The use of Deoxyribonucleic Acid (DNA) testing has revolutionized the Philippine judicial system, transforming how courts establish identity, filiation, and guilt or innocence. Governed primarily by the Rule on DNA Evidence (A.M. No. 06-11-5-SC), which took effect in 2007, the legal framework provides a scientific and procedural roadmap for the appreciation of genetic profiling in both civil and criminal cases.

I. Legal Basis: The Rule on DNA Evidence

The Supreme Court of the Philippines promulgated the Rule on DNA Evidence to provide guidelines for the application, collection, and assessment of DNA samples. Before this rule, the court relied on the landmark case of People vs. Vallejo (2002), which first recognized the admissibility of DNA as a valid form of scientific evidence in the country.

II. The Application for a DNA Testing Order

DNA testing is not automatic; it must be sought through a court order. The application may be filed by the prosecution, the defense, or the court motu proprio.

Requirements for the Application:

Existence of a biological sample: There must be a biological sample critical to the case.
Relevance: The sample must be relevant to the determination of a fact in issue (e.g., identity of the perpetrator or biological father).
Probability of outcome: The testing must have the potential to exclude or include a person as the source of the sample.
Standard methods: The testing must use scientifically valid and accepted methods.

III. Guidelines for Admissibility

The court does not blindly accept DNA results. Under Section 7 of the Rule, the court must assess the "probative value" of the DNA evidence by examining the following:

Chain of Custody: How the sample was collected, handled, and preserved. Any "break" in this chain can render the evidence unreliable.
The Laboratory: Whether the laboratory is accredited and follows standard operating procedures.
The Analyst: The qualifications, experience, and training of the person who performed the test.
The Process: The specific equipment used and the technical standards followed during the extraction and analysis.
Data Interpretation: The statistical significance of the results (e.g., the Likelihood Ratio).

IV. DNA Testing in Specific Cases

1. Criminal Cases

In criminal litigation, DNA is often the "silent witness." It is used to identify suspects in heinous crimes like rape or murder or to exonerate those wrongfully accused.

Post-Conviction DNA Testing: A convict may apply for DNA testing even after a final judgment, provided the evidence was not available during the trial and the results could lead to a reversal of the conviction.

2. Civil Cases (Paternity and Filiation)

DNA testing has largely replaced the "Physical Resemblance" test in determining paternity.

Presumption of Paternity: If the DNA result shows a 99.9% probability of paternity or higher, it creates a disputable presumption that the alleged father is indeed the biological parent.
Refusal to Undergo Testing: While the court cannot physically force an individual to undergo a blood draw, a refusal can be taken as an adverse inference against that party in some contexts.

V. Procedure for DNA Sample Collection

To ensure the integrity of the evidence, the following steps are mandated:

Consent or Court Order: Samples are taken via voluntary consent or through a court-mandated order.
Non-Invasive Methods: While blood samples are common, "buccal swabs" (scraping the inside of the cheek) are preferred for being non-invasive.
Documentation: Every person who touches the sample must be documented in a log to maintain the chain of custody.

VI. Evaluation of DNA Results

The result of a DNA test generally falls into three categories:

Inclusion: The profile of the subject matches the evidentiary sample. This suggests the subject is the source.
Exclusion: The profiles do not match. This conclusively proves the subject is not the source.
Inconclusive: The sample is degraded, contaminated, or insufficient to provide a clear result.

VII. Reliability and Constitutional Challenges

The Supreme Court has consistently ruled that the compulsory taking of DNA samples does not violate the Right Against Self-Incrimination. The privilege applies only to "testimonial" evidence—forcing someone to speak or write. Since DNA is a physical, "real" piece of evidence extracted from the body, it is treated similarly to fingerprints or photographs.

Summary Table: DNA Evidence Assessment

Factor	Legal Requirement
Primary Law	A.M. No. 06-11-5-SC (Rule on DNA Evidence)
Probability Threshold	99.9% for a presumption of paternity
Chain of Custody	Must be unbroken from collection to courtroom
Self-Incrimination	Not applicable; DNA is physical, not testimonial
Post-Conviction	Allowed if it may lead to acquittal