X and V Factor Test
A diagnostic guide for determining X (hemin) and V (NAD) factor requirements to differentiate and identify Haemophilus species.
XV Plates A-D
Observe the growth around the paper disks impregnated with X, V, and XV factors. Growth only occurs where required accessory growth factors have diffused.
Requires both X and V factors; growth is restricted to a clear halo around the XV disk only.
Requires V factor only; growth occurs around the V disk and the XV disk.
Requires X factor only; growth occurs around the X disk and the XV disk.
No requirement for X or V factors; confluent growth occurs over the entire surface of the agar.
Requires both X and V factors; growth is restricted to a clear halo around the XV disk only.
Requires V factor only; growth occurs around the V disk and the XV disk.
Requires X factor only; growth occurs around the X disk and the XV disk.
No requirement for X or V factors; confluent growth occurs over the entire surface of the agar.
Growth Patterns & Access Key
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Halo around XV only | Requires both hemin (X) and NAD (V) | Haemophilus influenzae |
| Halo around V and XV | Requires nicotinamide adenine dinucleotide (V) only | Haemophilus parainfluenzae |
| Halo around X and XV | Requires hemin (X) only | Haemophilus ducreyi |
| Confluent lawn over plate | No requirement for external X or V growth factors | Aggregatibacter aphrophilus |
Haemophilus Diagnostic Matrix
| Pattern | What to call it | How to think about it |
|---|---|---|
| H. influenzae | Growth around XV only | Pathogen; causes meningitis, otitis media, epiglottitis, and pneumonia. |
| H. parainfluenzae | Growth around V and XV | Common respiratory tract flora; occasionally opportunistic. |
| H. ducreyi | Growth around X and XV | Sexually transmitted pathogen; causes chancroid (soft chancre). |
| A. aphrophilus | Growth over entire plate | Oral flora; can cause endocarditis; formerly H. aphrophilus. |
- The X factor is hemin (heat-stable iron compound), and the V factor is coenzyme I / nicotinamide adenine dinucleotide (NAD, heat-labile).
- Trypticase Soy Agar (TSA) or heart infusion agar serves as the nutrient base because it lacks both factors, unlike Chocolate Agar which has both.
- XV disks contain both hemin and NAD; therefore, any factor-requiring species will grow around the XV disk.
XV disk supports all Haemophilus. V disk supports parainfluenzae. X disk supports ducreyi. Negative means growth everywhere.