In many court­rooms all across the United States today, a state ana­lyst opines under oath that a unknown leafy green veg­etable like sub­stance is in fact mar­i­juana. The ques­tion becomes, in the typ­i­cal case, is this a valid con­clu­sion? Is it the whole sci­en­tific truth?

In this series of posts we are going to exam­ine these seemly sim­ple questions:

1. What is the goal and the pur­pose of test­ing of unknowns gen­er­ally? How do we best design a test for marijuana?
2. How is most mar­i­juana test­ing con­ducted in the United States?
3. What is micro­scopic mor­pho­log­i­cal exam­i­na­tion? Is it a “good” test?
4. What is the mod­i­fied Duquenois-Levine test? Is it a “good” test?
5. What is Thin Layer Chro­matog­ra­phy? Is it a “good” test?
6. Is the com­bi­na­tion of all three tests cre­ate a “good” test­ing scheme?
7. Is there a bet­ter way to test for marijuana?

Part 1: What is the goal and the pur­pose of test­ing of unknowns gen­er­ally? How do we best design a test for marijuana?

Remem­ber that the goal of all ana­lyt­i­cal test­ing is to pro­duce a valid result. A valid result is the epit­ome of good science.

A valid result is made up of three dis­tinct features:

1. It is a spe­cific measurement,
2. using a tech­nique or series of tech­niques that are part of a val­i­dated procedure
3. that is verifiable.

Fea­tures of a spe­cific test

The first step is that the test­ing must be unique and spe­cific qual­i­ta­tive mea­sure­ment. The dif­fer­ence between speci­ficity and selec­tiv­ity is very impor­tant. To be spe­cific in terms of qual­i­ta­tive mea­sure­ment means that there is an iden­ti­fi­ca­tion of a par­tic­u­lar com­pound and exclu­sively that one spe­cific com­pound to the exclu­sion of every other com­pound or sub­stance in the uni­verse. It is unequiv­o­cal iden­ti­fi­ca­tion based upon objec­tive cri­te­rion and val­i­dated meth­ods or tech­niques that are ver­i­fi­able. In short, it is what it is and could be noth­ing else.

On the other hand, if the iden­ti­fi­ca­tion is selec­tive, then it is not spe­cific. The iden­ti­fi­ca­tion that results from selec­tive test­ing or meth­ods means that it could be that sub­stance but it also could be some­thing else. There could be cross-reactivity, co-elution or just poor sen­si­tiv­ity. There could be false pos­i­tives. In essence and fun­da­men­tally, we do not know what it is defin­i­tively, uniquely, and specif­i­cally. We may not know what it is, but we can learn what it is not.

Gen­er­ally, the dif­fer­ence between a spe­cific test­ing régime and that of a selec­tive one is the dif­fer­ence between con­fir­ma­tory data and pre­sump­tive data and the dif­fer­ence between the sci­en­tific weight of the opin­ions expressed based upon them. A pre­sump­tive test is some­thing that is char­ac­ter­ized by a high rate of false pos­i­tives and cross-reactivity. It is not at all spe­cific and may or may not be use­fully selec­tive. A con­fir­ma­tory test is one that results in a near spe­cific mea­sure­ment with a very, very low rate of false pos­i­tives. A con­fir­ma­tory test or even a com­bi­na­tion of con­fir­ma­tory tests can never be truly 100% cor­rect. There is still room for error. Noth­ing is perfect.

Selec­tive ver­sus Spe­cific is kind of like Sesame Street’s “one of these things is not like the other things”

The Inter­na­tional Union of Pure and Applied Chem­istry (IUPAC), which is the world author­ity on chem­i­cal nomen­cla­ture, ter­mi­nol­ogy, stan­dard­ized meth­ods for mea­sure­ment, atomic weights and other crit­i­cally eval­u­ated data and oth­ers have defined the dif­fer­ence between these often con­fused terms as follows:

A spe­cific reac­tion or test is one that occurs only with the sub­stance of inter­est, while a selec­tive reac­tion or test is one that can occur with other sub­stances but exhibits a degree of pref­er­ence for the sub­stance of inter­est.  Few reac­tions are spe­cific, but many “exhibit selectivity”.

Other com­mon def­i­n­i­tions include:

Selec­tiv­ity gives an indi­ca­tion of how strongly the result is affected by other com­po­nents in the sample.

and also

Selec­tiv­ity refers to the extent to which the method can be used to deter­mine par­tic­u­lar ana­lytes in mix­tures or matri­ces with­out inter­fer­ences from other com­po­nents of sim­i­lar behavior.

A selec­tive test may be not a spe­cific test due to cross-reactivity, inter­fer­ence, coelu­tion, or codetermination.

Fea­tures of a val­i­dated tech­nique or series of techniques

We wrote a whole 6 part series on method val­i­da­tion and what it takes to be valid result.

1. Part 1: Introduction-Is it valid, invalid or non-validated?
2. Part 2: What is method validation?
3. Part 3: Can we use some­one else’s val­i­dated method?
4. Part 4: What trig­gers ver­i­fi­ca­tion, re-validation or out right new val­i­da­tion of a method?
5. Part 5: What are the essen­tial terms in method validation?
6. Part 6: What is qual­ity assur­ance and qual­ity control?

Fea­tures of a ver­i­fi­able test

A ver­i­fi­able test is one in which oth­ers who were not present for the test­ing can look at some­thing to basi­cally dou­ble check what test­ing was pre­formed and its result. With­out ver­i­fi­ca­tion, then there can­not be a val­i­dated result.

The end result in the courtroom

If we, as a soci­ety, sin­cerely mean the prin­ci­ples of the pre­sump­tion of inno­cence, proof beyond a rea­son­able doubt, and the bur­den of pro­duc­tion and per­sua­sion, then in the case of iden­ti­fi­ca­tion of an unknown when it is pre­sented in court and if the gov­ern­ment can only prove selec­tiv­ity, then the jury’s result should always be not guilty.