle	Measures fluorescence during each amplification cycle	Divides sample into thousands–millions of tiny reactions; each partition is scored as positive or negative
Quantification	Relative or absolute (requires standard curve for absolute quantification)	Absolute quantification (no standard curve required)
Output	Ct (Cycle threshold/Cq) value	Copies/µL or copies/mL using Poisson statistics
Need for Standard Curve	Yes (for absolute quantification)	No
Sensitivity	High	Very high
Detection of Low Copy Numbers	Good	Excellent
Precision	Moderate	Excellent
Tolerance to PCR Inhibitors	Lower	Higher
Detection of Rare Mutations	Limited	Excellent (can detect mutant allele fractions <1%)
Reproducibility	Good	Excellent
Dynamic Range	Very wide (≈7–8 logs)	Moderate (≈4–5 logs)
Multiplexing	Easier	More limited (instrument dependent)
Speed	Faster	Slightly slower
Cost per Test	Lower	Higher
Equipment Cost	Lower	Higher
Routine Clinical Use	Very common	Increasing but still specialised

Principle

qPCR

DNA
 ↓
PCR amplification
 ↓
Fluorescence measured every cycle
 ↓
Ct value
 ↓
Standard curve
 ↓
Calculate viral load

Fluorescence increases continuously during amplification.

Digital PCR

DNA sample
      ↓
Partition into 20,000 droplets
□□□□□□□□□□□□□□□□□□□□□□

Each droplet contains:
0 copy
or
1 copy
or
2 copies

↓

PCR in every droplet

↓

Positive droplets = Fluorescent
Negative droplets = Dark

↓

Count positives

↓

Poisson calculation

↓

Absolute copies

Digital PCR is essentially counting molecules, rather than estimating amplification kinetics.

Output

qPCR

Cycle

Fluorescence
│
│          /
│        /
│      /
│    /
│  /
│/
└──────────────────────
        Ct

Earlier Ct = more target DNA.

Digital PCR

20,000 droplets

■■■■□■■□□□■■■■□□□□■■■

■ Positive
□ Negative

Positive fraction
↓

Poisson equation

↓

Copies/µL

No Ct value is produced.

Advantages of qPCR

• Fast

• High throughput

• Relatively inexpensive

• Wide dynamic range

• Excellent for routine diagnostics

• Well established in clinical laboratories

Advantages of Digital PCR

• Absolute quantification

• No calibration standards required

• Detects very low viral loads

• Detects rare mutations

• Better precision

• Better reproducibility

• Less affected by inhibitors

• Excellent for minimal residual disease and resistance mutation detection

Limitations

qPCR

• Requires standard curve for absolute quantification

• Ct varies between platforms

• Lower precision at low copy numbers

• More affected by inhibitors

Digital PCR

• Expensive instruments and consumables

• Lower throughput

• Smaller dynamic range

• Longer workflow

• Less widely available

Clinical Applications

qPCR

Routine diagnosis of:

• SARS-CoV-2

• HIV viral load

• HBV viral load

• HCV viral load

• CMV monitoring

• EBV monitoring

• Influenza

• Respiratory multiplex assays

• Mycobacterium tuberculosis detection

• Bacterial PCR assays

Digital PCR

Ideal for:

• Minimal residual disease (MRD)

• Rare mutation detection

• Low-level HIV reservoir studies

• Cell-free DNA analysis

• Oncology liquid biopsy

• Transplant donor-derived cell-free DNA

• Absolute viral load quantification

• Detection of antimicrobial resistance genes present at very low abundance

• Environmental and wastewater surveillance

Example

Patient A

qPCR:

Ct = 36

Interpretation:
Low viral load

Digital PCR:

Absolute result

18 copies/mL

Digital PCR provides a direct measurement without relying on a calibration curve, making it more reliable at very low concentrations.

FRCPath Question

Examiner: Why can digital PCR quantify DNA without a standard curve?

Answer:

Digital PCR partitions the sample into thousands of individual PCR reactions, each containing zero, one, or a few target molecules. After amplification, partitions are classified as positive or negative, and the proportion of positive partitions is converted into an absolute copy number using Poisson statistics. Because it counts target molecules directly rather than inferring quantity from amplification kinetics, no external standard curve is required.

When should a Consultant Microbiologist prefer Digital PCR?

Choose digital PCR when:

• Precise quantification of very low copy numbers is required.

• Monitoring minimal residual disease or residual viral burden.

• Detecting rare resistance mutations or minority variants.

• Accurate measurement is needed despite PCR inhibitors.

• A validated absolute copy number is clinically or scientifically important.

Choose qPCR for most routine diagnostic microbiology because it is faster, less expensive, scalable, and provides sufficient sensitivity for the majority of infectious disease applications.

💎 FRCPath High-Yield Takeaway

qPCR estimates the amount of nucleic acid by measuring fluorescence during amplification and typically uses a Ct value with a standard curve for absolute quantification.

Digital PCR counts individual target molecules after partitioning the sample into thousands of reactions, providing absolute quantification with higher precision, particularly for low-copy-number targets and rare variants.