Dispatch of specimens to a reference laboratory

The peripheral laboratory sends specimens to reference laboratories or more specialized laboratories for examinations that cannot be carried out locally. For example, serological examinations for treponemal infection or typhoid; culture of stools for detection of cholera vibrio; and histological examination of biopsy material.

Table 3.2 shows, for each type of specimen and each examination:

— which container and preservative (where necessary) to use;

— how much of the specimen to send;

— how long the specimen will keep.

1. Packing specimens for dispatch 

Always observe the regulations in force in your country. 

Double pack specimens. Place the specimen in the bottle or tube and seal her- metically (fixing the stopper with sticking-plaster; see Fig. 3.73). 

Check that the bottle is labelled with the patient’s name and the date of collection of the specimen. Then place the sealed bottle in an aluminium tube with a screw cap. Wedge it in the tube with absorbent cotton wool. 

Wrap the request form around the metal tube (Fig. 3.74). It should show: 

— the patient’s name (written in capital letters) and date of birth; 

— the nature of the specimen; 

— the date of collection of the specimen;

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Disposal of laboratory waste

1. Disposal of specimens and contaminated material 

Any clinical material brought into the laboratory and any apparatus used to handle this material must be considered as infectious. To avoid laboratory accidents, make sure that priority is given to correct handling and disposal of specimens and contaminated material. 

2. Incineration of disposable materials 

Making an incinerator (Fig. 3.70) 

An old metal drum is suitable for this purpose. 

1. Fix a strong metal grating (G) firmly about one-third of the way up the drum. 

2. Cut a wide opening or vent (V) below the level of the grating. 

3. Find a removable lid (L) for the drum.

Using an incinerator 

● At the end of each morning’s and each afternoon’s work, place all used stool and sputum boxes on the grating of the incinerator (Fig. 3.71).

Fig. 3.70 Components of an incinerator G: metal grating; L: lid; V: vent.

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Cleaning, disinfection and sterilization in the laboratory

1. Cleaning glassware and reusable syringes and needles

Instructions for cleaning:

— glass containers (Erlenmeyer flasks, beakers, test-tubes)

— pipettes

— microscope slides

— coverslips

— reusable syringes and needles.

Glass containers

New glassware

Glassware that has never been used may be slightly alkaline. In order to neutralize it:

● Prepare a bowl containing 3 litres of water and 60 ml of concentrated hydrochlo- ric acid (i.e. a 2% solution of acid).

● Leave the new glassware completely immersed in this solution for 24 hours.

● Rinse twice with ordinary water and once with demineralized water.

● Dry.

Dirty glassware

Preliminary rinsing

Rinse twice in cold or lukewarm water (never rinse bloodstained tubes in hot water).

If the glassware has been used for fluids containing protein, it should be rinsed immediately and then washed (never allow it to dry before rinsing).

Soaking in detergent solution

Prepare a bowl of water mixed with washing powder or liquid detergent. Put the rinsed glassware in the bowl and brush the inside of the containers with a test-tube brush (Fig. 3.57). Leave to soak for 2–3 hours.

 

Fig. 3.57 Cleaning dirty glassware

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Measurement and dispensing of liquids

Many of the liquids handled in the laboratory are either infectious, corrosive or poisonous. It is important for the prevention of accidents that the correct procedures for the measurement and dispensing of these liquids are clearly under- stood and are followed conscientiously.

Many of the new procedures for analysis require very small volumes of fluid and various pipetting and dispensing devices are available to enable small volumes to be measured with great precision.

Large volumes can be measured using a measuring cylinder or a volumetric flask. A measuring cylinder measures various volumes of fluid but is not very accurate. A volumetric flask measures a single volume of fluid, e.g. 1 litre, accurately.

Small volumes of fluid (0.1–10 ml) can be dispensed rapidly and accurately using one of the following methods:

● A fixed or variable volume dispenser attached to a reservoir made of glass or polypropylene. Various volumes from 0.1 to 1.0 ml and from 2.0 to 10.0 ml can be dispensed.

● A calibrated pipette with a rubber safety bulb.

1. Pipettes 

Types of pipette 

Graduated pipettes

Graduated pipettes have the following information marked at the top (Fig. 3.44):

— the total volume that can be measured;

— the volume between two consecutive graduation marks. There are two types of graduated pipette (Fig. 3.45):

● A pipette with graduations to the tip (A). The total volume that can be measured is contained between the 0 mark and the tip.

● A pipette with graduations not extending to the tip (B). The total volume is contained between the 0 mark and the last mark before the tip (this type is re- commended for quantitative chemical tests).

Various volumes can be measured using graduated pipettes. For example:

— a 10-ml pipette can be used to measure 8.5 ml;

— a 5-ml pipette can be used to measure 3.2 ml;

— a 1-ml pipette can be used to measure 0.6 ml.

Fig. 3.44 A graduated pipette

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Hyponatraemia: assessment and management

Clinical assessment 

Clinicians assessing a patient with hyponatraemia should ask themselves several questions.

- Am I dealing with dangerous (life-threatening) hyponatraemia?

- Am I dealing with water retention or sodium loss?

- How should I treat this patient?

To answer these questions, they must use the patient’s history, the findings from clinical examination, and the results of laboratory investigations. Each of these may provide valuable clues.

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Hyponatraemia: pathophysiology

Hyponatraemia is defined as a serum sodium concentration below the reference interval of 133–146 mmol/ L. It is the electrolyte abnormality most frequently encountered in clinical biochemistry.

Development of hyponatraemia

The serum concentration of sodium is simply a ratio, of sodium (in millimoles) to water (in litres), and hyponatraemia can arise either because of loss of sodium ions or retention of water. 

- Loss of sodium. Sodium is the main extracellular cation and plays a critical role in the maintenance of blood volume and pressure, by osmotically regulating the passive movement of water. Thus when significant sodium depletion occurs, water is lost with it, giving rise to the characteristic clinical signs associated with ECF compartment depletion. Primary sodium depletion should always be actively considered if only to be excluded; failure to do so can have fatal consequences. 

- Water retention. Retention of water in the body compartments dilutes the constituents of the extracellular space including sodium, causing hyponatraemia. Water retention occurs much more frequently than sodium loss, and where there is no evidence of fluid loss from history or examination, water retention as the mechanism becomes a near-certainty.

Water retention

The causes of hyponatraemia due to water retention are shown in Figure 8.1.

Fig 8.1 The causes of hyponatraemia

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Gram-Negative Bacilli

Asporogenous Gram-negative bacilli of clinical importance can be divided into two major groups. Glucose-fermenting, oxidase-negative, and catalase-positive members constitute one group, called Enterobacteriaceae. Several members of this group are normally present in human intestines and others are causal agents of serious infec- tions. The second group, somewhat more heterogeneous, usually called nonfermentative Gram-negative bacilli, are glucose nonfermenters. They are widely distributed in nature and prefer aquatic habitats. However, several members of this group are frequently isolated from human sources and known to cause serious infections. A simple and practical scheme for the grouping of important pathogenic Gram-negative bacteria is depicted at the end of the previous chapter.

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