How did U.S. find chlorine exposure in Douma residents by testing urine and blood?

From NBC News, April 12:

WASHINGTON — The U.S. now has blood and urine samples from last Saturday's deadly attack in Syria that have tested positive for chemical weapons, according to two U.S. officials familiar with the intelligence. The samples suggested the presence of both chlorine gas and an unnamed nerve agent, two officials said. Typically, such samples are obtained through hospitals and collected by U.S. or foreign intelligence assets on the ground. The officials said they were "confident" in the intelligence, though not 100 percent sure.

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I venture that 100 percent certainty would be hard, considering that blood and urine tests are not used to determine chlorine exposure.

From the New York State Department of Health fact sheet on chemical terrorism and chlorine:

The diagnosis of chlorine poisoning will primarily be based upon clinical examination and patient history, not laboratory testing. While laboratory testing for chlorine exposure is not useful in assisting treatment decisions, routine studies to monitor blood count, glucose and electrolytes may be useful for evaluating complications.

The same observations are made by other medical authorities such as the CDC, NIH, etc., also published online. Blood and urine are simply not tested to determine chlorine exposure. 

I don't hold it against NBC for not doing basic fact-checking. They were simply disseminating propaganda as was CNN, which picked up on the NBC story. But I was struck by this sentence in the NBC story:

U.S. officials say the Assad regime has conducted multiple chlorine attacks on rebels during the past six months.

On what basis did the officials make the determination? By analyzing blood and urine samples?

Regarding the "unnamed nerve agent" in the samples that officials discussed with NBC -- whether or not the agent is claimed to be sarin, one fact towers above Western government claims that the Syrian government used chemical weapons after it agreed in 2013 to destroy its chemical weapons stockpiles (1):

Anyone familiar with the symptoms of sarin gas poisoning only needs to glance at video/ photographic evidence of alleged victims of the Khan Sheikhoun 'sarin gas' incident in 2017 to know those people were not suffering from sarin gas poisoning. It was the same with the Ghouta incident in 2013. 

There is no way around this fact, even though the Syrian government's alleged use of sarin gas in Khan Sheikhoun (or Shaykhun) was the prextent for the Trump regime to bomb a Syrian airbase.  

Then what about laboratories that found evidence of sarin gas poisoning in the Ghouta and Khan Sheikhoun incidents? Either they were lying or examining evidence that had been tampered with. 

In short, there is so much misinformation and disinformation concerning the use of chemical WMDs in Syria that it would take an investigation along the lines of the Church Committee to even hope to get at the truth. 

What is completely clear at this time is that a great many agencies, governmental and non-governmental, have a vested interest in looking at Syria, and President Bashar al-Assad, with their eyes wide shut. 

1)  See for example Syrian chemical weapons: how lab tests uncover evidence of sarin gas; September 5, 2013; The Guardian

and 

Exclusive: Tests link Syrian government stockpile to largest sarin attack - sources;  January 30, 2018; Reuters

See also:

UN on Khan Sheikhoun - Victims Hospitalized BEFORE Claimed Incident Happened; October 29, 2017; Moon of Alabama

ISIS Used Chemical Arms at Least 52 Times in Syria and Iraq, Report Says; November 21, 2016; The New York Times

“Mosul was at the center of the Islamic State’s chemical weapons production,” Mr. Strack said. “But most of the equipment and experts were probably evacuated to Syria in the weeks and months leading up to the Mosul offensive ..."

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