Answering Your Questions

Oct. 1, 2011

Handling values that are not ordered 

I am the medical director for a centralized laboratory serving 25 hospitals and many outreach clients and nursing homes. Two of our three hospital systems are migrating to Epic in the near future, and, as part of that process, the following question was raised: If a physician in our system orders only a hemoglobin and hematocrit (or platelet count), the analyzer performs the whole CBC without differential. Can we ethically and legally only report the Hg and Ht? What if there are abnormal or critical values in the remainder of the CBC? Are there compliance concerns? Currently, we have no orderable test for just an H and H or platelet count because of these concerns.


Within our institution we do have an orderable for H/H only or platelet-only which we offer for the OR, where a rapid hemoglobin or platelet count is often desired without having to worry about the WBC or differential count “flagging” and causing the sample to be reviewed or results to be otherwise delayed. I don't know that I have all the answers, but we have handled this internally by creating a policy for handling critical values that are not ordered (a critical platelet value for a patient with an H/H order). In these cases (which are rare) we call the result like any other critical, and also order and result (but do not charge) the critical analyte. We do not worry about abnormals, just criticals. I have never seen guidelines for this and I don't know that there are any regulations that cover this situation, other than perhaps College of American Pathologists (CAP), Joint Commission and other requirements that labs have a documented policy and procedure for handling critical results. One might argue that this creates a compliance problem by performing tests which are not ordered. However I believe that if you have an institutional policy for this, and assuming that you do not charge for the tests not ordered, you could make a good argument for reporting the “unordered” test as part of the process necessary to complete the test that has been ordered directly by the physician.

There may be a compliance/regulatory issue with NOT offering these analytes separately, as CMS requires that you offer an option to order as individual tests any tests bundled in a panel or performed as a reflex. CBCs are difficult to interpret with regard to this requirement, but in general most labs do offer CBC with and without differential to meet compliance requirements. Whether this would apply to a hemoglobin or platelet value only is difficult to say. If the charge for an H/H or platelet only would be significantly different from that of a CBC, then there could be a compliance issue, and that would provide a reason to offer these tests alone when needed.

On the whole I would say that if there is a need for an H/H or platelet count only within your practice, for compliance and medical reasons I would lean towards offering it. Whether this raises ethical issues in a more complicated question, but if the physician has ordered only an H/H then I do not feel it is unethical to report only those values, even if other parts of the CBC are abnormal (as defined by the reference ranges for those parameters). If the physician had needed that information, he or she could have ordered it directly.

—Brad S. Karon MD, PhD, Director
Hospital Clinical Laboratories
Mayo Clinic, Rochester, MN

Blood specimens from patients receiving TPN

Our inquiry relates to TPN therapy through a line, and nurses accessing and wasting 20cc, then collecting blood specimens from lines the TPN therapy has been continuously running. We are having questionable lab results due to the TPN interferences. Please clarify to nursing management and staff the correct blood collection techniques for patients receiving TPN therapy to remove the interferences and increase the reliability of the lab tests collected.

Should the TPN be stopped for 30 min. to 60 min. and then collected directly from peripheral vein?

Does the TPN need to be stopped at all prior to collecting from peripheral vein?

Can collection from the TPN infusion site ever be used for blood collection without interference from residual TPN product?

Should blood specimens always be collected from peripheral vein when patient is on TPN therapy?


Over the years we have collected pieces of information and data sets in an attempt to deal with the issue of collecting blood specimens from patients receiving total parenteral nutrition (TPN). I can attempt to answer all your questions, but realize that the information is based upon our experience and limited data rather than on concrete recommendations or guidelines. TPN is a lipid-rich nutritional mixture and thus tends to stick to indwelling catheters used to administer the mixture, leading to an increased risk of sample contamination compared to other medications administered intravenously. Besides lipids, TPN often contains glucose, electrolytes (Na, K, Cl, Ca, Mg, phosphorus), vitamins, insulin and other constituents that patients who will not be eating for some period of time will need. Thus the timing between TPN administration and laboratory draws depends to some extent on what things you wish to measure, relative to the constituents of the typical TPN solution and the time required to reach a “steady state” concentration of that analyte. In addition, nutritional support of infants and adults differs. Let me try to answer your questions by discussing some of these issues separately.

