The Blood Film and Count (Part 2)

Haemopoietic cells

Peripheral blood cells are produced in the bone marrow. Their precursors are referred to as haemopoietic cells (Fig. 1.12). The only significant function of haemopoietic cells is the production of mature end cells. Recognizable haemopoietic precursors are present in the circulating blood of healthy subjects but, except in the neonatal period and during pregnancy, they are quite uncom- mon and are not often noted in a blood film. They are much commoner in patients with leukaemia or other haematological disorders and in patients with severe infection or other serious systemic diseases.

Myeloblasts

Myeloblasts (Fig. 1.13) are very rare in the blood of healthy subjects. They are larger than lymphocytes but often smaller than monocytes. They have a high nucleocytoplasmic ratio and scanty to moderate amounts of cytoplasm, which varies from weakly to moderately basophilic. (Basophilic in this context in- dicates a blue colour consequent on the uptake of basic dyes.) The nucleus is approximately round, nuclear chromatin is dif- fuse and nucleoli may be apparent. In patients with leukaemia and related disorders, the cytoplasm may contain small numbers of azurophilic granules or other inclusions or vacuoles (see page 91). Myeloblasts are precursors of neutrophils, eosinophils and basophils.

Promyelocytes

Promyelocytes (Fig. 1.14) are rare in the blood of healthy people. They are larger than myeloblasts with more plentiful cytoplasm and consequently a lower nucleocytoplasmic ratio. The cyto- plasm is more basophilic than that of a myeloblast and contains azurophilic (pinkish-purple) primary granules. Sometimes there is a more lightly staining zone in the cytoplasm adjacent to the nucleus, which represents the Golgi apparatus, where granules are produced. The nucleus is round or oval, is usually eccentric, shows some chromatin condensation and has a

Fig. 1.12  A diagram showing the  relationship of haemopoietic precursors to each  other and  to the  end  cells  into which they differentiate. Proliferation of cells  occurs simultaneously with maturation or differentiation so that one  myeloblast is likely to give rise  to 16 mature granulocytes and  one  proerythroblast to 16 red cells.  Myeloblasts, promyelocytes and  myelocytes are all cells capable of cell  division or mitosis. Metamyelocytes and  all later cells  are non-dividing cells.  All red cell  precursors with the exception of late  erythroblasts are dividing cells.  Myeloblasts differentiate not  only  into neutrophils, as shown in the  diagram, but also  into eosinophils and  basophils.

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Plumbing: simple procedures in laboratory

A fault in the plumbing of the laboratory (a dripping tap, a blocked sink, etc.) can hamper laboratory work considerably. Some simple remedies are described below, in case a plumber is not readily available.

Fig. 2.15 Tools and materials for plumbing repairs

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Electricity in laboratory

A reliable energy supply should be available to ensure continuity of the work in a laboratory. The energy can be provided from the following sources:

— mains electricity supply

— generators

— solar energy supply system.

Remote laboratories often have problems in ensuring a continuous supply of elec- trical power and may need to generate electricity by using a local generator or a solar energy supply system.

1. Sources of electricity

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The interpretation of results

It can take considerable effort, and expense, to produce what may seem to be just numbers on pieces of paper or on a computer screen. Understanding what these numbers mean is of crucial importance if the correct diagnosis is to be made, or if the patient’s treatment is to be changed.

How biochemical results are expressed

Most biochemical analyses are quantita- tive, although simple qualitative or semi- quantitative tests, such as those for the presence of glucose in urine, are com- monly encountered methods used for point of care testing. Many tests measure the amount of the analyte in a small volume of blood, plasma, serum, urine or some other fluid or tissue. Results are reported as concentrations, usually in terms of the number of moles in one litre (mol/ L) (Table 3.1).

The concept of concentration is illus- trated in Figure 3.1. The concentration of any analyte in a body compartment is a ratio: the amount of the substance

Fig 3.1 Understanding concentrations. Concentration is always dependent on two factors: the amount of solute and the amount of solvent. The concentration of the sugar solution in the beaker can be increased from 1 spoon/ beaker (a) to 2 spoons/beaker by either decreasing the volume of solvent (b) or increasing the amount of solute (c)

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The use of the laboratory

Every biochemistry analysis should attempt to answer a question that the clinician has posed about the patient. Obtaining the correct answers can often seem to be fraught with difficulty. 

Specimen collection 

In order to carry out biochemical analy- ses, it is necessary that the laboratory be provided with both the correct speci- men for the requested test, and also information that will ensure that the right test is carried out and the result returned to the requesting clinician with the minimum of delay. As much detail as possible should be included on the request form to help both laboratory staff and the clinician in the interpreta- tion of results. This information can be very valuable when assessing a patient’s progress over a period, or reassessing a diagnosis. Patient identification must be correct, and the request form should include some indication of the suspected pathology. The requested analyses should be clearly indicated. Request forms differ in design. Clinical biochem- istry forms in Europe are conventionally coloured green.

A variety of specimens are used in biochemical analysis and these are shown in Table 2.1.

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The clinical biochemistry laboratory

Clinical biochemistry, chemical pathology and clinical chemistry are all names for the subject of this book, that branch of laboratory medicine in which chemical and biochemical methods are applied to the study of disease (Fig 1.1). While in theory this embraces all non- morphological studies, in practice it is usually, though not exclusively, con- fined to studies on blood and urine because of the relative ease in obtaining such specimens. Analyses are made on other body fluids, however, such as gastric aspirate and cerebrospinal fluid. Clinical biochemical tests comprise over one-third of all hospital laboratory investigations.

Fig 1.1 The place of clinical biochemistry  in medicine

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Clinical Presentation of Fungal Keratitis

The early lesions of fungal keratitis are quite characteristic. The symptoms are less than what the size would warrant. The symptoms are also less than that of bacterial keratitis of similar size. Some of the characteristic clinical manifestations include, dry raised surface, feathery margins, satellite lesions and posterior corneal abscess. Of these, feathery margins are very typical of fungal corneal ulcers (Figs 2.1 to 2.21).

Fig. 2.1: An early fungal ulcer presenting with very mild congestion and few symptoms. In fungal keratitis the signs are disproportionately higher than the symptoms

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