No traces of drugs were found in the urine samples of Siddhartha Das and Mugdha Das, who were arrested in connection with the killing of four members of the same family in Rangpur.
However, according to the police, the confessional statements the two accused gave to the court mentioned that they had taken yaba on the night of the incident.
These two pieces of information give rise to a question: if their statements mention drug use, why did their test results come back negative? The answer lies in the timing of sample collection.
On the night of 20 August, retired teacher Ganapati Chakraborty, his wife Pritilata Chakraborty, daughter Agami Chakraborty and son Ayush Chakraborty were killed in the Machuapara area of Rangpur city.
Police recovered the bodies of the four victims on the night of 22 August. Siddhartha and Mugdha were arrested in the early hours of 23 August. However, their urine samples were collected on 30 August—nearly 10 days after the time of the killings, according to the police.
The Rangpur Metropolitan Police Detective Branch (DB) received the test report the following day. The report found no traces of drugs in either of the two samples. From a forensic science perspective, these two pieces of information cannot be described as contradictory.
Prothom Alo spoke to seven people, including physicians, relevant experts, a criminologist, police officials and officials from the Department of Narcotics Control, about these issues. Their comments indicate that after a person takes a drug, the drug itself or its metabolites do not remain in every type of biological sample for the same length of time. A substance that may no longer be detectable in urine after a few days can remain detectable in hair or nail samples for much longer.
What does a “negative” result actually mean?
A “negative” result on a drug test does not always mean that the person concerned has never taken drugs. Put simply, it means that when the sample was tested, the drug in question or the substance produced in the body as a result of taking it was not present at a level high enough to be detected.
Bangladesh’s Drug Detection Testing in Biological Samples (Dope Test) Rules, 2026 also defines a “false negative” result. The rules state that even if a person has taken drugs, the test may produce a negative result if the drug or its metabolites in their biological sample are below the laboratory’s prescribed threshold at the time of testing.
Experts say that at least four factors need to be determined when interpreting a negative result: which drug was taken, how long ago it was taken, what type of sample was tested and what the detection threshold of the test was.
Is it unusual not to detect the substance 10 days later
The principal drug component in yaba is methamphetamine. It generally remains detectable in urine for two to three days. However, this period can vary depending on factors such as regular drug use, an individual’s metabolism, the acidity or alkalinity of the urine and the detection threshold of the testing method.
Scientific reviews of various drugs have found that, following a single instance of drug use, most drugs generally remain detectable in urine for one and a half to four days. In the case of regular users, some drugs may remain detectable for a week or longer.
Professor Mohammad Rashedul Haque, head of the Department of Biochemistry at Rangpur Medical College, told Prothom Alo that it was not unusual for methamphetamine not to be detected in a urine sample collected nearly 10 days later.
In other words, the drugs tested for were not present in the urine samples collected on 30 August at detectable levels. However, this result cannot conclusively establish whether the two accused took yaba on the night of 20 August.
Urine, blood, saliva, hair—which sample is useful and when?
Section 24 of the Narcotics Control Act, 2018 provides for various physical examinations when necessary for investigation or searches. Subsection 4 of the same section states that drug tests should be conducted in accordance with the rules to identify people who are dependent on drugs.
Based on this law, the government issued the Drug Detection Testing in Biological Samples (Dope Test) Rules, 2026 on 16 February this year.
The rules define biological samples to include urine, saliva, blood, hair, nails and breath, as well as organs, parts of organs and other bodily fluids where necessary. However, they specify separate procedures for collecting urine, blood, saliva, hair and nail samples. There are also internationally recognised protocols governing such testing.
Urine: One of the most commonly used samples for detecting drug use in the preceding few days. Methamphetamine can generally be detected within two to three days. However, even when drugs are detected in urine, it is not possible to determine exactly when the person took them.
Blood: More useful for establishing the time between drug use and testing. According to a review by Alain Verstraete, a researcher at Ghent University in Belgium, most drugs can generally be detected in blood or blood plasma for one to two days. However, this period can be considerably shorter depending on the drug.
Saliva: Effective for detecting very recent drug use. In general, various drugs can be detected in saliva for between five and 48 hours. However, studies have also found that, in exceptional cases, drugs may remain detectable for longer among people who use them in high doses or over extended periods.
