Interpret A BLAST Result E-value, Coverage

Use molecular-test evidence to interpret a blast result (e-value, coverage) accurately.

Builds on (2 levels back)inferred · high confidence

Control logic: Molecular results need positive and negative controls.
Signal interpretation: Bands, colors, curves, and E-values must be compared to a rule.

Prerequisites are inferred: pending teacher review.

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

Use molecular-test evidence to interpret a blast result (e-value, coverage) accurately.

Step 1: Learn the key

Read the control [blank], compare the sample signal to the [blank], and report the result with one [blank].

BLAST field	Meaning
Coverage	percent of query aligned
E-value	chance-like match score; lower is stronger
Identity	matching bases in aligned region

Step 2: Use the model

Read the figure, table, control, range, or protocol before choosing an answer.

Step 3: Name the limit

Say what the evidence can support and what it cannot prove yet.

Practice

Two BLAST hits are shown. Remember: a lower E-value means a stronger match, and higher coverage means more of your sequence lined up. Which hit is the more confident match to your sample?

Reviewed

Hit	E-value	Coverage
Hit A	2e-80	99%
Hit B	0.04	41%

A.Hit A
B.Hit B
C.Both are equally confident
D.Neither can be judged

Show the worked solution ▾

Answer: A. Hit A

Step 1: Compare E-values: 2e-80 is far lower than 0.04, so Hit A's match is much stronger.
Step 2: Compare coverage: 99% coverage (Hit A) is higher than 41% (Hit B).
Step 3: Decide: Lower E-value and higher coverage both point to Hit A.

Why it's right: Hit A has both a much lower E-value (2e-80 vs 0.04) and higher coverage (99% vs 41%), so it is the more confident match.

Why the others miss:

B: Hit B has a higher E-value and lower coverage, so it is the weaker match.
C: They differ a lot in both E-value and coverage, so they are not equal.
D: Both metrics are given, so the hits can be compared and judged.

Aligned to Biotechnology Research and Experiments · reading level ~grade 9

Where you'd see this

In Unit 1.1 Pathogen ID (BLAST), this skill turns class evidence into a result another person can check.

Careers that use this

Molecular Technologist: Uses interpret a blast result (e-value, coverage) to make a decision from evidence without overclaiming.

Video library

Watch: Interpret A BLAST Result E-value, Coverage

BLAST Results: Expect Values, Part 1

National Library of Medicine

Guided notes

Fill these in as you work through the lesson.

Big idea: A BLAST match is judged by its E-value (lower means a stronger, less-random match) and its coverage (how much of your sequence lined up).

Key terms: write the meaning

E-value (chance-like match score; lower is stronger):
Coverage (percent of your sequence that aligned):
Query (the sequence you are searching with):
Hit (a database sequence BLAST matched to your query):

The rule

A E-value means a stronger match, and a coverage means more of your sequence lined up.

Check yourself

Between E-value 1e-50 and 0.04, which is the stronger match?
Hit A: 2e-80, 99%. Hit B: 0.04, 41%. Which hit is more confident?
If a hit has a low E-value but 38% coverage, what does that low coverage mean?

Work one example

Hit A (E-value 2e-80, coverage 99%) vs Hit B (E-value 0.04, coverage 41%). Hit A has the lower E-value and higher coverage, so Hit A is the more confident match.