Q&A column

Editor: Frederick L. Kiechle, MD, PhD

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Q. I have heard that in the United States there is a shortage of tuberculin skin test antigen used for detecting Mycobacterium tuberculosis infection. Is there an alternative that can be used?

A. August 2019—There are two purified protein derivative (PPD) tuberculin antigens—Aplisol (Par Pharmaceuticals) and Tubersol (Sanofi Pasteur)—licensed by the FDA for use in performing tuberculin skin tests (TST). Par Pharmaceuticals had announced that, beginning in June 2019, it anticipated a three- to 10-month shortage of Aplisol. Based on this announcement, the Centers for Disease Control and Prevention recommends the following¹:

• Substitute interferon-gamma release assay (IGRA) blood tests for TSTs. However, clinicians should note that the criteria for IGRA blood test interpretation are different from the criteria for interpreting TSTs. (Interpretation guidelines were published in the CDC’s MMWR Recommendations and Reports in 2010.²) The CDC warns physicians that switching between PPD skin test products or between TSTs and blood tests in serial testing may cause apparent conversions of results from negative to positive or reversions from positive to negative. This may be due to inherent interproduct or intermethod discordance rather than change in M. tuberculosis infection status.
• Substitute Tubersol for Aplisol. Concordance between the two products has been demonstrated in controlled studies.
• “Prioritize allocation of TSTs, in consultation with state and local public health authorities. Prioritization might require the deferment of testing some persons. CDC recommends testing only for persons who are at risk for TB. Groups at high risk for TB infection include 1) persons who are recent contacts exposed to persons with TB disease; 2) those born in or who frequently travel to countries where TB disease is common; 3) those who currently or previously lived in large group settings (such as homeless shelters or correctional facilities); 4) persons with compromised immune systems, including those with health conditions or taking medications that might alter immunity; and 5) children, especially those aged <5 years, if they are in one of the risk groups noted above.”

1. Centers for Disease Control and Prevention. Nationwide shortage of tuberculin skin test antigens: CDC recommendations for patient care and public health practice. MMWR Morb Mortal Wkly Rep. 2019;68(24):552–553.
2. Mazurek GH, Jereb J, Vernon A, et al. Updated guidelines for using interferon gamma release assays to detect Mycobacterium tuberculosis infection—United States, 2010. MMWR Recomm Rep. 2010;59(RR-5):1–25.

Scott J. Zimmerman, DrPH, MPH, HCLD (ABB)
Senior Director
Department of Science and Technology
Laboratory Corporation of America Holdings
Burlington, NC

In this “Best of Q&A” series, we reprint select coagulation-related questions and answers. All have been chosen for their timeliness and relevance today. The following question and answer were published in June 2016.

Q. Our coagulation department staff is debating the usefulness of incubated mixing studies for prolonged prothrombin times that do not correct. We incubate only for APTT studies. Even though none of the prothrombin time factors are time dependent (despite rumblings concerning factor V), we still see on CAP participant summary reports a significant number of labs that perform incubated prothrombin time studies. Half of the staff want to do incubated PTs to make sure we are not missing important information, but none can explain the value of doing so. What is the value of doing incubated mixing studies on prolonged prothrombin times?

A. There are few data in the literature regarding the performance of mixing studies and even fewer that address prothrombin time (PT) mixing studies specifically. In general, PT mixing studies can be performed similarly to activated partial thromboplastin time (APTT) mixing studies and for similar indications (i.e. to determine if clotting time prolongation is more likely due to a factor deficiency versus an inhibitor). Most commonly, a 1:1 mixture of patient plasma and normal pooled plasma is assayed in the test system showing initial prolongation (PT or APTT). The results are interpreted as correction (suggestive of a factor deficiency) or non-correction (suggestive of an inhibitor), with mixing study correction defined and validated by each individual laboratory. The current CLSI guideline discussing APTT and PT (document H47-A2) states that PT mixing studies are less commonly performed since PT prolongation is rarely due to lupus anticoagulants or factor inhibitors. The guideline notes, though, that if a PT prolongation is suspected to be due to an inhibitor, then a mixing study is recommended and both immediate and incubated mixing studies “can” be performed, similar to the APTT.

In practice, there are few instances in which an incubated PT mixing study may add value to the immediate mixing study. Factor V inhibitors have rarely been reported to demonstrate time dependence; however, the majority of factor V inhibitors will demonstrate their effects in an immediate mixing study. For laboratories that handle secondary specimen aliquots, incubated PT mixing studies may provide supporting evidence that a sample represents potassium EDTA plasma rather than sodium citrate plasma. However, the presence of EDTA can be determined using other simple and widely available laboratory methods (namely, measuring calcium and potassium in the sample). Based on the limited available evidence, routine performance of incubated PT mixing studies does not appear to offer a significant incremental benefit over the performance of immediate PT mixing studies. In cases in which there is a strong clinical and/or laboratory suspicion of factor V inhibitor but correction of the immediate mixing study, incubated PT mixing studies may provide a clue to the correct diagnosis. However, laboratories would do well to remember that mixing studies are screening tests for inhibitors, and if the level of clinical or laboratory suspicion for an inhibitor is high, proceeding directly to specific factor assays and inhibitor titers (measured in Bethesda units) for the factor(s) suspected to be involved would also be a reasonable step.

1. Clinical Laboratory and Standards Institute. One-Stage Prothrombin Time (PT) Test and Activated Partial Thromboplastin Time (APTT) Test; Approved Guideline—Second Edition (H47-A2). May 30, 2008.
2. Ledford-Kraemer M. All mixed up about mixing studies. Clotting Times: The Official Newsletter of CLOT-ED. 2004;3(4):1–11.
3. Rodgers GM, Lehman CM. Hemostasis screening assays. In: Bennett ST, Lehman CM, Rodgers GM, eds. Laboratory Hemostasis: A Practical Guide for Pathologists. 2nd ed. Cham, Switzerland: Springer International Publishing; 2015:74.
4. Lipshitz J, Chelliah T, Aledort L. A case of factor V inhibitor with complete correction of the PT and aPTT upon mixing. Am J Hematol. 2012;87(3):313–315.
5. Ashizawa M, Kimura S, Wada H, et al. Acquired factor V inhibitor associated with life-threatening bleeding and a mixing test result that indicated coagulation factor defi­ciency. Hematology. 2013;18(5):300–304.
6. Franchini M, Lippi G. Acquired factor V inhibitors: a systematic review. J Thromb Thrombolysis. 2011;31(4):449–457.
7. Moser KA, Adcock Funk DM. Pitfalls in special coagulation testing: three illustrative case studies. Int J Lab Hematol. 2013;35(3):​334–338.

Karen A. Moser, MD
Currently Assistant Professor
Department of Pathology
University of Utah School of Medicine Medical Director
Hemostasis/Thrombosis Laboratory
ARUP Laboratories
Salt Lake City
Member of the CAP Coagulation
Resource Committee at time
of original publication