Hair: Hair can provide a much longer history of drug use than urine, blood or saliva. It takes some time for drugs to reach the hair and emerge as a testable segment. Therefore, hair is not a suitable sample for detecting drug use during the most recent few days. Generally, hair samples can help establish a history of drug use from around one week earlier to several months in the past.
Nails: Like hair, nails can also provide information about long-term drug use. A major study found that drugs could still be detected in nails three to six months after consumption. However, no definitive timeframe has yet been established for how long specific drugs can be reliably detected in nails.
In other words, there is no single answer to which sample is the “best”. The choice of sample depends on the period of drug use that needs to be determined.
Not all drugs have same detection window
Another reason for confusion over drug tests is that not all drugs leave the body at the same rate. Methamphetamine can generally be detected in urine for several days.
However, in long-term cannabis users, metabolites of tetrahydrocannabinol (THC), the principal active component of cannabis, can remain detectable in urine for several weeks. On the other hand, the metabolites of some drugs remain detectable for only a few hours.
For this reason, information such as “testing after three days” or “testing after seven days” alone cannot determine whether a test was conducted at the appropriate time. It is first necessary to know which drug the test is looking for.
Moreover, a single drug test cannot establish the presence of every drug. The prescribed preliminary test report under the drug-testing rules has separate fields for opioids, methamphetamine, cannabinoids, benzodiazepines and alcohol. There is also a separate field for “other” drugs.
In other words, simply saying that a “drug test was negative” does not provide the complete picture. It is necessary to know which drugs or classes of drugs the test was designed to detect.
A standard test may detect certain substances from one class of drugs while failing to detect another drug from the same group. Where necessary, a separate test must be conducted for a specific drug.
Drug testing generally takes place in two stages—an initial screening test and a confirmatory test. Under the 2026 rules, recognised testing instruments, kits or strips may be used for the initial screening.
If the result is positive, the same sample must undergo confirmatory testing using internationally recognised methods. Therefore, a positive result from the initial screening alone cannot be regarded as conclusive.
Caution is also needed when collecting samples
Urine samples must be collected in internationally acceptable containers. Hands must be washed before collecting the sample, and the container must be sealed in a way that prevents tampering or alteration.
Under the rules, the person concerned must rest for at least 20 minutes before a blood sample is collected. A volume of 5 to 10 millilitres of blood must then be collected using a sterile procedure and placed in a specialised blood collection tube.
A person must refrain from consuming food and beverages for at least 30 minutes before providing a saliva sample. A special sample-collection swab must be rotated four to eight times along the inside of both cheeks to collect the sample.
For hair testing, at least 10 strands of hair must be collected from the head or a designated part of the body, wrapped in clean paper and placed in a sealed envelope. For nail testing, the rules require two samples in total—one portion of a fingernail and one portion of a toenail.
The purpose of these procedures is to ensure the chain of custody of the sample—establishing whose sample it is, when it was collected and whether it remained intact until it reached the laboratory.
Capacity constraints in hair and nail testing
Manjurul Islam, deputy director of the Department of Narcotics Control (DNC), told Prothom Alo that laboratory capacity remains limited in implementing the new rules.
According to Manjurul Islam, the Department of Narcotics Control’s own laboratories currently conduct mainly preliminary tests on urine and blood samples. If the preliminary result is positive, further testing is conducted using gas chromatography–mass spectrometry or high-performance liquid chromatography.
Sources at the Department of Narcotics Control said its own laboratories do not yet have the capacity to conduct comprehensive drug testing on hair and nail samples. Work is under way to expand the capacity to test these samples in accordance with the new rules.
Professor Mohammad Rashedul Haque, head of the Department of Biochemistry at Rangpur Medical College, also told Prothom Alo that their hospital has facilities for urine testing for drug screening, but does not have facilities to test hair samples.
The recent incident in Rangpur has highlighted an important limitation of drug testing. Conducting a test alone is not enough; samples suitable for detecting drug use during the relevant period must be collected as quickly as possible.
If there is a delay, the test may be conducted properly, but the sample may no longer contain the evidence needed to answer the question being investigated.