Dark Lines Under Fingernails

What Causes Dark Lines Under Fingernails?

Dark lines under fingernails can have various causes, just like most other symptoms.  Finding the true cause means ruling out or confirming each possibility – in other words, diagnosis.

Diagnosis is usually a complex process due to the sheer number of possible causes and related symptoms.  In order to diagnose dark lines under fingernails, we could:
  • Research the topic
  • Find a doctor with the time
  • Use a diagnostic computer system.
The process is the same, whichever method is used.

Step 1: List all Possible Causes

We begin by identifying the disease conditions which have "dark lines under fingernails" as a symptom.  For example, melanoma.

Step 2: Build a Symptom Checklist

We then identify all possible symptoms and risk factors of each possible cause, and check the ones that apply:
bleeding/enlarging/tender moles
melanoma in family members
mouth sores
light hair
black/brown fingernails
having many common moles
lighter/paler skin color
very many irregular dark moles
melanoma
light eye color
history of melanoma
... and so on

Step 3: Rule Out or Confirm each Possible Cause

A differential diagnosis of your symptoms and risk factors finds the likely cause of dark lines under fingernails.

Arriving at a Correct Diagnosis

The Analyst™ is our online diagnosis tool that learns all about you through a straightforward process of multi-level questioning, providing diagnosis at the end.

If you indicate abnormal fingernails and toenails, The Analyst™ will ask further questions including this one:
Do you have any dark lines/streaks below the nails, running in the direction of nail growth?
Possible responses:
→ No / don't know
→ Yes
Based on your response to this question, which may indicate dark lines under nails, The Analyst™ will consider possibilities such as Melanoma. Dark lines beneath the nail should be investigated as soon as possible.