Reference range for random glucose
Q
Our hospital lab uses the American Diabetes Association (ADA) recommended reference range of 70 mg/dL to 99 mg/dL for fasting serum/plasma glucose.
We are struggling to come up with a valid reference range for a random (not fasting) serum/plasma glucose run in the main lab.
This test is usually ordered as a part of the chem8 or chem14 panel on diabetic and non-diabetic patients.
More vexing is the reference-range dilemma for capillary whole-blood glucometer glucose, since this point-of-care test should only be done on diabetic patients for glycemia and/or insulin management.
A
There are basically two ways to go about determining a reference range for random glucose. You could measure glucose values in some large number of random patient samples and come up with a mathematical reference range. This is the traditional way of determining reference ranges for most tests, though it may not make sense for random glucose. One question to consider is, how did you come up with your reference range for fasting glucose?
Most likely, the reference range for fasting glucose is based on a clinical action limit — usually derived from guidelines published by the ADA. Accrediting and regulatory agencies such as the College of American Pathologists, or CAP, and Clinical Laboratory Improvement Amendments, or CLIA, require either reference values or action limits be reported for all tests. Fasting glucose and cholesterol are examples of tests for which most labs choose to provide action limits rather than true reference values. That is, the 70 mg/dL to 99 mg/dL does not represent the distribution of fasting-glucose values in your patient population, nor does <200 mg/dL represent the distribution of cholesterol values. In these instances, your laboratory (and every other laboratory for that matter) substitutes a clinical-action limit from a reputable source (ADA and National Cholesterol Education Program for glucose and cholesterol, respectively) rather than reporting a true reference interval.
The same approach can be — and probably should be — taken for random glucose, though there are no good published guidelines to use in this instance. A question to ask yourself and clinical colleagues in your practice may be: “What levels of random glucose do providers need to be aware of to take clinical action?” In our practice, we collaborated with endocrinologists and critical-care physicians to come up with a range of 70 mg/dL to 140 mg/dL. Other practices use 70 mg/dL to 99 mg/dL, while a few others use 70 mg/dL to 180 mg/dL. On the low end, most practices believe providers should take note when random glucose goes below the action limit for fasting glucose — which is why 70 mg/dL on the low end is probably reasonable. On the high end, it depends upon the medical practice in your hospital or community. For most patients in your facility, at what random glucose level would physicians, nurses, and others consider the values abnormal and worthy of action or follow-up? For institutions doing a lot of glycemic control, that value is likely to be between 140 mg/dL to 180 mg/dL on the high end. Institutions less interested in glycemic control might even choose a higher value, say 200 mg/dL. We have set reference values for random glucose, both plasma and capillary, at 70 mg/dL to 140 mg/dL after consultation with our clinical colleagues.
— Brad Karon, MD, PhD
Devices for central-line blood draws
Q
There is a debate at my institution regarding whether to use a syringe or vacuum tube for central-line blood draws.
I would like to reduce blood exposure for nurses and stop syringe use by going to a vacuum tube with a waste tube.
According to the representative for our catheters, the maximum pressure limit for the catheter is 12.5 psi but the vacuum tube holds vacuum of 25 psi.
Do you know of any studies that discuss catheter damage from vacuum tubes? Our value-analysis team is concerned about catheter damage,
but other facilities report using a vacuum tube draw system without any difficulties. Any information regarding this subject would be greatly appreciated.
A
Most institutions do not understand the physics behind collecting blood through a catheter tip using a vacuum tube. Blood-collection tubes do not hold a vacuum of 25 psi. A key factor is the pressure difference between atmospheric pressure, and the pressure in the tube. One can estimate the pressure in a vacuum tube from the dimensions and the draw volume using the ideal gas law. The pressure in any given tube will vary slightly depending on its size and draw volume. The estimate for a 13-mm x 75-mm, 4-mL draw tube is ~3 psi. Atmospheric pressure at the point of draw is 14.3 psi. The difference in pressure would be 11.3 psi which is below the 12.5-psi threshold supplied by the manufacturer of the catheters. The estimated pressure difference is greatest initially after the tube is placed on the needle. As the blood fills into the tube, the pressure differential decreases.
There are several studies that have investigated hemolysis rates from catheter draws. Those using full-draw vacuum tubes tend to exhibit greater hemolysis1-3 than those using direct venous collection4 or partial draw tubes.5 The reason for this is the sheer stress on the cells when pulled through a device with a tortuous path. Hemolysis is minimized using a straight needle and vacuum tube as compared to a catheter tip and vacuum tube. Otherwise, the combination of the catheter tip and vacuum tube would be favored due to the reduction to blood exposure, as identified above.
Some institutions, however, have found success with catheter collections using partial-draw tubes. The tube manufacturer should be able to provide more information.
—Valerie Bush, PhD, Director
Clinical Laboratory and Point-of-Care Testing
Bassett Healthcare
Cooperstown, NY
References
- Raisky F, Gauthier C, Marchal A, Blum D. Haemolyzed samples: responsibility of short catheters. Ann Biol Clin. 1994; 52:523-527.
