Understanding Results

Point of Care Test kits (POCT)

There is more to understanding a POCT result than just reading the test lines. There are factors that can impact POCT results. Understanding what factors can impact results may help you avoid problems in the future..

FACTORS THAT IMPACT LINE INTENSITY

LIGHT / FAINT TEST LINES

There are many reasons why POCT test line intensity levels will vary from test to test and donor to donor.  Light test lines can significantly impact POCT accuracy if not careful. Unless you are using SmartReader or some other digitally assisted reader, you are going to have to learn how to deal with light lines. The 5-minute rule will correct some portion of light line issues. According to manufacturers and industry standards, a light line represents a negative result. But at what point will a light line really mean positive?

First, it is important to understand what can cause light lines other than the target drug. Light lines can be caused by:

 

  • Ignoring the 5-minute rule

  • Chemistry of the donor

  • Using cold POCTs without bringing to room temperature

  • A variety of adulterants

  • Damage to POCT or packaging malfunction

  • Inverting or shaking POCT with specimen

  • Drug Imposters 

DRUG IMPOSTERS VS ACTUAL DRUG

THE DIFFERENCE CAN BE HARD TO SPOT

There are large and growing number of chemical compounds on the market that look almost identical to illegal drugs, yet are common over the counter medications or supplements. These compounds do not always cause false positives, but may impact line intensity. When these compounds are present, test results can be more sensitive to “Flux” and may shift the “Window of Accuracy” timeline by several minutes longer.

 

In some cases, the only way to tell the difference is to conduct a lab confirmation test. The images below are diagrams for two drugs. One is an over the counter cold medication and the other is crystal meth. Notice how their structure is similar. Both molecules will impact the methamphetamine assay on a rapid test. One will show positive and the other will produce a light line. If read early, the light line may not be visible yet, causing a false-positive reading.

 

NOTE: Methamphetamine (right-side) and pseudoephedrine HCL (left-side)

POCT FLUX

POCTS AND LABS USE DIFFERENT SCIENCE

A vast majority of the time, your POCT result and lab result will agree. (If the POCT is used properly). But it is important to remember that POCTs and Labs do not use the same technology to get a result. 

On a POCT package insert, you will notice that each drug (assay) is able to detect a list of drugs and/or drug metabolites. Look at the Benzodiazepine 200 ng cut-off below. The target drug is Oxazepam which is listed at 200 ng. But look at all the other drugs. If there is 130 ng of Alprazolam

the POCT will also turn positive.

 

So what happens to the Oxazepam cut-off level when there Alprazolam is present at 65 ng? It will lower the cut-off level for Oxazepam by 50%. 

Here is a simplified version that may be easier to understand. 

Imagine a test for blueberry jelly beans with a cut-off of 50 jellybeans. If there were a test for blueberry jelly beans, the package insert might show the test detects more than just blueberry. For example:

 

  • blueberry 50 

  • strawberry 500 

  • peppermint 20,000 

If a donor sample contained 250 strawberry (which is 50% of the strawberry cut-off) jellybeans then it would only take 25 blueberry jellybeans to show positive. The way POCTs work is that all of the drugs and metabolites listed "cocktail" together and all contribute to the POCT's detection.

 

The 250 strawberry represents 1/2 the cut-off and if there were only 25 blueberry jelly beans the test would show positive for blueberry jellybeans. 

Laboratory technology is more "specific" and can target just blueberry jellybeans. The presence of strawberry jellybeans has no impact on the lab results. So the same donor with 250 strawberry and 25 blueberry jellybeans would be reported by the lab as negative.

This is called FLUX and can occur with samples at or near the cut-off level. This is one of the main reasons why the lab will confirm a sample at a lower cut-off level than the initial screen. The reduction in the cut-off is designed to eliminate flux.

But if you are not informing your lab that the sample sent has been screened already, they will not know to lower the cut-off when the lab confirms your sample. In this case, the POCT will appears to produce false positives when in fact it has not.

The FLUX on some assays can be really small. Look again at the package insert. (below). Look at the BUP assay. There is only one drug which means there is no "FLUX" impact. In this case the screen and confirmation for BUP can be at the same cut-off. The Flux of an assay will change from POCT to POCT and assay to assay.