Leukocyte Alkaline Phosphatase Stain (LAP)
Optimizing Laboratory Testing for Hematologic
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Published: September 2010Print Record of Viewing
Dr. Hanson is presenting the first topic in our series "Optimizing Laboratory Testing for Hematologic Disorders." This series will address guidelines for appropriate laboratory test utilization in hematologic disorders and represents the combined effort of the Divisions of Hematopathology and Laboratory Genetics. This first presentation will address the leukocyte alkaline phosphatase stain (LAP), a traditional laboratory assay that no longer has value in clinical practice.
Presenter: Curtis A. Hanson, MD
- Division of Hematopathology at Mayo Clinic
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Welcome to Mayo Medical Laboratories' Hot Topics. These presentations provide short discussions of current topics and may be helpful to you in your practice.
Our speaker for this program is Dr. Curtis Hanson, from the Division of Hematopathology at Mayo Clinic. Dr. Hanson is presenting the first topic in our series "Optimizing Laboratory Testing for Hematologic Disorders." This series will address guidelines for appropriate laboratory test utilization in hematologic disorders and represents the combined effort of the Divisions of Hematopathology and Laboratory Genetics. This first presentation will address the leukocyte alkaline phosphatase stain (LAP), a traditional laboratory assay that no longer has value in clinical practice.
Optimizing Laboratory Testing for Hematologic Disorders Series
This is a first in a series of hot topics that will address the issue of appropriate laboratory test utilization in hematologic disorders. This is a combined effort of the Divisions of Hematopathology and Laboratory Genetics with the primary focus of identifying the correct algorithmic testing approach for various hematologic diseases and secondly to demonstrate how the laboratory can and should be engaged in reducing unnecessary testing.
The Big Picture: We Need to Focus On...
So why do we need to discuss this topic? The big picture is that we need to understand and react to three large drivers in health care. The first driver is quality of care. If the wrong test is ordered, there is an obvious impact on overall quality as is also the converse — if the right test is not ordered. Certainly as laboratorians we understand the impact of not correlating our sophisticated test results which clearly puts the clinician at risk of not responding correctly to our laboratory studies. The second driver is the impact of inappropriate test utilization on the cost of care, whether the impact is on the patient, on the laboratories or the institution in which those laboratories reside, and in how we best utilize the limited laboratory resources that we have.
Finally, healthcare reform, whether done today or in the future, will impact the laboratory. Clearly, rising healthcare costs will force us to focus on how to best utilize limited laboratory resources. As bundled payments become increasingly the primary mechanism of payment for patient care, clearly there will be incentives to do the right test at the right time as opposed to performing volumes of tests. All of these big picture issues really point to the realities of healthcare — both for today and into the future.
LAP: A Test That Time Has Passed By
Today we will be talking about leukocyte alkaline phosphatase stain, or LAP, as a test that time has truly passed by. Occasionally the LAP may be confused with other tests. The LAP is not a serum alkaline phosphatase which is used to evaluate liver, bone, intestinal, and other diseases. Another test that is sometimes mistakenly confused with the LAP is leucine aminopeptidase. This enzyme is marketed as a potential useful marker of hepatic function and may be abnormal in some cancers. Both of these assays may be the intended goal of the clinician, but by mistake, a LAP is ordered instead.
For today’s talk, I have to use a variety of abbreviations. LAP, which you have already heard me talk about. Chronic myelogenous leukemia or CML; polycythemia vera or PV; essential thrombocythemia or ET; primary myelofibrosis or PMF; the generic category of myeloproliferative neoplasms or MPN; and the generic category of myelodysplastic syndromes or MDS.
LAP: Assay History
The LAP has had a presence in the clinical hematology laboratory for several decades. In 1955, Kaplow described a cytochemical staining technique for assessing LAP activity on a peripheral blood smear, and by the late 1950's, a low LAP score was known to be associated with CML. By the mid 1960's, an LAP score of >100 was included as one of the diagnostic criteria for PV.
LAP: Original Clinical Utility
The original clinical utility of LAP was very important in its day. A low LAP score was one of the key recognition criteria for CML and clearly was very helpful in the era before cytogenetic karyotyping became readily available. Conversely, a high LAP score supported the diagnosis of an infectious or bacterial process or as a diagnostic indicator of PV. More importantly, it was used to exclude the diagnosis of CML. Again, this was clearly helpful in the pre‑serum erythropoietin and pre-JAK2 testing era.
LAP Procedure—an Inexact Science!
The LAP assay is clearly a subjective test based on an inexact science. The procedure involves counting 100 neutrophils including lobed and band forms but excluding other left-shifted granulocytes, eosinophils, and basophils. The reaction is scored from 0 to 4 depending on the number of stained granules and the intensity of the stain. The number of cells is multiplied by the score and added up with a normal range being from 40 to 100.
Typical Staining Type Reaction
This slide points out the typical staining type reaction and highlights a neutrophil with a score of 4 versus a neutrophil with a score of 0.
