Clinical pathology

Author: Mikael Häggström, M.D.

Memorization-worthy: For the most likely types of cases and/or questions that you may be responsible for, know where to find local policies and procedures, and have a good idea of whom to ask for further advice. Make sure you have access and/or contact details for whenever and wherever you are likely to need them. Preferably have at least a quick look at policies and procedures so that you have an idea of what kind of answers you will find there when needed.

Hematopathology

For learning hematopathology, focus on the aspects of it that you may be expected to handle rather independently, mainly including peripheral blood smears and general screening of lymph nodes. Otherwise, a hematopathology workup is generally a very complex matter that should be performed by subspecialists thereof. In the unlikely event that you will end up at a workplace without an internal arrangement for how to consult a hematopathologist, make such arrangement yourself, since it will save you a lot of time compared to even trying to learn yourself how to perform a modern hematology workup.

Sample tube types

The following is an overview of the main types of sample tube types, organized by order of draw (the recommended sequence by which fluid is drawn) during blood draws:

Tube cap color or type in order of draw	Additive	Usage and comments
Blood culture bottle	Sodium polyanethol sulfonate (anticoagulant) and growth media for microorganisms	Usually drawn first for minimal risk of contamination. Two bottles are typically collected in one blood draw; one for aerobic organisms and one for anaerobic organisms.
Light blue	Sodium citrate (anticoagulant)	Coagulation tests such as prothrombin time (PT) and partial thromboplastin time (PTT) and thrombin time (TT). Tube must be filled 100%.
Plain red	No additive	Serum: Total complement activity, cryoglobulins
Gold (sometimes red and grey "tiger top")	Clot activator and serum separating gel	Serum-separating tube: Tube inversions promote clotting. Most chemistry, endocrine and serology tests, including hepatitis and HIV.
Dark green	Sodium heparin (anticoagulant)	Chromosome testing, HLA typing, ammonia, lactate
Light green	Lithium heparin (anticoagulant)	Plasma. Tube inversions prevent clotting
Lavender ("purple")	EDTA (chelator / anticoagulant)	Whole blood: CBC, ESR, Coombs test, platelet antibodies, flow cytometry, blood levels of tacrolimus and cyclosporin
Pink	EDTA (chelator / anticoagulant)	Blood typing and cross-matching, direct Coombs test, HIV viral load
Royal blue	EDTA (chelator / anticoagulant)	Trace elements, heavy metals, most drug levels, toxicology
Tan	EDTA (chelator / anticoagulant)	Lead
Gray	Sodium fluoride (glycolysis inhibitor) Potassium oxalate (anticoagulant)	Glucose, lactate
Yellow	Acid-citrate-dextrose A (anticoagulant)	Tissue typing, DNA studies, HIV cultures
Pearl ("white")	Separating gel and (K2)EDTA	PCR for adenovirus, toxoplasma and HHV-6

Lab management

Lab management is essentially about handling each of the extremely various lab-related situations that arise, and can generally be achieved by:

• Common sense
• Gathering enough information before a decision

• Identifying what questions needs answering
• Asking proper expertise and/or looking up relevant information in proper sources (see learning pathology), which may include local protocols as well as policies of accrediting organizations of the department (which are generally more stringent than the national or regional laws).

For location-specific issues, it generally helps to personally come and see the location at hand.

Test question Retention time (You may skip this question if you don't expect to ever be part of laboratory management in the US.) You work in a pathology department in the United States, which is accredited by the College of American Pathologists (CAP). Your local procedure manual states that non-forensic paraffin-embedded blocks must be retained for at least 10 years before being thrown away. In order to save storage space, one suggestion that gets brought up is to reduce the retention time to 5 years. You look up the issue, and find that federal U.S. law (the Clinical Laboratory Improvement Amendments; CLIA) states that such blocks must be retained for at least 2 years. Is it acceptable to finish the look-up here, and agree to reduce the retention time of non-forensic paraffin-embedded blocks to 5 years in this department? Answer: This pathology department is accredited by CAP, whose requirements commonly exceed those of CLIA, in this case stating at least 10 years for non-forensic paraffin-embedded blocks. Therefore, it is not acceptable to relax minimum retention times without first having had a look at CAP requirements, if the lab is accredited by it (don't assume it isn't without checking). Thus, the answer to this question is: no. Furthermore, state laws need to be considered as well, as they may exceed those of CLIA. For quick look-up in the future, the following are the most relevant retention times, as given by CLIA as well as by CAP (more comprehensive lists are available in their sources): Microscopy slides Histology and non-forensic autopsy 10 years Forensic autopsy Indefinitely Cytology, fine needle aspiration 10 years Cytology, apart from fine needle aspiration 5 years Paraffin-embedded blocks Non-forensic 2 or 10 years Forensic Indefinitely Requisition forms and test reports Pathology reports 10 years Other 2 years Blood bank records Quality control records 5 years Donor and recipient records 10 years Records of indefinitely deferred donors Indefinitely Wet tissues Until report is completed or 2 weeks thereafter Proficiency testing records and quality management/quality control records (except for blood bank) 2 years Discontinued procedures 2 years Blood smears and other body fluid smears, microbiology slides (including Gram stains) 7 days Flow cytometry plots 10 years

Test indication

In simplified terms, the indication for a test on an individual is primarily determined by its overall positive impact, referred to as the net benefit. Tests are selected when the anticipated benefit outweighs the expected harm. The net benefit may roughly be estimated by:

• b_n = Λp * r_i * ( b_i - h_i ) - h_t

, where:

• b_n is the net benefit of conducting a test
• Λp which represents the absolute difference between the pre- and posttest probabilities of certain conditions, like diseases, that the test is expected to detect. This absolute difference is significantly influenced by the test's power, characterized by parameters such as sensitivity, specificity, or likelihood ratio. Additionally, the pre-test probability plays a role, with lower pre-test probabilities resulting in diminished absolute differences. This means that powerful tests may show a low absolute difference for unlikely conditions, while less powerful tests can still have a substantial impact on highly suspected conditions.
• r_i is the rate of probability differences leading to changes in interventions. For instance, if a medical test primarily affects the likelihood of one disease over another but both diseases have the same treatment (or no available treatment), the impact on interventions is minimal, and the test may lack value in that aspect.
• b_i is the benefit of changes in interventions for the individual
• h_i is the harm of such changes for the individual, including side effects of medical treatment
• h_t is the harm caused by the test itself.

Several other considerations impacting the decision on whether to conduct a medical test include factors such as the test's cost, the accessibility of supplementary tests, potential interference with subsequent tests (for example, abdominal palpation inducing intestinal sounds that may disrupt abdominal auscultation), the time required for the test, and various other practical or administrative aspects. It is also essential to evaluate the potential benefits of a diagnostic test in relation to the costs associated with unnecessary tests, subsequent follow-ups, and possibly unwarranted treatment of incidental findings.

Other clinical pathology articles

• Common on-call topics

• Blood bank

• Kleihauer–Betke test
• Genetics questions
• Thromboelastography

• Platelet aggregation study
• Cytology
• Microbiology

Main page

• Main page of Patholines