- Kennedy C, Angenmuller S, King R, Noviello S, et al. A comparison of hemolysis rates using intravenous catheters versus venipuncture tubes for obtaining blood samples. J Emerg Nurs. 1996;22(6):566-569.
- Grant MS. The effect of blood drawing techniques and equipment on the hemolysis of ED laboratory blood samples. J Emerg Nurs. 2003; 29(2):116-121.
- Seeman S, Reinhardt A. Blood sample collection from a peripheral catheter system compared with phlebotomy. J Intraven Nursing. 2000; 23:290-297.
- Sixsmith DM, Weinbaum F, Chan SYA, Nussabaum M, Magdich K. Reduction of hemolysis of blood specimens drawn from ED patients for routine chemistry tests by use of low vacuum collection tubes. Academic Emerg Med. 2000;7:524.
Anticoagulant testing
Q
Our pharmacy has started running partial thromboplastin time (PTT) tests on patients on a drug called argatroban.
It is a direct thrombin inhibitor for patients the pharmacy thinks have developed heparin-induced thrombocytopenia.
The pharmacist says they use the PTT as a monitoring indicator to adjust dosing. Do you have any information on this?
A
Patients with heparin-induced thrombocytopenia (HIT) are often anticoagulated with direct thrombin inhibitors (DTIs) until long-term anticoagulation with warfarin is achieved.
There are now a variety of DTIs used for the treatment of HIT including argatroban, bivalirudin, and hirudin.1
The most common way to monitor argatroban is using the activated partial thromboplastin time (aPTT) test,
which is also recommended by the manufacturer for patients receiving prophylaxis or treatment for HIT.2 A baseline aPTT value is obtained prior to starting an intravenous argatroban
infusion of 2
mg/kg/min titrated until a target aPTT ratio (aPTT on DTI divided by baseline aPTT) of 1.5 to 3.0 is obtained.2
The aPTT has limited reliability, however, due to the non-linear response to argatroban dose and the influence that coagulation factor deficiencies and inhibitors have.
Ecarin clotting time (ECT) assays can be used to monitor DTI levels, including argatroban. ECT assays have shown improved performance over aPTT because they respond
linearly with argatroban concentrations up to 8
mg/mL unlike aPTT which was only linear up to 1.2
mg/mL.3 Ecarin is a metalloproteinase isolated from the venom of the viper — Echis carinatus — that activates
prothrombin converting it into meizothrombin, which is a thrombin precursor that is inhibited by DTIs. Unbound meizothrombin converts fibrinogen into fibrin, whereby the clotting
time is proportional to the DTI concentration.1
ECT is not influenced by heparin, oral anticoagulants, and coagulation inhibitors. The disadvantage of the ECT assay is its dependence on prothrombin and fibrinogen
concentrations in the sample.4 More recently, this disadvantage has been overcome by development of the ecarin chromogenic assay (ECA) which has been described as a
simple, rapid, precise, reliable, and specific method for the quantitative determination of DTIs.4 Unfortunately, there are no FDA-approved assays for either ECT or ECA at the present time.
There is particular concern for patients undergoing cardiovascular surgery due to the increased DTI concentrations required for anticoagulation. The manufacturer of argatroban
recommends monitoring patients undergoing percutaneous coronary intervention, or PCI, using activated clotting time (ACT) with a therapeutic goal of 300 seconds to 450 seconds.3
For that reason, most labs have used aPTT and ACT to monitor DTI concentrations during cardiovascular surgery, recognizing that the relationship between either ACT or aPTT and DTI
concentration is rather poor. Several case reports have been published where DTI use in cardiovascular surgery resulted in massive blood loss, further emphasizing the need for
caution in using these agents.
The trend has been toward re-evaluating patients with HIT antibodies to determine which patients might safely be administered heparin for surgery, but a review of HIT management
is beyond the scope of this article.
—Darci R. Block, PhD
Clinical Chemistry Fellow
Mayo Clinic, Rochester, MN
References
- Castellone DD, Van Cott EM. Laboratory monitoring of new anticoagulants. Am J Hematol. 2010; 85:185-187.
- Argatroban. Prescribing information. Brentford, Middlesex, United Kingdom: GlaxoSmithKline; 2009.
-
Gosselin RC, King JH, Janatpour KA, Dager WE, Larkin EC, Owings JT. Comparing direct thrombin inhibitors using aPTT, ecarin clotting times, and thrombin inhibitor
management testing. Ann Pharmacother. 2004;38:1383-1388. - Lange U, Nowak G, Bucha E. Ecarin chromogenic assay — A new method for quantitative determination of direct thrombin inhibitors like hirudin. Pathophysiol Haemost Thromb. 2003;33:184-191.
Brad S. Karon, MD, PhD, is associate professor of laboratory medicine and pathology, and director of the Hospital Clinical Laboratories, point-of-care testing, and phlebotomy services at Mayo Clinic in Rochester, MN.