Subjective Nature of the Scoring Process
This slide, again, points out the subjective nature of the scoring process ranging from a 4 to a 3 to a 2 to a 1. I am sure you can appreciate the inexact nature of this scoring process.
LAP Scoring Example
As I mentioned before, the numbers are multiplied and added up and, in this particular example, a LAP score of 90 is obtained. You can also appreciate that there are various combinations that would enable you to get the same score, which clearly highlights the variability in the evaluation and interpretation process.
LAP Discontinued at Mayo Clinic
The LAP was discontinued here at Mayo Clinic in 2007 after it was reviewed with multiple clinical groups and was decided that it had no current clinical value. Mayo Medical Laboratories will be discontinuing the availability of this test to clients beginning September 2010.
Low LAP Scores* Not Specific for CML: A Mayo 1-Year Experience
The next four slides summarize a one-year experience that we had here looking at outcomes in patients who had LAPs ordered on Mayo Clinic patients. If we look at patients who had low LAP scores, there were a variety of myeloid malignancies that were associated with a low LAP. This included six cases of CML but a variety of MPN and MDS disorders were also identified with low LAP scores. This clearly points out the lack of specificity of a low LAP score for CML.
Low LAP Scores* Do Not Indicate Myeloid Malignancy: A Mayo 1-Year Experience
In addition, a low LAP score did not always indicate a myeloid malignancy. As you can see, there were a variety of non-myeloid malignancies and non-malignant conditions that were associated with a low LAP score, again pointing out the lack of diagnostic specificity of a low LAP score.
High LAP Scores* Not Specific for PV: A Mayo 1-Year Experience
Likewise, a high LAP score was not limited to PV amongst the myeloid malignancies and was found in all of the MPNs as well some cases of MDS and in a single case of bcr-abl-positive CML.
High LAP Scores* Do Not Indicate Myeloid Malignancy: A Mayo 1-Year Experience
In addition, a high LAP score did not always indicate a myeloid malignancy or even an infectious process. As you can see, there were a variety of non-malignant, non-infectious etiologies that were associated with a high LAP score, again pointing out the lack of diagnostic specificity of a high LAP score.
LAP: A Test Whose Time Has Passed
To summarize why are we discontinuing the LAP: First of all, a low LAP score is not specific for CML and can be found in patients with MPN, MDS, and even benign or reactive conditions. A high LAP score is not specific for PV and can be identified in patients with other MPNs, MDS, and numerous, benign, non-infectious conditions. Likewise, a high LAP score is not sensitive for PV as not all PV cases will have a high LAP score.
Consequences of an Abnormal LAP
The outcome of an abnormal LAP score is certainly not inconsequential. It may lead to a bone marrow procedure as well as cytogenetic studies, FISH and RT-PCR studies for BCR/ABL , as well as JAK2 V617F mutation analysis. All in all, an abnormal LAP may result in a series of procedures and assays that may not be needed or it will have simply delayed appropriate test selection at the initial evaluation.
Alternatives to LAP
What are testing alternatives to LAP? Clearly, it depends on the clinical picture. If the patient has an unexplained elevation in counts and/or unexplained splenomegaly, then a myeloproliferative disorder will likely be considered in the differential diagnosis. If CML is a consideration, then BCR/ABL testing is the appropriate route, whereas if PV or other MPNs are in the differential, then JAK2 testing is required.
Alternatives to LAP: BCR/ABL
At Mayo Clinic, we offer a variety of assays for both of these scenarios. For BCR/ABL at diagnosis, #8506 or chromosome analysis for hematologic disorders in bone marrow, #89006 for RT‑PCR detection of BCR/ABL as a qualitative diagnostic assay, and #80578 for FISH studies looking for BCR/ABL translocation t(9;22) can be used to evaluate for a possible CML.
Alternatives to LAP: JAK2
Likewise, if JAK2 is needed at diagnosis for evaluation, one of these tests can be ordered depending on the tissue source. JAK2 testing for the exon 12 mutation, #89189 and #60025, should only be done if PV is the likely diagnosis and the initial V617F mutation is negative.
In conclusion, LAP is a test that time has clearly passed by. It is a test that may result in nonspecific and potentially misleading information and, thus, we strongly believe that the LAP provides no clinical value for patient care. Therefore, the LAP test will be discontinued September 2010. If other testing is needed to diagnose or exclude CML or other MPN, then BCR/ABL or JAK2 assays should be considered.
Thank you for the opportunity to present to you and hopefully I have been able to introduce you to the concept of how the laboratory can help control utilization of ancillary testing in hematologic diseases. Although the LAP assay will not be a significant assay in any test utilization strategy, the approach we used does show how a laboratory can use data analysis to help convince clinicians on the lack of clinical value for a potentially obsolete test. Our second goal today was more practical as we wanted to share why we will no longer be offering LAP testing through Mayo Medical Laboratories. I look forward in future Hot Topics to continue our discussion of the laboratory’s and pathologist’s roles in appropriate laboratory test utilization in hematologic disorders.