In infants TPN is often administered with lipid emulsion, an even more (compared to TPN) lipid-rich compound. Lipid emulsion has been shown to interfere with the analysis of common laboratory tests in two distinct ways. Lipid emulsion can cause an analytical interference due to the light-scattering effects of phospholipids, and this has been demonstrated for many common chemical tests performed on neonates receiving this therapy.1 The free phospholipid content of lipid emulsion also has been shown to cause hemolysis in samples contaminated with this substance, an effect which is greater in samples collected in vacutainer tubes compared to blood gas syringes.2 In addition, infants (usually very small premature infants who receive this therapy) have a small total blood volume, and thus large discard volumes prior to drawing samples are not possible. All these issues make drawing samples during TPN/lipid emulsion therapy for infants challenging.

Furthermore, many protocols call for frequent measurement of triglyceride concentrations in infants receiving TPN/lipid emulsion support. In our experience triglyceride concentrations are extremely high if drawn while TPN/lipid emulsion is being administered, and almost certainly do not reflect fasting or near-fasting concentrations. Primarily for this reason (to allow better assessment of triglyceride concentrations) various protocols have been developed that call for either a 4-hour “window” between TPN/lipid emulsion and laboratory draws, a 1-hour window, or no window at all (drawing labs during TPN/lipid emulsion administration), depending largely upon whether triglyceride concentrations will be measured. Unfortunately there is little data to support any one protocol or “window” period. Given all the issues that have been identified in this patient population however, a window of 1-4 hours seems reasonable between TPN/lipid emulsion administration and laboratory blood draws for neonates when possible.

In adults the use of lipid emulsion is much less common. For adult patients the issue of timing may depend more upon what tests are being requested. As discussed already, if lipids (cholesterol, triglycerides) will be measured, then a 4-hour window between TPN and testing is desired to approach a fasting state and allow reasonable interpretation of these values. Electrolytes, particularly Ca, Mg, and phosphorus, present an interesting dilemma. Intravenous electrolyte replacement protocols often call for repeat assessment of Ca, Mg and phosphorus at least four hours after infusion to insure that steady state concentrations are reached. However, these replacement protocols call for greater amounts of Ca, Mg or phosphorus to be given over shorter periods of time compared to TPN therapy. Thus it is not clear whether a window period is needed between TPN administration and assessment of Ca, Mg or phosphorus in adult patients, but some period of time between 1and 4 hours may be optimal. For most other common chemical and hematologic testing, discontinuing TPN administration for 2 to 3 minutes is probably sufficient to allow for appropriate interpretation of results (assuming that random rather than fasting glucose is being measured).

The question of whether a peripheral vein must always be used has no clear answer, but my sense is probably not. Because TPN is lipid-rich and tends to stick to catheters, collection of blood specimens from the same lumen of a catheter used to administer the solution is best avoided. However if the patient has a multiple-lumen catheter, or multiple catheters, then it should be possible to collect blood specimens from a lumen not used to administer TPN. In these situations (with the exceptions noted above), discontinuation of TPN administration for 2 to 3 minutes, followed by a reasonable flushing and discard, should allow for appropriate samples to be obtained. CLSI guidelines call for discard volume of at least 2X the dead space of the catheter for tests other than coagulation, and 5 mL or 6X the dead space of the catheter for coagulation tests. Because collection of unacceptable samples is more common in patients receiving TPN, some institutions choose to use the coagulation guidelines (5 mL or 6X dead space) for all TPN blood collections, though I know of no guidelines or recommendations that specifically support this practice. Note that these discard volumes are only appropriate for adult patients; institutions generally use smaller discard volumes for neonates and often return the discard to the patient after blood collection.

In summary, for adult patients the practice of collecting blood specimens from the same lumen used to administer TPN solutions is best avoided. There is an increased chance of sample contamination due to the lipid-rich nature of these solutions, and repeated attempts to clear catheters by discarding 10 to 20 mL of blood before each collection may not be appropriate for some patients. It is better in these instances to collect blood specimens from adults by venipuncture. For most tests collected from adult patients on TPN, discontinuing the infusion for 2 to 3 minutes prior to blood collection should be adequate. In premature neonates, however, QA venipuncture can be very difficult. For a neonate with only one catheter for medication infusion and blood sample collection, allowing a window period of 1 to 4 hours to wash out the TPN and following institutional procedures for blood collection from indwelling catheters may be the best approach. Certain tests (triglycerides, possibly Ca, Mg and phosphorus) are best collected 1 to 4 hours after TPN has been administered.

—Brad S. Karon MD, PhD, Director
Hospital Clinical Laboratories
Mayo Clinic, Rochester, MN

Further reading

  1. Dimeski G. A commentary on the effect of lipid emulsions on pathology tests. Anaesthesia. 2009;64:1031-1035.
  2. Jaben EA, Koch CD, Karon BS. Lipid emulsion solution: A novel cause of hemolysis in serum and plasma blood samples. Clin Biochem. 2011 Feb; 44(2-3):254-256.

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