Affectors of Laboratory Results: Part V

This week we will discuss the standards and controls that are used by analytical laboratories to establish their standard curves, which relate to the quantity of active drug, or analyte, in your samples. This will prepare us for next week where we will discuss the different methods of using standards which include internal standard, external standard, and standard addition methods. All these discussions are at the heart of selecting a good analytical laboratory and evaluating test results.

A standard solution is a solution of a pure analyte (drug) in a suitable solvent which is used to prepare a calibration curve and verify the analytical method. The calibration curve is determined by very carefully preparing a series of solutions, each with a known concentration, of the analyte. The response from the detector or detection system is plotted versus the concentration to obtain a standard curve. The sample is analyzed and its response is compared to the response in the standard curve where it is related to it�s concentration. As you can see, if the standard curve is off, your sample will be off and you will obtain erroneous results. First, however, what is a �standard�?

A standard is a chemical or drug of known purity and is used to prepare a calibration curve. It differs from a control which is used to check for interferences and determine any analyte loss or alteration during sample preparation and during the analytical process. A very specific type of standard, the Reference Standard, is a standard that has been subjected to extensive analytical testing in at least three different laboratories to confirm its purity and potency. USP Reference Standards are defined as authentic specimens that have been approved by the USP Reference Standards Committee as suitable for use as comparison standards in USP or NF tests and assays. When a test or assay calls for use of a compendial substance (USP monographed drug) rather than for a USP Reference Standard as a standard material, a substance meeting all of the compendial monograph requirements for that article is to be used. USP Reference Standards generally cost from $162 to about $550 for less than 1 mg up to 1 g of the substance.

What do laboratories use as standards? If they are analyzing for a drug that has a USP-NF monograph, they should be using USP Reference Standards. If not available, they should be using a drug substance that meets all the requirements of the USP monograph, if the drug has a monograph. If a monograph is not available, it is common practice to purchase a sample of the bulk drug powder and use it as the standard. As is evident, the USP Reference Standards are of the highest quality, standards meeting USP monographs would be next, followed by a purchased pure drug powder, as used in compounding or manufacturing. Another source which should not be used unless absolutely necessary, is a dosage form containing the drug with relatively few excipients, generally injections, etc.

Second, what is a control? A control is a known concentration of an analyte in a matrix which is analytically equivalent to the sample matrix. Analytically equivalent implies that analysis of a control solution should yield the same result after being subjected to the same sample preparation steps we have been discussing (e.g., filtration, extraction, evaporation, derivatization, etc.) and then analysis. Believe it or not, the most important control is the Zero Concentration Control, since it is at this concentration that interferences may appear. If during the analytical methods development and validation the analytical procedure yields 100% recovery of all the analytes from the sample matrices, it may be possible and more practical to substitute standard solutions for controls. Controls are important and should be analyzed frequently to continually �test the system�.

In evaluating any results that are received from a laboratory, especially those that are published, the complete analytical method, including methods development and validation procedures, use of standards and controls, must be made known for an informed and educated decision to be made as to their reliability and validity. This is done for all manuscripts published in the Peer Reviewed Section of IJPC.

Another thought to ponder and possibly a couple of questions that need to be addressed are:

1. If a standard solution is prepared and used for more than one day in the laboratory, what is the stability of that standard solution and how is it stored? As is evident, reference standards are quite expensive and in an effort to keep costs down, it is common practice to prepare standard solutions that are used over a period of time. These may be kept refrigerated, frozen, etc. Sometimes they are subdivided into smaller containers and frozen and removed as an individual unit when required. Regardless of which method is used, the laboratory must document the stability of their standards.
2. What is the stability of the standards and Reference Standards used by the laboratories. As dry powders, they are generally used up to their labeled expiration dates, if one is present. If not, they may be periodically re-certified. Also, the small quantities that are packaged helps to maintain a tight inventory and control of these and they must be stored properly.

One last somewhat related topic on expiration dates of bulk substances. With the difficulty in assigning expiration dates, packaged on dates, etc. on bulk drug substances by bulk substance repackagers, it would be of value for repackagers to have a system to adopt for expiration dating rather than an artibrary time, such as one year. This new effort has already been initiated with the help and support of the USP Reference Standards Division. This approach should be appropriate since these substances are packaged appropriately to meet monograph standards and when they reach the compounding pharmacy or the analytical laboratory, they are stored according to monograph requirements. The pharmacy would need to document storage conditions through proper SOPs, completed data forms, etc. This may help alleviate some of the artificially short dates that are being required by regulatory agencies that result in needless discarding of valuable and expensive drug substances into our environment. More on this topic later.

Next week, we will look at the different methods of using these standards to derive a numerical value for your samples that are submitted for analysis.

Loyd V. Allen, Jr., Ph.D., R.Ph
Editor-in-Chief

One of the most important resources compounding pharmacists need is literature to give to prescribers to substantiate the preparation of individualized dosage forms. Therefore CompoundingToday.com has built a dynamic system to pull citations and abstracts of relevant literature from more than 300 medical and pharmacy publications for our subscribers to search by keyword.

Ideally, CompoundingToday.com would be able to provide full-text articles of all the literature that is cited. However, providing full-text articles, either print or electronic, from other publications not owned by IJPC or CompoundingToday.com is a violation of copyright. What we have done to help subscribers actually get their hands on needed literature is provide a link to the publisher of the cited literature. Therefore, once an article needed is identified, it can be obtained by direct contact with the original publisher through the electronic link provided. Some articles may be free or a small fee will be charged. Articles can also be obtained from local libraries.

If an article needed has been published in IJPC, subscribers of IJPC have access to the IJPC archives based on the start date of their subscription. All articles published within the time frame of a subscription are free.

Over time CompoundingToday.com will hosts thousands upon thousands of abstracts and citations for pharmacists to access and provide to prescribers, include in presentations and research for their own knowledge.

Each week we add between 30 and 100 abstracts. Here are a sampling of the abstracts added this week.

• Classen AM, Wimbish GH, Kupiec TC. Stability of admixture containing morphine sulfate, bupivacaine hydrochloride, and clonidine hydrochloride in an implantable infusion system. J Pain Symptom Manage 2004; 28(6): 603-611
• Trissel LA. Other sources of stability research. Am J Health Syst Pharm 1999; 56(2): 179-180.
• Trissel LA, Gilbert DL, Martinez JF et al. Compatibility of medications with 3-in-1 parenteral nutrition admixtures. JPEN J Parenter Enteral Nutr 1999; 23(2): 67-74.
• Trissel LA, Zhang Y. Stability of methylprednisolone sodium succinate in autodose infusion system bags. J Am Pharm Assoc (Wash) 2002; 42(6): 868-870.
• Xu QA, Ahang YP, Trissel LA et al. Stability of cisatracurium besylate in vials, syringes, and infusion admixtures. Am J Health Syst Pharm 1998; 55(10): 1037-1041.

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